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The phytohormone auxin is polarly transported in plants by PIN-FORMED (PIN) transporters and controls virtually all growth and developmental processes. Canonical PINs possess a long, largely disordered cytosolic loop. Auxin transport by canonical PINs is activated by loop phosphorylation by certain kinases. The structure of the PIN transmembrane domains was recently determined, their transport properties remained poorly characterized, and the role of the loop in the transport process was unclear. Here, we determined the quantitative kinetic parameters of auxin transport mediated by Arabidopsis PINs to mathematically model auxin distribution in roots and to test these predictions in vivo. Using chimeras between transmembrane and loop domains of different PINs, we demonstrate a strong correlation between transport parameters and physiological output, indicating that the loop domain is not only required to activate PIN-mediated auxin transport, but it has an additional role in the transport process by a currently unknown mechanism. [Display omitted] • PIN-FORMED auxin exporters have different transport properties • The intrinsically disordered loop contributes to the transport process • Properties of the loop domains are interchangeable between PINs • Slow auxin transport in the PIN2 domain is required for gravitropic root growth Polar auxin transport regulates plant growth responses, and Janacek et al. establish that PIN-FORMED auxin exporters have transport properties unique to every individual PIN. These properties depend on the interplay of transmembrane and loop domains to precisely fine-tune auxin flow to mediate growth responses. [ABSTRACT FROM AUTHOR]

Summary Soybean, a staple crop on a global scale, frequently encounters challenges due to lodging under high planting densities, which results in significant yield losses. Despite extensive research, the fundamental genetic mechanisms governing lodging resistance in soybeans remain elusive. In this study, we identify and characterize the Creeping Stem 1 (CS1) gene, which plays a crucial role in conferring lodging resistance in soybeans. The CS1 gene encodes a HEAT‐repeat protein that modulates hypocotyl gravitropism by regulating amyloplast sedimentation. Functional analysis reveals that the loss of CS1 activity disrupts polar auxin transport, vascular bundle development and the biosynthesis of cellulose and lignin, ultimately leading to premature lodging and aberrant root development. Conversely, increasing CS1 expression significantly enhances lodging resistance and improves yield under conditions of high planting density. Our findings shed light on the genetic mechanisms that underlie lodging resistance in soybeans and highlight the potential of CS1 as a valuable target for genetic engineering to improve crop lodging resistance and yield. [ABSTRACT FROM AUTHOR]

In 'Hass' avocado (Persea americana), fruit presence reduces next season flowering. Recent fruit tree studies proposed that heavy fruit load (HFL) generates an auxin (indole-3-acetic acid, IAA) signal in the buds that represses flowering. However, the nature of this signal remains unknown. Here, we investigated the effect of avocado HFL on bud IAA accumulation and flowering transition. We found that IAA–aspartate and IAA–glutamate conjugate levels were significantly higher in buds from fully loaded ('on') than low-loaded ('off') trees, hinting that free IAA levels were higher in the former. Expression analysis showed that coinciding with flowering reduction, HFL induced the floral repressor PaTFL1 , and suggested that accumulation of IAA in buds as imposed by HFL was associated with its conjugation to aspartate and glutamate and resulted both from de novo IAA synthesis and from reduced IAA export. Accordingly, experiments involving radiolabelled [14C]IAA demonstrated that HFL reduced shoot basipetal IAA transport. Finally, we confirmed the negative effects of IAA on flowering, showing that IAA and polar auxin transport blocker (2,3,5-triiodobenzoic acid) treatments delayed 'off' trees' inflorescence development, reducing their inflorescence axis and inducing PaTFL1 expression. Together, our data indicate that avocado HFL generates IAA signalling in buds that induces PaTFL1 , leading to repression of inflorescence development. [ABSTRACT FROM AUTHOR]

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Significant progress has been made in the functions of auxin efflux transporter PIN-FORMED (PIN) genes for the regulation of growth and development in rice. However, knowledge on the roles of OsPIN genes in abiotic stresses is limited. We previously reported that the mutation of OsPIN1b alters rice architecture and root gravitropism, while the role of OsPIN1b in the regulation of rice abiotic stress adaptations is still largely elusive. In the present study, two homozygous ospin1b mutants (C1b-1 and C1b-2) were employed to investigate the roles of OsPIN1b in regulating abiotic stress adaptations. Low temperature gradually suppressed OsPIN1b expression, while osmotic stress treatment firstly induced and then inhibited OsPIN1b expression. Most OsPIN genes and auxin biosynthesis key genes OsYUC were up-regulated in ospin1b leaves, implying that auxin homeostasis is probably disturbed in ospin1b mutants. The loss of function of OsPIN1b significantly decreased rice chilling tolerance, which was evidenced by decreased survival rate, increased death cells and ion leakage under chilling conditions. Compared with the wild-type (WT), ospin1b mutants accumulated more hydrogen peroxide (H2O2) and less superoxide anion radicals (O 2 −) after chilling treatment, indicating that reactive oxygen species (ROS) homeostasis is disrupted in ospin1b mutants. Consistently, C-repeat binding factor (CBF)/dehydration-responsive element binding factor (DREB) genes were downregulated in ospin1b mutants, implying that OsDREB genes are implicated in OsPIN1b-mediated chilling impairment. Additionally, the mutation of OsPIN1b led to decreased sensitivity to abscisic acid (ABA) treatment in seed germination, impaired drought tolerance in the seedlings and changed expression of ABA-associated genes in rice roots. Taken together, our investigations revealed that OsPIN1b is implicated in chilling and drought tolerance in rice and provide new insight for improving abiotic stress tolerance in rice. [ABSTRACT FROM AUTHOR]

The auxin efflux transporter PIN-FORMED (PIN) family is one of the major protein families that facilitates polar auxin transport in plants. Here, we report that overexpression of OsPIN9 leads to altered plant architecture and chilling tolerance in rice. The expression profile analysis indicated that OsPIN9 was gradually suppressed by chilling stress. The shoot height and adventitious root number of OsPIN9-overexpressing (OE) plants were significantly reduced at the seedling stage. The roots of OE plants were more tolerant to N-1-naphthylphthalamic acid (NPA) treatment than WT plants, indicating the disturbance of auxin homeostasis in OE lines. The chilling tolerance assay showed that the survival rate of OE plants was markedly lower than that of wild-type (WT) plants. Consistently, more dead cells, increased electrolyte leakage, and increased malondialdehyde (MDA) content were observed in OE plants compared to those in WT plants under chilling conditions. Notably, OE plants accumulated more hydrogen peroxide (H2O2) and less superoxide anion radicals ( O 2 − ) than WT plants under chilling conditions. In contrast, catalase (CAT) and superoxide dismutase (SOD) activities in OE lines decreased significantly compared to those in WT plants at the early chilling stage, implying that the impaired chilling tolerance of transgenic plants is probably attributed to the sharp induction of H2O2 and the delayed induction of antioxidant enzyme activities at this stage. In addition, several OsRboh genes, which play a crucial role in ROS production under abiotic stress, showed an obvious increase after chilling stress in OE plants compared to that in WT plants, which probably at least in part contributes to the production of ROS under chilling stress in OE plants. Together, our results reveal that OsPIN9 plays a vital role in regulating plant architecture and, more importantly, is involved in regulating rice chilling tolerance by influencing auxin and ROS homeostasis. [ABSTRACT FROM AUTHOR]

