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Plants synthesize hundreds of small secretory peptides, which are perceived by the receptor-like kinase (RLK) family at the cell surface. Various signaling peptide-RLK pairs ensure plant adaptation to distinct environmental conditions. Here, we report that SERINE RICH ENDOGENOUS PEPTIDE (SCOOP) immune peptides modulate root growth and development by regulating PIN-FORMED (PIN)-regulated polar auxin transport in Arabidopsis. The SCOOP4 and SCOOP12 treatments impaired root gravitropic growth, auxin redistribution in response to gravistimulation, and PIN abundance in the PM. Furthermore, genetic and cell biological analyses revealed that these physiological and cellular effects of SCOOP4 and SCOOP12 peptides are mediated by the receptor MALE DISCOVERER1-INTERACTING RECEPTOR LIKE KINASE2 (MIK2) and the downstream mitogen-activated kinase MPK6. Biochemical evidence indicates that MPK6 directly phosphorylates the cytosolic loop of PIN proteins. Our work established a link between the immune signaling peptide SCOOPs and root growth pathways, providing insights into the molecular mechanisms underlying plant root adaptive growth in the defense response., (© 2024 Society for Experimental Biology and John Wiley & Sons Ltd.)

The phytohormone auxin plays central roles in many growth and developmental processes in plants. Development of chemical tools targeting the auxin pathway is useful for both plant biology and agriculture. Here we reveal that naproxen, a synthetic compound with anti-inflammatory activity in humans, acts as an auxin transport inhibitor targeting PIN-FORMED (PIN) transporters in plants. Physiological experiments indicate that exogenous naproxen treatment affects pleiotropic auxin-regulated developmental processes. Additional cellular and biochemical evidence indicates that naproxen suppresses auxin transport, specifically PIN-mediated auxin efflux. Moreover, biochemical and structural analyses confirm that naproxen binds directly to PIN1 protein via the same binding cavity as the indole-3-acetic acid substrate. Thus, by combining cellular, biochemical, and structural approaches, this study clearly establishes that naproxen is a PIN inhibitor and elucidates the underlying mechanisms. Further use of this compound may advance our understanding of the molecular mechanisms of PIN-mediated auxin transport and expand our toolkit in auxin biology and agriculture., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

Lateral root initiation requires the accumulation of auxin in lateral root founder cells, yielding a local auxin maximum. The positioning of auxin maxima along the primary root determines the density and spacing of lateral roots. The GOLVEN6 (GLV6) and GLV10 signaling peptides and their receptors have been established as regulators of lateral root spacing via their inhibitory effect on lateral root initiation in Arabidopsis. However, it was unclear how these GLV peptides interfere with auxin signaling or homeostasis. Here, we show that GLV6/10 signaling regulates the expression of a subset of auxin response genes, downstream of the canonical auxin signaling pathway, while simultaneously inhibiting the establishment of auxin maxima within xylem-pole pericycle cells that neighbor lateral root initiation sites. We present genetic evidence that this inhibitory effect relies on the activity of the PIN3 and PIN7 auxin export proteins. Furthermore, GLV6/10 peptide signaling was found to enhance PIN7 abundance in the plasma membranes of xylem-pole pericycle cells, which likely stimulates auxin efflux from these cells. Based on these findings, we propose a model in which the GLV6/10 signaling pathway serves as a negative feedback mechanism that contributes to the robust patterning of auxin maxima along the primary root., (© The Author(s) 2023. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

The root cap is a small tissue located at the tip of the root with critical functions for root growth. Present in nearly all vascular plants, the root cap protects the root meristem, influences soil penetration, and perceives and transmits environmental signals that are critical for root branching patterns. To perform these functions, the root cap must remain relatively stable in size and must integrate endogenous developmental pathways with environmental signals, yet the mechanism is not clear. We previously showed that low pH conditions altered root cap development, and these changes are mediated by the NIN LIKE PROTEIN 7 (NLP7) transcription factor, a master regulator of nitrate signaling. Here we show that in Arabidopsis NLP7 integrates nitrate signaling with auxin pathways to regulate root cap development. We found that low nitrate conditions promote aberrant release of root cap cells. Nitrate deficiency impacts auxin pathways in the last layer of the root cap, and this is mediated in part by NLP7. Mutations in NLP7 abolish the auxin minimum in the last layer of the root cap and alter root cap expression of the auxin carriers PIN-LIKES 3 (PILS3) and PIN-FORMED 7 (PIN7) as well as transcription factors that regulate PIN expression. Together, our data reveal NLP7 as a link between endogenous auxin pathways and nitrate signaling in the root cap., (© The Author(s) 2023. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