Summary: PIN‐FORMED auxin efflux transporters, a subclass of which is plasma membrane‐localised, mediate a variety of land‐plant developmental processes via their polar localisation and subsequent directional auxin transport. We provide the first characterisation of PIN proteins in liverworts using Marchantia polymorpha as a model system. Marchantia polymorpha possesses a single PIN‐FORMED gene, whose protein product is predicted to be plasma membrane‐localised, MpPIN1.To characterise MpPIN1, we created loss‐of‐function alleles and produced complementation lines in both M. polymorpha and Arabidopsis. In M. polymorpha, gene expression and protein localisation were tracked using an MpPIN1 transgene encoding a translationally fused fluorescent protein.Overexpression of MpPIN1 can partially complement loss of an orthologous gene, PIN‐FORMED1, in Arabidopsis. In M. polymorpha, MpPIN1 influences development in numerous ways throughout its life cycle. Most notably, MpPIN1 is required to establish gemmaling dorsiventral polarity and for orthotropic growth of gametangiophore stalks, where MpPIN1 is basally polarised.PIN activity is largely conserved within land plants, with PIN‐mediated auxin flow providing a flexible mechanism to organise growth. Specifically, PIN is fundamentally linked to orthotropism and to the establishment of de novo meristems, the latter potentially involving the formation of both auxin biosynthesis maxima and auxin‐signalling minima. [ABSTRACT FROM AUTHOR]

SUMMARY: The supply of boron (B) alleviates the toxic effects of aluminum (Al) on root growth; however, the mechanistic basis of this process remains elusive. This study filled this knowledge gap, demonstrating that boron modifies auxin distribution and transport in Al‐exposed Arabidopsis roots. In B‐deprived roots, treatment with Al induced an increase in auxin content in the root apical meristem zone (MZ) and transition zone (TZ), whereas in the elongation zone (EZ) the auxin content was decreased beyond the level required for adequate growth. These distribution patterns are explained by the fact that basipetal auxin transport from the TZ to the EZ was disrupted by Al‐inhibited PIN‐FORMED 2 (PIN2) endocytosis. Experiments involving the modulation of protein biosynthesis by cycloheximide (CHX) and transcriptional regulation by cordycepin (COR) demonstrated that the Al‐induced increase of PIN2 membrane proteins was dependent upon the inhibition of PIN2 endocytosis, rather than on the transcriptional regulation of the PIN2 gene. Experiments reporting on the profiling of Al3+ and PIN2 proteins revealed that the inhibition of endocytosis of PIN2 proteins was the result of Al‐induced limitation of the fluidity of the plasma membrane. The supply of B mediated the turnover of PIN2 endosomes conjugated with indole‐3‐acetic acid (IAA), and thus restored the Al‐induced inhibition of IAA transport through the TZ to the EZ. Overall, the reported results demonstrate that boron supply mediates PIN2 endosome‐based auxin transport to alleviate Al toxicity in plant roots. Significance Statement: Aluminum toxicity and boron deficiency are limiting factors for agricultural production in acid soils, which make up almost half of the world's arable land. Boron fertilization alleviates aluminum toxicity but the mechanism remains unclear. The study shows that boron supply releases aluminum‐induced root growth arrest by facilitating endosome‐based polar auxin transport via PIN2 polar localization in the upper half of the plasma membrane (shootward). We provide solid support for boron fertilizer in remediating aluminum toxicity in acid soils. [ABSTRACT FROM AUTHOR]

SUMMARY: Auxin is indispensable to the fertilization‐induced coordinated development of the embryo, endosperm, and seed coat. However, little attention has been given to the distribution pattern, maintenance mechanism, and function of auxin throughout the process of seed development. In the present study, we found that auxin response signals display a dynamic distribution pattern during Arabidopsis seed development. Shortly after fertilization, strong auxin response signals were observed at the funiculus, chalaza, and micropylar integument where the embryo attaches. Later, additional signals appeared at the middle layer of the inner integument (ii1′) above the chalaza and the whole inner layer of the outer integument (oi1). These signals peaked when the seed was mature, then declined upon desiccation and disappeared in the dried seed. Auxin biosynthesis genes, including ASB1, TAA1, YUC1, YUC4, YUC8, and YUC9, contributed to the accumulation of auxin in the funiculus and seed coat. Auxin efflux carrier PIN3 and influx carrier AUX1 also contributed to the polar auxin distribution in the seed coat. PIN3 was expressed in the ii1 (innermost layer of the inner integument) and oi1 layers of the integument and showed polar localization. AUX1 was expressed in both layers of the outer integument and the endosperm and displayed a uniform localization. Further research demonstrated that the accumulation of auxin in the seed coat regulates seed size. Transgenic plants that specifically express the YUC8 gene in the oi2 or ii1 seed coat produced larger seeds. These results provide useful tools for cultivating high‐yielding crops. Significance Statement: The present study revealed the dynamic auxin distribution pattern and its establishment and maintenance mechanism throughout seed development in Arabidopsis thaliana and further demonstrated that the accumulation of auxin in the seed coat regulates seed size. [ABSTRACT FROM AUTHOR]

Significant progress has been made in the functions of auxin efflux transporter PIN-FORMED (PIN) genes for the regulation of growth and development in rice. However, knowledge on the roles of OsPIN genes in abiotic stresses is limited. We previously reported that the mutation of OsPIN1b alters rice architecture and root gravitropism, while the role of OsPIN1b in the regulation of rice abiotic stress adaptations is still largely elusive. In the present study, two homozygous ospin1b mutants (C1b-1 and C1b-2) were employed to investigate the roles of OsPIN1b in regulating abiotic stress adaptations. Low temperature gradually suppressed OsPIN1b expression, while osmotic stress treatment firstly induced and then inhibited OsPIN1b expression. Most OsPIN genes and auxin biosynthesis key genes OsYUC were up-regulated in ospin1b leaves, implying that auxin homeostasis is probably disturbed in ospin1b mutants. The loss of function of OsPIN1b significantly decreased rice chilling tolerance, which was evidenced by decreased survival rate, increased death cells and ion leakage under chilling conditions. Compared with the wild-type (WT), ospin1b mutants accumulated more hydrogen peroxide (H2O2) and less superoxide anion radicals (O2−) after chilling treatment, indicating that reactive oxygen species (ROS) homeostasis is disrupted in ospin1b mutants. Consistently, C-repeat binding factor (CBF)/dehydration-responsive element binding factor (DREB) genes were downregulated in ospin1b mutants, implying that OsDREB genes are implicated in OsPIN1b-mediated chilling impairment. Additionally, the mutation of OsPIN1b led to decreased sensitivity to abscisic acid (ABA) treatment in seed germination, impaired drought tolerance in the seedlings and changed expression of ABA-associated genes in rice roots. Taken together, our investigations revealed that OsPIN1b is implicated in chilling and drought tolerance in rice and provide new insight for improving abiotic stress tolerance in rice.

Locoweeds are leguminous forbs known for their toxicity to livestock caused by the endophytic fungi Alternaria sect. Undifilum. Unlike the defensive mutualisms reported in many toxin-producing endophytes and their plant hosts, the benefits that A. sect. Undifilum can confer to it host plants remains unclear. Here, we conducted physiological and genetic analyses to show that A. (sect. Undifilum) oxytropis influences growth, especially root development, in its locoweed host Oxytropis ochrocephala and Arabidopsis. The presence of A. oxytropis significantly decreased primary root length while increasing the numbers of lateral roots and root hairs, and increasing plant leaf area and fresh weight. The fungus also increased the concentrations of plant endogenous auxin, and the expression of key genes for auxin biosynthesis, signaling, and transport. These effects on root development were abolished in mutants deficient in auxin signaling and polar transport. Alternaria oxytropis down-regulated expression of PIN1 but increased expression of PIN2 , PIN7 , and AUX1 , which might reflect alterations in the spatial accumulation of auxin responsible for the changes in root architecture. Plant growth was insensitive to A. oxytropis when naphthylphthalamic acid was applied. Our findings indicate a function of A. oxytropis in promoting the growth and development of Arabidopsis via the regulation of auxin, which in turn suggests a possible role in benefiting its locoweed hosts via a process independent of its toxin production. [ABSTRACT FROM AUTHOR]

PIN-FORMED- (PIN) mediated polar auxin transport plays a predominant role in most auxin-triggered organogenesis in plants. Global control of PIN polarity at the plasma membrane contributes to the essential establishment of auxin maxima in most multicellular tissues. However, establishment of auxin maxima in single cells is poorly understood. Cotton fibers, derived from ovule epidermal cells by auxin-triggered cell protrusion, provide an ideal model to explore the underlying mechanism. Here, we report that cell-specific degradation of GhPIN3a, which guides the establishment of the auxin gradient in cotton ovule epidermal cells, is associated with the preferential expression of GhROP6 GTPase in fiber cells. In turn, GhROP6 reduces GhPIN3a abundance at the plasma membrane and facilitates intracellular proteolysis of GhPIN3a. Overexpression and activation of GhROP6 promote cell elongation, resulting in a substantial improvement in cotton fiber length. [ABSTRACT FROM AUTHOR]

Growth and development, and polar auxin transport in epicotyls of etiolated pea (Pisum sativum L. cv Alaska) seedlings grown under hypergravity conditions produced by centrifugation were studied. Seeds were allowed to germinate and grow under various magnitude of hypergravity conditions for 3.5 days in the dark. Hypergravity at up to 50 G had little effect on epicotyl elongation growth, but increasing gravity force up to 300 G resulted in a reduction of elongation growth, the reduction at 100 G and 200 G being ca. 30% and 50% of control, respectively. Although expressions of genes playing an important role in polar auxin transport, auxin efflux carrier PsPINs and influx carrier PsAUX1, were not affected by hypergravity conditions, polar auxin transport activities of the epicotyls were substantially reduced, those at 100 G and 200 G being ca. 80% and 60% of control, respectively. In addition, close relationships between inhibition of elongation growth and polar auxin transport of the epicotyls were observed. These results suggest that hypergravity-regulated polar auxin activity is involved in inhibition of elongation growth of epicotyls of etiolated pea seedlings. The mechanisms to regulate elongation growth and polar auxin transport are discussed. [ABSTRACT FROM AUTHOR]

SUMMARY: Somatic embryogenesis (SE), or embryo development from in vitro cultured vegetative explants, can be induced in Arabidopsis by the synthetic auxin 2,4‐dichlorophenoxyacetic acid (2,4‐D) or by overexpression of specific transcription factors, such as AT‐HOOK MOTIF NUCLEAR LOCALIZED 15 (AHL15). Here, we explored the role of endogenous auxin [indole‐3‐acetic acid (IAA)] during 2,4‐D and AHL15‐induced SE. Using the pWOX2:NLS‐YFP reporter, we identified three distinct developmental stages for 2,4‐D and AHL15‐induced SE in Arabidopsis, with these being (i) acquisition of embryo identity; (ii) formation of pro‐embryos; and (iii) somatic embryo patterning and development. The acquisition of embryo identity coincided with enhanced expression of the indole‐3‐pyruvic acid auxin biosynthesis YUCCA genes, resulting in an enhanced pDR5:GFP‐reported auxin response in the embryo‐forming tissues. Chemical inhibition of the indole‐3‐pyruvic acid pathway did not affect the acquisition of embryo identity, but significantly reduced or completely inhibited the formation of pro‐embryos. Co‐application of IAA with auxin biosynthesis inhibitors in the AHL15‐induced SE system rescued differentiated somatic embryo formation, confirming that increased IAA levels are important during the last two stages of SE. Our analyses also showed that polar auxin transport, with AUXIN/LIKE‐AUX influx and PIN‐FORMED1 efflux carriers as important drivers, is required for the transition of embryonic cells to proembryos and, later, for correct cell fate specification and differentiation. Taken together, our results indicate that endogenous IAA biosynthesis and its polar transport are not required for the acquisition of embryo identity, but rather to maintain embryonic cell identity and for the formation of multicellular proembryos and their development into histodifferentiated embryos. Significance Statement: We show that the induction of embryo identity during 2,4‐D or transcription factor‐induced somatic embryogenesis in Arabidopsis does not rely on an increase in auxin biosynthesis, or the auxin efflux and influx machinery. Rather, auxin biosynthesis and auxin influx are required for the maintenance of embryonic cell identity and formation of pro‐embryos, as well as both auxin biosynthesis and its polar transport for the further development of these pro‐embryos into differentiated somatic embryos. [ABSTRACT FROM AUTHOR]

The plant hormone auxin is certainly the most studied developmental regulator in plants. The many functions of auxin during development, from the embryo to the root and shoot construction, are mediated by an ever-growing collection of molecular regulators, with an overwhelming degree of both ubiquity and complexity that we are still far from fully understanding and that biological experiments alone cannot grasp. In this review, we discuss how bioinformatics and computational modeling approaches have helped in recent years to explore this complexity and to push the frontiers of our understanding of auxin biology. We focus on how analysis of massive amounts of genomic data and construction of computational models to simulate auxin-regulated processes at different scales have complemented wet experiments to increase the understanding of how auxin acts in the nucleus to regulate transcription and how auxin movement between cells regulates development at the tissular scale. [ABSTRACT FROM AUTHOR]

Auxin is a hormone that delays ripening in part by reducing anthocyanin content and impairing color development. Auxin content declines during the ripening process, whereas sugars accumulate from pre-veraison onwards. The spatio-temporal distribution of this hormone depends in part on polar auxin transport. On the other hand, sugar, acting as a signal molecule, modulates auxin distribution in model organisms; however, its effect on polar auxin transport during the coloring process in grapevine berries has not been investigated. To address this issue, we characterized auxin transport and sugar variations during the ripening process and performed treatments intended to alter auxin homeostasis in grape fruits. We found that polar auxin transport declines in concert with increasing sugar content prior to and at veraison. Moreover, N-1-napthylphthalamic acid (NPA; a polar auxin transport inhibitor) and glucose treatment increased berry coloration, reduced polar auxin transport and VvPIN1 transcript abundance at pre-veraison, and combined NPA and glucose treatment further increased berry color compared to glucose and NPA alone. Indole-3-acetic acid (IAA) treatment prevented the negative effect of glucose on auxin transport, suggesting that auxin homeostasis might be relevant for glucose modulation of berry ripening in grapevine. Impaired auxin transport is associated with increased ethylene sensitivity in several plant processes, including fruit abscission. For exploring a potential involvement of the ethylene pathway during the coloring process, we analyzed the transcript abundance of the putative ethylene receptor, VvETR2, a possible negative regulator of the ethylene pathway. We found that glucose plus NPA treatment reduced VvETR2 transcript abundance, whose expression is reported to decline from veraison onwards. Our results suggest a possible mechanism in which a rise in glucose contributes to auxin transport inhibition in coloring berries. As glucose has been reported to promote ripening and IAA inhibits berry coloring, our results further support an antagonistic switch between IAA and glucose, that could also involve changes in the expression of the VvETR2 gene. [ABSTRACT FROM AUTHOR]

Summary: Shade affects all aspects of plant growth and development, including seed germination, hypocotyl elongation, petiole growth, leaf hyponasty, and flowering time. Here, we found that mutations in the key Arabidopsis karrikins signal perception‐associated KARRIKIN INSENSITIVE 2 (KAI2) gene, encoding an α/β‐fold hydrolase, and the MORE AXILLARY GROWTH 2 (MAX2) gene, encoding an F‐box protein, led to greater hypocotyl elongation under shade avoidance conditions.We further verified that these phenotypes were caused by perception of the endogenous KAI2‐ligands (KLs), and that this phenotype is independent of strigolactone biosynthetic or signaling pathways. Upon perception of a KL, it is probable that the target protein forms a complex with the KAI2/MAX2 proteins, which are degraded through the action of the 26S proteasome.We demonstrated that SUPPRESSOR OF MAX2‐1 (SMAX1) is the degradation target for the KAI2/MAX2 complex in the context of shade avoidance. KAI2 and MAX2 require SMAX1 to limit the hypocotyl growth associated with shade avoidance. Treatment with l‐kynurenine, an inhibitor of auxin accumulation, partially restored elongation of kai2 mutant hypocotyls under simulated shade. Furthermore, KAI2 is involved in regulating auxin accumulation and polar auxin transport, which may contribute to the hypocotyl shade response.In addition, SMAX1 gene overexpression promoted the hypocotyl shade response. RNA‐sequencing analysis revealed that SMAX1‐overexpression affected the expression of many auxin homeostasis genes, especially under simulated shade. Altogether, our data support the conclusion that KL signaling regulates shade avoidance by modulating auxin homeostasis in the hypocotyl. [ABSTRACT FROM AUTHOR]

We analyze an ‘up-the-gradient’ model for the formation of transport channels of the phytohormone auxin, through auxin-mediated polarization of the PIN1 auxin transporter. We show that this model admits a family of travelling wave solutions that is parameterized by the height of the auxin-pulse. We uncover scaling relations for the speed and width of these waves and verify these rigorous results with numerical computations. In addition, we provide explicit expressions for the leading-order wave profiles, which allows the influence of the biological parameters in the problem to be readily identified. Our proofs are based on a generalization of the scaling principle developed by Friesecke and Pego to construct pulse solutions to the classic Fermi–Pasta–Ulam–Tsingou model, which describes a one-dimensional chain of coupled nonlinear springs. [ABSTRACT FROM AUTHOR]

Arabidopsis mitochondrial-localized heat shock protein 70-1 (mtHSC70-1) modulates vegetative growth by assisting mitochondrial complex IV assembly and maintaining reactive oxygen species (ROS) homeostasis. In addition, mtHSC70-1 affects embryo development, and this effect is mediated by auxin. However, whether mtHSC70-1 regulates vegetative growth through auxin and knowledge of the link between ROS homeostasis and auxin distribution remain unclear. Here, we found that mtHSC70-1 knockout seedlings (mthsc70-1a) displayed shortened roots, decreased fresh root weight and lateral root number, increased root width and abnormal root morphology. The introduction of the mtHSC70-1 gene into mthsc70-1a restored the growth and development of roots to the level of the wild type. However, sugar and auxin supplementation could not help the mutant roots restore to normal. Moreover, mthsc70-1a seedlings showed a decrease in meristem length and activity, auxin transport carrier (PINs and AUX1) and auxin abundances in root tips. The application of exogenous reducing agents upregulated the levels of PINs in the mutant roots. The introduction of antioxidant enzyme genes (MSD1 or CAT1) into the mthsc70-1a mutant rescued the PIN and local auxin abundances and root growth and development. Taken together, our data suggest that mtHSC70-1 regulates polar auxin transport through ROS homeostasis in Arabidopsis roots. [ABSTRACT FROM AUTHOR]

To reveal the mechanisms underlying root adaptation to drought stress, we isolated and characterized an Arabidopsis mutant, dig5 (drought inhibition of lateral root growth 5), which exhibited increased sensitivity to the phytohormone abscisic acid (ABA) for the inhibition of lateral root growth. The dig5 mutant also had fewer lateral roots under normal conditions and the aerial parts were yellowish with a lower level of chlorophylls. The mutant seedlings also displayed phenotypes indicative of impaired auxin transport, such as abnormal root curling, leaf venation defects, absence of apical hook formation, and reduced hypocotyl elongation in darkness. Auxin transport assays with [3H]-labeled indole acetic acid (IAA) confirmed that dig5 roots were impaired in polar auxin transport. Map-based cloning and complementation assays indicated that the DIG5 locus encodes a chloroplast-localized tRNA adenosine deaminase arginine (TADA) that is involved in chloroplast protein translation. The levels of flavonoids, which are naturally occurring auxin transport inhibitors in plants, were significantly higher in dig5 roots than in the wild type roots. Further investigation showed that flavonoid biosynthetic genes were upregulated in dig5. Introduction of the flavonoid biosynthetic mutation transparent testa 4 (tt4) into dig5 restored the lateral root growth of dig5. Our study uncovers an important role of DIG5/TADA in retrogradely controlling flavonoid biosynthesis and lateral root development. We suggest that the DIG5-related signaling pathways, triggered likely by drought-induced chlorophyll breakdown and leaf senescence, may potentially help the plants to adapt to drought stress through optimizing the root system architecture. [ABSTRACT FROM AUTHOR]

The root cortex transports water and nutrients absorbed by the root epidermis into the vasculature and stores substances such as starch, resins, and essential oils. The cortical cells are also deeply involved in determining epidermal cell fate. In Arabidopsis thaliana roots, the cortex is composed of a single cell layer generated by a single round of periclinal division of the cortex/endodermis initials. To further explore cortex development, we traced the development of the cortex by counting cortical cells. Unlike vascular cells, whose number increased during the development of root apical meristem (RAM), the number of cortical cells did not change, indicating that cortical cells do not divide during RAM development. However, auxin-induced cortical cell division, and this finding was confirmed by treatment with the auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) and examining transgenic plants harboring CO2:ARF5, in which cortical expression of truncated AUXIN RESPONSE FACTOR5 (ARF5) induces auxin responses. NPA-induced cortical auxin accumulation and CO2:ARF5-mediated cortical auxin response induced anticlinal and periclinal cell divisions, thus increasing the number of cortical cells. These findings reveal a tight link between auxin and cortical cell division, suggesting that auxin is a key player in determining root cortical cell division. [ABSTRACT FROM AUTHOR]

The auxin efflux transporter PIN-FORMED (PIN) family is one of the major protein families that facilitates polar auxin transport in plants. Here, we report that overexpression of OsPIN9 leads to altered plant architecture and chilling tolerance in rice. The expression profile analysis indicated that OsPIN9 was gradually suppressed by chilling stress. The shoot height and adventitious root number of OsPIN9-overexpressing (OE) plants were significantly reduced at the seedling stage. The roots of OE plants were more tolerant to N-1-naphthylphthalamic acid (NPA) treatment than WT plants, indicating the disturbance of auxin homeostasis in OE lines. The chilling tolerance assay showed that the survival rate of OE plants was markedly lower than that of wild-type (WT) plants. Consistently, more dead cells, increased electrolyte leakage, and increased malondialdehyde (MDA) content were observed in OE plants compared to those in WT plants under chilling conditions. Notably, OE plants accumulated more hydrogen peroxide (H2O2) and less superoxide anion radicals (O2−) than WT plants under chilling conditions. In contrast, catalase (CAT) and superoxide dismutase (SOD) activities in OE lines decreased significantly compared to those in WT plants at the early chilling stage, implying that the impaired chilling tolerance of transgenic plants is probably attributed to the sharp induction of H2O2 and the delayed induction of antioxidant enzyme activities at this stage. In addition, several OsRboh genes, which play a crucial role in ROS production under abiotic stress, showed an obvious increase after chilling stress in OE plants compared to that in WT plants, which probably at least in part contributes to the production of ROS under chilling stress in OE plants. Together, our results reveal that OsPIN9 plays a vital role in regulating plant architecture and, more importantly, is involved in regulating rice chilling tolerance by influencing auxin and ROS homeostasis.

The distribution and content of auxin within plant tissues affect a variety of important growth and developmental processes. Polar auxin transport (PAT), mainly mediated by auxin influx and efflux transporters, plays a vital role in determining auxin maxima and gradients in plants. The auxin efflux carrier PIN-FORMED (PIN) family is one of the major protein families involved in PAT. Rice (Oryza sativa L.) genome possesses 12 OsPIN genes. However, the detailed functions of OsPIN genes involved in regulating the rice architecture and gravity response are less well understood. In the present study, OsPIN1b was disrupted by CRISPR/Cas9 technology, and its roles in modulating rice architecture and root gravitropism were investigated. Tissue-specific analysis showed that OsPIN1b was mainly expressed in roots, stems and sheaths at the seedling stage, and the transcript abundance was progressively decreased during the seedling stages. Expression of OsPIN1b could be quickly and greatly induced by NAA, indicating that OsPIN1b played a vital role in PAT. IAA homeostasis was disturbed in ospin1b mutants, as evidenced by the changed sensitivity of shoot and root to NAA and NPA treatment, respectively. Mutation of OsPIN1b resulted in pleiotropic phenotypes, including decreased growth of shoots and primary roots, reduced adventitious root number in rice seedlings, as well as shorter and narrower leaves, increased leaf angle, more tiller number and decreased plant height and panicle length at the late developmental stage. Moreover, ospin1b mutants displayed a curly root phenotype cultured with tap water regardless of lighting conditions, while nutrient solution culture could partially rescue the curly root phenotype in light and almost completely abolish this phenotype in darkness, indicating the involvement of the integration of light and nutrient signals in root gravitropism regulation. Additionally, amyloplast sedimentation was impaired in the peripheral tiers of the ospin1b root cap columella cell, while it was not the main contributor to the abnormal root gravitropism. These data suggest that OsPIN1b not only plays a vital role in regulating rice architecture but also functions in regulating root gravitropism by the integration of light and nutrient signals. [ABSTRACT FROM AUTHOR]

Summary: De novo shoot organogenesis is a prerequisite for numerous applications in plant research and breeding but is often a limiting factor, for example, in genome editing approaches. Class III homeodomain‐leucine zipper (HD‐ZIP III) transcription factors have been characterized as crucial regulators of shoot specification, however up‐stream components controlling their activity during shoot regeneration are only partially identified.In a chemical genetic screen, we isolated ZIC2, a novel activator of HD‐ZIP III activity. Using molecular, physiological and hormone transport analyses in Arabidopsis and sunflower (Helianthus annuus), we examined the molecular mechanism by which the drug promotes HD‐ZIP III expression.ZIC2‐dependent upregulation of HD‐ZIP III transcription promotes shoot regeneration in Arabidopsis and is accompanied by the induction of shoot specifying factors WUS and RAP2.6L and a subset of cytokinin biosynthesis enzymes. ZIC2's effect on HD‐ZIP III expression and regeneration is based on its ability to limit polar auxin transport. We further provide evidence that chemical modulation of auxin efflux can enhance de novo shoot formation in the regeneration recalcitrant species sunflower.Activation of HD‐ZIP III transcription during shoot regeneration depends on the local distribution of auxin and chemical modulation of auxin transport can be used to overcome poor shoot organogenesis in tissue culture. [ABSTRACT FROM AUTHOR]

The apical hook is formed by dicot seedlings to protect the tender shoot apical meristem during soil emergence. Regulated by many phytohormones, the apical hook has been taken as a model to study the crosstalk between individual signaling pathways. Over recent decades, the roles of different phytohormones and environmental signals in apical hook development have been illustrated. However, key regulators downstream of canonical hormone signaling have rarely been identified via classical genetics screening, possibly due to genetic redundancy and/or lethal mutation. Chemical genetics that utilize small molecules to perturb and elucidate biological processes could provide a complementary strategy to overcome the limitations in classical genetics. In this review, we summarize current progress in hormonal regulation of the apical hook, and previously reported chemical tools that could assist the understanding of this complex developmental process. We also provide insight into novel strategies for chemical screening and target identification, which could possibly lead to discoveries of new regulatory components in apical hook development, or unidentified signaling crosstalk that is overlooked by classical genetics screening. [ABSTRACT FROM AUTHOR]

Apple species are the unique naturally rich source of dihydrochalcones, phenolic compounds with an elusive role in planta, but suggested auto-allelochemical features related to "apple replant disease" (ARD). Our aim was to elucidate the physiological basis of the phytotoxic action of dihydrochalcone phloretin in the model plant Arabidopsis and to promote phloretin as a new prospective eco-friendly phytotoxic compound. Phloretin treatment induced a significant dose-dependent growth retardation and severe morphological abnormalities and agravitropic behavior in Arabidopsis seedlings. Histological examination revealed a reduced starch content in the columella cells and a serious disturbance in root architecture, which resulted in the reduction in length of meristematic and elongation zones. Significantly disturbed auxin metabolome profile in roots with a particularly increased content of IAA accumulated in the lateral parts of the root apex, accompanied by changes in the expression of auxin biosynthetic and transport genes, especially PIN1, PIN3, PIN7, and ABCB1, indicates the role of auxin in physiological basis of phloretin-induced growth retardation. The results reveal a disturbance of auxin homeostasis as the main mechanism of phytotoxic action of phloretin. This mechanism makes phloretin a prospective candidate for an eco-friendly bioherbicide and paves the way for further research of phloretin role in ARD. [ABSTRACT FROM AUTHOR]

Auxin is the earliest discovered plant hormone, which plays important roles in each aspect of plant growth and development. There are two main pathways for auxin to be transported from the synthetic site (such as young leaves and terminal buds) to the active site. First, auxin is transported over long distances through phloem in an unfixed direction throughout the whole plant. Second, short-distance polar transport between cells requires the participation of auxin carriers, including unidirectional transport from stem tip to root and local unidirectional transport between tissues. Polar transport is critical to the establishment and maintenance of the auxin concentration gradient, which specifically regulates plant growth and development and responds to environmental changes. In this article, we reviewed the research progress of auxin transporters AUX1/LAX, PIN, and ABCB families, and some potential auxin transporters in rice growth and development, which provide information for the interpretation of biological functions of polar auxin transport families and lay a foundation for the genetic improvement of important agronomic traits in rice. [ABSTRACT FROM AUTHOR]

BIG, a regulator of polar auxin transport, is necessary to regulate the growth and development of Arabidopsis. Although mutations in the BIG gene cause severe root developmental defects, the exact mechanism remains unclear. Here, we report that disruption of the BIG gene resulted in decreased quiescent center (QC) activity and columella cell numbers, which was accompanied by the downregulation of WUSCHEL-RELATED HOMEOBOX5 (WOX5) gene expression. BIG affected auxin distribution by regulating the expression of PIN-FORMED proteins (PINs), but the root morphological defects of big mutants could not be rescued solely by increasing auxin transport. Although the loss of BIG gene function resulted in decreased expression of the PLT1 and PLT2 genes, genetic interaction assays indicate that this is not the main reason for the root morphological defects of big mutants. Furthermore, genetic interaction assays suggest that BIG affects the stem cell niche (SCN) activity through the SCRSCARECROW (SCR)/SHORT ROOT (SHR) pathway and BIG disruption reduces the expression of SCR and SHR genes. In conclusion, our findings reveal that the BIG gene maintains root meristem activity and SCN integrity mainly through the SCR/SHR pathway. [ABSTRACT FROM AUTHOR]

The plant root is an important storage organ that stores indole-3-acetic acid (IAA) from the apical meristem, as well as nitrogen, which is obtained from the external environment. IAA and nitrogen act as signaling molecules that promote root growth to obtain further resources. Fluctuations in the distribution of nitrogen in the soil environment induce plants to develop a set of strategies that effectively improve nitrogen use efficiency. Auxin integrates the information regarding the nitrate status inside and outside the plant body to reasonably distribute resources and sustainably construct the plant root system. In this review, we focus on the main factors involved in the process of nitrate- and auxin-mediated regulation of root structure to better understand how the root system integrates the internal and external information and how this information is utilized to modify the root system architecture.

The wild population of Madhuca pasquieri (Dubard) H. J. Lam is currently dwindling; its understory seedlings are rare, and there is a lack of molecular studies, which impedes the conservation of this species. This study exploited second-generation sequencing and widely targeted metabolomics analysis to uncover the dynamic changes in differentially expressed genes (DEGs) and differentially accumulated metabolites (DAMs) in five post-germination stages of M. pasquieri whole organism. Notably, the weighted gene co-expression network analysis (WGCNA), transcriptome, and metabolome association analyses all indicated significant enrichment of the flavonoid biosynthesis pathway in stage 4 (two-leaf), and an upregulation of the genes encoding flavonol biosynthesis in this stage. In stage 5 (nine-leaf), the flavonols were significantly accumulated, indicating that the changes in metabolites were driven at the transcript level. According to the significant changes in gene expression encoding auxin transport carriers and their correlation with flavonols during stage 5, the flavonols were speculated to have a direct inhibitory effect on the expression of PIN4 encoding gene, which may inhibit the process of polar auxin transport. The results provided important insights into the molecular network relationships between the transcription and metabolism of this rare and endangered species during the post-germination stages and explained the reasons for the slow growth of its seedlings at the molecular level.

The plant root is an important storage organ that stores indole-3-acetic acid (IAA) from the apical meristem, as well as nitrogen, which is obtained from the external environment. IAA and nitrogen act as signaling molecules that promote root growth to obtain further resources. Fluctuations in the distribution of nitrogen in the soil environment induce plants to develop a set of strategies that effectively improve nitrogen use efficiency. Auxin integrates the information regarding the nitrate status inside and outside the plant body to reasonably distribute resources and sustainably construct the plant root system. In this review, we focus on the main factors involved in the process of nitrate- and auxin-mediated regulation of root structure to better understand how the root system integrates the internal and external information and how this information is utilized to modify the root system architecture. [ABSTRACT FROM AUTHOR]

The wild population of Madhuca pasquieri (Dubard) H. J. Lam is currently dwindling; its understory seedlings are rare, and there is a lack of molecular studies, which impedes the conservation of this species. This study exploited second-generation sequencing and widely targeted metabolomics analysis to uncover the dynamic changes in differentially expressed genes (DEGs) and differentially accumulated metabolites (DAMs) in five post-germination stages of M. pasquieri whole organism. Notably, the weighted gene co-expression network analysis (WGCNA), transcriptome, and metabolome association analyses all indicated significant enrichment of the flavonoid biosynthesis pathway in stage 4 (two-leaf), and an upregulation of the genes encoding flavonol biosynthesis in this stage. In stage 5 (nine-leaf), the flavonols were significantly accumulated, indicating that the changes in metabolites were driven at the transcript level. According to the significant changes in gene expression encoding auxin transport carriers and their correlation with flavonols during stage 5, the flavonols were speculated to have a direct inhibitory effect on the expression of PIN4 encoding gene, which may inhibit the process of polar auxin transport. The results provided important insights into the molecular network relationships between the transcription and metabolism of this rare and endangered species during the post-germination stages and explained the reasons for the slow growth of its seedlings at the molecular level. [ABSTRACT FROM AUTHOR]

The eukaryotic AGC protein kinase subfamily (protein kinase A/ protein kinase G/ protein kinase C-family) is involved in regulating numerous biological processes across kingdoms, including growth and development, and apoptosis. PDK1(3-phosphoinositide-dependent protein kinase 1) is a conserved serine/threonine kinase in eukaryotes, which is both a member of AGC kinase and a major regulator of many other downstream AGC protein kinase family members. Although extensively investigated in model plant Arabidopsis, detailed reports for tobacco PDK1s have been limited. To better understand the functions of PDK1s in tobacco, CRISPR/CAS9 transgenic lines were generated in tetraploid N. tabacum, cv. Samsun (NN) with 5–7 of the 8 copies of 4 homologous PDK1 genes in tobacco genome (NtPDK1a/1b/1c/1d homologs) simultaneously knocked out. Numerous developmental defects were observed in these NtPDK1a/1b/1c/1d CRISPR/CAS9 lines, including cotyledon fusion leaf shrinkage, uneven distribution of leaf veins, convex veins, root growth retardation, and reduced fertility, all of which reminiscence of impaired polar auxin transport. The severity of these defects was correlated with the number of knocked out alleles of NtPDK1a/1b/1c/1d. Consistent with the observation in Arabidopsis, it was found that the polar auxin transport, and not auxin biosynthesis, was significantly compromised in these knockout lines compared with the wild type tobacco plants. The fact that no homozygous plant with all 8 NtPDK1a/1b/1c/1d alleles being knocked out suggested that knocking out 8 alleles of NtPDK1a/1b/1c/1d could be lethal. In conclusion, our results indicated that Nt PDK1s are versatile AGC kinases that participate in regulation of tobacco growth and development via modulating polar auxin transport. Our results also indicated that CRISPR/CAS9 technology is a powerful tool in resolving gene redundancy in polyploidy plants. • Four PDK1 homologous genes in tetraploid Nicotiana tabacum were selectively knocked out by the CRISPR/CAS9 technology. • The NtPDK1a/1b/1c/1d partial knockout lines displayed growth and development defects in tetraploid N. tabacum. • The severity of these growth/development defects was correlated with the number of alleles of NtPDK1s been knocked out. • The growth and development defects displayed by the NtPDK1a/1b/1c/1d knockout lines resulted from impaired polar auxin transport. [ABSTRACT FROM AUTHOR]

Arabidopsis mitochondrial-localized heat shock protein 70-1 (mtHSC70-1) modulates vegetative growth by assisting mitochondrial complex IV assembly and maintaining reactive oxygen species (ROS) homeostasis. In addition, mtHSC70-1 affects embryo development, and this effect is mediated by auxin. However, whether mtHSC70-1 regulates vegetative growth through auxin and knowledge of the link between ROS homeostasis and auxin distribution remain unclear. Here, we found that mtHSC70-1 knockout seedlings (mthsc70-1a) displayed shortened roots, decreased fresh root weight and lateral root number, increased root width and abnormal root morphology. The introduction of the mtHSC70-1 gene into mthsc70-1a restored the growth and development of roots to the level of the wild type. However, sugar and auxin supplementation could not help the mutant roots restore to normal. Moreover, mthsc70-1a seedlings showed a decrease in meristem length and activity, auxin transport carrier (PINs and AUX1) and auxin abundances in root tips. The application of exogenous reducing agents upregulated the levels of PINs in the mutant roots. The introduction of antioxidant enzyme genes (MSD1 or CAT1) into the mthsc70-1a mutant rescued the PIN and local auxin abundances and root growth and development. Taken together, our data suggest that mtHSC70-1 regulates polar auxin transport through ROS homeostasis in Arabidopsis roots.

Auxin is the earliest discovered plant hormone, which plays important roles in each aspect of plant growth and development. There are two main pathways for auxin to be transported from the synthetic site (such as young leaves and terminal buds) to the active site. First, auxin is transported over long distances through phloem in an unfixed direction throughout the whole plant. Second, short-distance polar transport between cells requires the participation of auxin carriers, including unidirectional transport from stem tip to root and local unidirectional transport between tissues. Polar transport is critical to the establishment and maintenance of the auxin concentration gradient, which specifically regulates plant growth and development and responds to environmental changes. In this article, we reviewed the research progress of auxin transporters AUX1/LAX, PIN, and ABCB families, and some potential auxin transporters in rice growth and development, which provide information for the interpretation of biological functions of polar auxin transport families and lay a foundation for the genetic improvement of important agronomic traits in rice.

Adventitious roots (ARs) are produced from non‐root tissues in response to different environmental signals, such as abiotic stresses, or after wounding, in a complex developmental process that requires hormonal crosstalk. Here, we characterized AR formation in young seedlings of Solanum lycopersicum cv. 'Micro‐Tom' after whole root excision by means of physiological, genetic and molecular approaches. We found that a regulated basipetal auxin transport from the shoot and local auxin biosynthesis triggered by wounding are both required for the re‐establishment of internal auxin gradients within the vasculature. This promotes cell proliferation at the distal cambium near the wound in well‐defined positions of the basal hypocotyl and during a narrow developmental window. In addition, a pre‐established pattern of differential auxin responses along the apical‐basal axis of the hypocotyl and an as of yet unknown cell‐autonomous inhibitory pathway contribute to the temporal and spatial patterning of the newly formed ARs on isolated hypocotyl explants. Our work provides an experimental outline for the dissection of wound‐induced AR formation in tomato, a species that is suitable for molecular identification of gene regulatory networks via forward and reverse genetics approaches. We found that wound‐induced adventitious root formation in tomato (Solanum lycopersicum cv. 'Micro‐Tom') shoot explants requires both long‐range and short‐range regulation of auxin levels through polar auxin transport and local auxin biosynthesis. [ABSTRACT FROM AUTHOR]

Glutathione-dependent root growth in Arabidopsis is linked to polar auxin transport (PAT). Arabidopsis mutants with reduced glutathione (GSH) levels also show reduced PAT. To gain an insight into the relationship between PAT and GSH level, we analyzed tomato polycotyledon mutant, pct1-2, which has enhanced PAT. Microarray analysis of gene expression in pct1-2 mutant revealed underexpression of several genes related to glutamate and glutathione metabolism. In consonance with microarray analysis, enzymatic as well as in vivo assay revealed higher glutathione levels in the early phase of pct1-2 seedling growth than WT. The inhibition of auxin transport by 2,3,5-triiodobenzoic acid (TIBA) reduced both GSH level and PIN1 expression in pct1-2 root tips. The reduction of in vivo GSH accumulation in pct1-2 root tips by buthionine sulfoximine (BSO) stimulated elongation of the short root of pct1-2 mutant akin to TIBA. The rescue of the short root phenotype of the pct1-2 mutant was restricted to TIBA and BSO. The other auxin transport inhibitors 1-N-naphthylphthalamic acid (NPA), 2-[4-(diethylamino)-2-hydroxybenzoyl] benzoic acid (BUM), 3-chloro-4-hydroxyphenylacetic acid (CHPAA), brefeldin and gravacin inhibited root elongation in both WT and pct1-2 mutant. Our results indicate a relationship between PAT and GSH levels in tomato akin to Arabidopsis. Our work also highlights that TIBA rescues the short root phenotype of the pct1-2 mutant by acting on a PAT component distinct from the site of action of other PAT inhibitors. [ABSTRACT FROM AUTHOR]

In many fruit trees, heavy fruit load in one year reduces flowering in the following year, creating a biennial fluctuation in yield termed alternate bearing (AB). In subtropical trees, where flowering induction is mostly governed by the accumulation of chilling hours, fruit load is thought to generate a signal (AB signal) that blocks the perception of cold induction. Fruit removal during a heavy-fruit-load year is effective at inducing flowering only if performed one to a few months before the onset of the flowering induction period. We previously showed that following fruit removal, the content of the auxin indoleacetic acid (IAA) in citrus buds is reduced, suggesting that the hormone plays a role in the AB signal. Here, we demonstrate that fruit presence generates relatively strong polar auxin transport in citrus and olive stems. Upon fruit removal, polar auxin transport is reduced and allows auxin release from the bud. Furthermore, using immunolocalization, hormone, and gene expression analyses, we show that in citrus, IAA level in the bud and specifically in the apical meristem is reduced upon fruit removal. Overall, our data provide support for the notion that fruit presence generates an auxin signal in the bud, which may affect flowering induction. [ABSTRACT FROM AUTHOR]

Summary: The widely used non-steroidal anti-inflammatory drugs (NSAIDs) are derivatives of the phytohormone salicylic acid (SA). SA is well known to regulate plant immunity and development, whereas there have been few reports focusing on the effects of NSAIDs in plants. Our studies here reveal that NSAIDs exhibit largely overlapping physiological activities to SA in the model plant Arabidopsis. NSAID treatments lead to shorter and agravitropic primary roots and inhibited lateral root organogenesis. Notably, in addition to the SA-like action, which in roots involves binding to the protein phosphatase 2A (PP2A), NSAIDs also exhibit PP2A-independent effects. Cell biological and biochemical analyses reveal that many NSAIDs bind directly to and inhibit the chaperone activity of TWISTED DWARF1, thereby regulating actin cytoskeleton dynamics and subsequent endosomal trafficking. Our findings uncover an unexpected bioactivity of human pharmaceuticals in plants and provide insights into the molecular mechanism underlying the cellular action of this class of anti-inflammatory compounds.

Adventitious root (AR) formation is a complex developmental process controlled by a plethora of endogenous and environmental factors. Based on fossil evidence and genomic phylogeny, AR formation might be considered the default state of plant roots, which likely evolved independently several times. The application of next-generation sequencing techniques and bioinformatics analyses to non-model plants provide novel approaches to identify genes putatively involved in AR formation in multiple species. Recent results uncovered that the regulation of shoot-borne AR formation in monocots is an adaptive response to nutrient and water deficiency that enhances topsoil foraging and improves plant performance. A hierarchy of transcription factors required for AR initiation has been identified from genetic studies, and recent results highlighted the key involvement of additional regulation through microRNAs. Here, we discuss our current understanding of AR formation in response to specific environmental stresses, such as nutrient deficiency, drought or waterlogging, aimed at providing evidence for the integration of the hormone crosstalk required for the activation of root competent cells within adult tissues from which the ARs develop.

SUMMARY: Root nutation indicates the behavior that roots grow in a waving and skewing way due to unequal growth rates on different sides. Although a few developmental and environmental factors have been reported, genetic pathways mediating this process are obscure. We report here that the Arabidopsis CrRLK1L family member FERONIA (FER) is critical for root nutation. Functional loss of FER resulted in enhanced root waviness on tilted plates or roots forming anti‐clockwise coils on horizontal plates. Suppressing polar auxin transport, either by pharmacological treatment or by introducing mutations at PIN‐FORMED2 (PIN2) or AUXIN RESISTANT1 (AUX1), suppressed the asymmetric root growth (ARG) in fer‐4, a null mutant of FER, indicating that FER suppression of ARG depends on polar auxin transport. We further showed by pharmacological treatments that dynamic microtubule organization and Ca2+ signaling are both critical for FER‐mediated ARG. Results presented here demonstrate a key role of FER in mediating root nutating growth, through PIN2‐ and AUX1‐mediated auxin transport, through dynamic microtubule organization, and through Ca2+ signaling. Significance Statement: Root nutation indicates the behavior that roots grow in a waving and skewing way due to unequal growth rates on different sides, which likely enhances plant fitness by efficiently sampling soil for better growth environments. We report that the Arabidopsis FERONIA (FER) is critical for root nutation. Polar auxin transport, dynamic microtubule organization, and Ca2+ signaling are critical for FER‐mediated root nutation. [ABSTRACT FROM AUTHOR]

Adventitious root (AR) formation is a complex developmental process controlled by a plethora of endogenous and environmental factors. Based on fossil evidence and genomic phylogeny, AR formation might be considered the default state of plant roots, which likely evolved independently several times. The application of next-generation sequencing techniques and bioinformatics analyses to non-model plants provide novel approaches to identify genes putatively involved in AR formation in multiple species. Recent results uncovered that the regulation of shoot-borne AR formation in monocots is an adaptive response to nutrient and water deficiency that enhances topsoil foraging and improves plant performance. A hierarchy of transcription factors required for AR initiation has been identified from genetic studies, and recent results highlighted the key involvement of additional regulation through microRNAs. Here, we discuss our current understanding of AR formation in response to specific environmental stresses, such as nutrient deficiency, drought or waterlogging, aimed at providing evidence for the integration of the hormone crosstalk required for the activation of root competent cells within adult tissues from which the ARs develop. [ABSTRACT FROM AUTHOR]

Adventitious roots form only at the proximal cut surface (PCS) but not at the distal cut surface (DCS) of mango cotyledon segments. In this study, mango embryos treated with indole-3-butyric acid (IBA) showed significantly increased adventitious root formation, while those treated with 2, 3, 5-triiodobenzoic acid (TIBA) demonstrated complete inhibition of adventitious rooting. Mango embryos treated with auxin influx inhibitors demonstrated lower inhibition of adventitious roots than those treated with TIBA. The endogenous indol-3-acetic acid (IAA) content on the PCS and DCS was similar at 0 h, then increased on both surfaces after 6 h, and IAA content on the PCS were always higher than those on the DCS. We cloned three genes encoding auxin efflux carriers (i.e., MiPIN2-4) and examined their temporal and spatial expression patterns under different treatments. Relative expression of all MiPINs studied was very low at 0 h but significantly increased on both PCS and DCS from 1 d to 10 d, to varying degrees. We overexpressed MiPIN1-4 in Arabidopsis plants and found a significant increase in adventitious root quantity in MiPIN1 and MiPIN3 transgenic lines. Immunofluorescence results showed that MiPIN1 and MiPIN3 are primarily localized in the vascular tissues and the cells adjacent to abaxial surface. In conclusion, we propose that in mango cotyledon segments, wounding stimulates IAA biosynthesis, the transcription levels of PIN genes were significantly increased in different magnitudes on the PCS and DCS, resulting in polar IAA transport from the DCS to PCS via the vascular tissues, thereby triggering adventitious root formation. • TIBA had greater suppression of rooting in mango cotyledon segments than auxin influx inhibitors. • IAA content increased but IBA content changed little during adventitious root formation. • MiPIN1 and MiPIN3 are primarily located in vascular tissues and the cells adjacent to the abaxial surface. • MiPIN1 and MiPIN3 might play important roles in the formation of adventitious roots in mango cotyledon segments. [ABSTRACT FROM AUTHOR]

Contamination of soils with heavy metals, such as nickel (Ni), is a major environmental concern due to increasing pollution from industrial activities, burning of fossil fuels, incorrect disposal of sewage sludge, excessive manure application and the use of fertilizers and pesticides in agriculture. Excess Ni induces leaf chlorosis and inhibits plant growth, but the mechanisms underlying growth inhibition remain largely unknown. A detailed analysis of root development in Arabidopsis thaliana in the presence of Ni revealed that this heavy metal induces gravitropic defects and locally inhibits root growth by suppressing cell elongation without significantly disrupting the integrity of the stem cell niche. The analysis of auxin-responsive reporters revealed that excess Ni inhibits shootward auxin distribution. Furthermore, we found that PIN2 is very sensitive to Ni, as the presence of this heavy metal rapidly reduced PIN2 levels in roots. A transcriptome analysis also showed that Ni affects the expression of many genes associated with plant cell walls and that Ni-induced transcriptional changes are largely independent of iron (Fe). In addition, we raised evidence that excess Ni increases the accumulation of reactive oxygen species and disturbs the integrity and orientation of microtubules. Together, our results highlight which processes are primarily targeted by Ni to alter root growth and development. [ABSTRACT FROM AUTHOR]

Main conclusion: Endogenous auxin determines the pattern of adventitious shoot formation. Auxin produced in the dominant shoot is transported to the internodal segment and suppresses growth of other shoots. Adventitious shoot formation is required for the propagation of economically important crops and for the regeneration of transgenic plants. In most plant species, phytohormones are added to culture medium to induce adventitious shoots. In ipecac (Carapichea ipecacuanha (Brot.) L. Andersson), however, adventitious shoots can be formed without phytohormone treatment. Thus, ipecac culture allows us to investigate the effects of endogenous phytohormones during adventitious shoot formation. In phytohormone-free culture, adventitious shoots were formed on the apical region of the internodal segments, and a high concentration of IAA was detected in the basal region. To explore the relationship between endogenous auxin and adventitious shoot formation, we evaluated the effects of auxin transport inhibitors, auxin antagonists, and auxin biosynthesis inhibitors on adventitious shoot formation in ipecac. Auxin antagonists and biosynthesis inhibitors strongly suppressed adventitious shoot formation, which was restored by exogenously applied auxin. Auxin biosynthesis and transport inhibitors significantly decreased the IAA level in the basal region and shifted the positions of adventitious shoot formation from the apical region to the middle region of the segments. These data indicate that auxin determines the positions of the shoots formed on internodal segments of ipecac. Only one of the shoots formed grew vigorously; this phenomenon is similar to apical dominance. When the largest shoot was cut off, other shoots started to grow. Naphthalene-1-acetic acid treatment of the cut surface suppressed shoot growth, indicating that auxin produced in the dominant shoot is transported to the internodal segment and suppresses growth of other shoots. [ABSTRACT FROM AUTHOR]

We have isolated germacranolide-type sesquiterpene lactones with an α-methylene-γ-lactone moiety, dehydrocostus lactone (DHCL), costunolide, santamarine, and a novel compound denoted artabolide [3-hydroxy-4,6,7(H)-germacra-1(10),11(13)-dien-6,12-olide] from oriental medicinal Asteraceae plants as novel naturally occurring inhibitors of polar auxin transport detected by the radish hypocotyl bioassay. To investigate the mode of action of natural sesquiterpene lactones on the inhibition of polar auxin transport as well as its relation to the growth of seedlings, the function of DHCL on growth and auxin dynamics in etiolated pea seedlings was studied intensively. DHCL reduced polar auxin transport in a dose-dependent manner together with the inhibition of the accumulation of mRNA of PsAUX1 and PsPIN1 genes encoding influx and efflux carrier proteins of auxin, respectively. DHCL applied to the apical hook region as a lanolin paste substantially inhibited elongation growth in the subapical region of epicotyls in intact etiolated pea seedlings, coupled with a significant reduction of endogenous levels of indole-3-acetic acid (IAA). DHCL also revealed the inhibition of IAA-induced cell elongation in etiolated pea epicotyl segments by affecting IAA-induced changes in the mechanical properties of cell walls. These facts suggest that germacranolide-type sesquiterpene lactones with an α-methylene-γ-lactone moiety affect the expression of PsAUX1 and PsPINs genes, and then inhibit polar auxin transport and reduce endogenous levels of IAA necessary for stem growth in etiolated pea seedlings. These compounds are also suggested to show the inhibitory effects on auxin action in pea stem growth.

Flooding is a severe limitation for crop production worldwide. Unlike other crop plants, rice (Oryza sativa L.) is well adapted to partial submergence rendering it a suitable crop plant to understand flooding tolerance. Formation of adventitious roots (ARs), that support or replace the main root system, is a characteristic response to flooding. In rice, AR emergence is induced by ethylene and in the dark where roots grow upward. We used the synthetic auxins 2,4-D and α-NAA, and the auxin transport inhibitor naphthylphtalamic acid (NPA) to study emergence, growth rate and growth angle of ARs. While α-NAA had no effect, NPA and 2,4-D reduced the root elongation rate and the angle with a stronger effect on root angle in the dark than in the light. Furthermore, NPA delayed emergence of AR primordia suggesting that efflux carrier-mediated auxin transport is required for all aspects of directed AR growth. Expression analysis using OsPIN:GUS reporter lines revealed that OsPIN1b and OsPIN1c promoters were active in the stele and root cap in accord with their predicted role in acropetal auxin transport. OsPIN2 was expressed at the root tip and was reduced in the presence of NPA. Auxin activity, detected with DR5:VENUS, increased in primordia following growth induction. By contrast, auxin activity was high in epidermal cells above primordia and declined following growth induction suggesting that auxin levels are antagonistically regulated in AR primordia and in epidermal cells above AR primordia suggesting that auxin signaling contributes to the coordinated processes of epidermal cell death and AR emergence.

Summary: Plants sense mechanical stimuli to recognise nearby obstacles and change their growth patterns to adapt to the surrounding environment. When roots encounter an obstacle, they rapidly bend away from the impenetrable surface and find the edge of the barrier. However, the molecular mechanisms underlying root−obstacle avoidance are largely unknown.Here, we demonstrate that PIN‐FORMED (PIN)‐mediated polar auxin transport facilitates root bending during obstacle avoidance. We analysed two types of bending after roots touched barriers. In auxin receptor mutants, the rate of root movement during first bending was largely delayed. Gravity‐oriented second bending was also disturbed in these mutants.The reporter assays showed that asymmetrical auxin responses occurred in the roots during obstacle avoidance. Pharmacological analysis suggested that polar auxin transport mediates local auxin accumulation. We found that PINs are required for auxin‐assisted root bending during obstacle avoidance.We propose that rapid root movement during obstacle avoidance is not just a passive but an active bending completed through polar auxin transport. Our findings suggest that auxin plays a role in thigmotropism during plant−obstacle interactions. See also the Commentary on this article by Zhang & Friml, 225: 1049–1052. [ABSTRACT FROM AUTHOR]