The meristem is the most functionally dynamic part in a plant. The shaping of the meristem requires constant cell division and elongation, which are influenced by hormones and the cytoskeletal component, actin. Although the roles of hormones in modulating meristem development have been extensively studied, the role of actin in this process is still elusive. Using the single and double mutants of the vegetative class actin, we demonstrate that actin isovariant ACT7 plays an important role in root meristem development. In the absence of ACT7, but not ACT8 and ACT2, depolymerization of actin was observed. Consistently, the act7 mutant showed reduced cell division, cell elongation, and meristem length. Intracellular distribution and trafficking of auxin transport proteins in the actin mutants revealed that ACT7 specifically functions in the root meristem to facilitate the trafficking of auxin efflux carriers PIN1 and PIN2, and consequently the transport of auxin. Compared with act7, the act7act8 double mutant exhibited slightly enhanced phenotypic response and altered intracellular trafficking. The altered distribution of auxin in act7 and act7act8 affects the response of the roots to ethylene, but not to cytokinin. Collectively, our results suggest that ACT7-dependent auxin-ethylene response plays a key role in controlling Arabidopsis root meristem development., (© The Author(s) 2022. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Root hair (RH) growth to increase the absorptive root surface area is a key adaptation of plants to limiting phosphate availability in soils. Despite the importance of this trait, especially for seedling survival, little is known about the molecular events connecting phosphate starvation sensing and RH growth regulation. KARRIKIN INSENSITIVE2 (KAI2), an α/β-hydrolase receptor of a yet-unknown plant hormone ("KAI2-ligand" [KL]), is required for RH elongation. 1 KAI2 interacts with the F-box protein MORE AXILLIARY BRANCHING2 (MAX2) to target regulatory proteins of the SUPPRESSOR of MAX2 1 (SMAX1) family for degradation. 2 Here, we demonstrate that P i starvation increases KL signaling in Arabidopsis roots through transcriptional activation of KAI2 and MAX2. Both genes are required for RH elongation under these conditions, while smax1 smxl2 mutants have constitutively long RHs, even at high P i availability. Attenuated RH elongation in kai2 mutants is explained by reduced shootward auxin transport from the root tip resulting in reduced auxin signaling in the RH zone, caused by an inability to increase localized accumulation of the auxin importer AUXIN TRANSPORTER PROTEIN1 (AUX1) and the auxin exporter PIN-FORMED2 (PIN2) upon P i starvation. Consistent with AUX1 and PIN2 accumulation being mediated via ethylene signaling, 3 expression of 1-AMINOCYCLOPROPANE-1-CARBOXYLATE SYNTHASE 7 (ACS7) is increased at low P i in a KAI2-dependent manner, and treatment with an ethylene precursor restores RH elongation of acs7, but not of aux1 and pin2. Thus, KAI2 signaling is increased by phosphate starvation to trigger an ethylene- AUX1/PIN2-auxin cascade required for RH elongation., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Melatonin has been characterized as a growth regulator in plants. Melatonin shares tryptophan as the precursor with the auxin indole-3-acetic acid (IAA), but the interplay between melatonin and IAA remains controversial. In this study, we aimed to dissect the relationship between melatonin and IAA in regulating Arabidopsis primary root growth. We observed that melatonin concentrations ranging from 10-9 to 10-6 M functioned as IAA mimics to promote primary root growth in Arabidopsis wild type, as well as in pin-formed (pin) single and double mutants. Transcriptome analysis showed that changes in gene expression after melatonin and IAA treatment were moderately correlated. Most of the IAA-regulated genes were co-regulated by melatonin, indicating that melatonin and IAA regulated a similar subset of genes. Melatonin partially rescued primary root growth defects in pin single and double mutant plants. However, melatonin treatment had little effect on primary root growth in the presence of high concentrations of auxin biosynthesis inhibitors, or polar transport inhibitor, and could not rescue the root length defect of the IAA biosynthesis quintuple mutant yucQ. Therefore, we propose that melatonin promotes primary root growth in an IAA-dependent manner., (© The Author(s) 2021. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Cell and tissue polarization is fundamental for plant growth and morphogenesis. The polar, cellular localization of Arabidopsis PIN-FORMED (PIN) proteins is crucial for their function in directional auxin transport. The clustering of PIN polar cargoes within the plasma membrane has been proposed to be important for the maintenance of their polar distribution. However, the more detailed features of PIN clusters and the cellular requirements of cargo clustering remain unclear. Here, we characterized PIN clusters in detail by means of multiple advanced microscopy and quantification methods, such as 3D quantitative imaging or freeze-fracture replica labeling. The size and aggregation types of PIN clusters were determined by electron microscopy at the nanometer level at different polar domains and at different developmental stages, revealing a strong preference for clustering at the polar domains. Pharmacological and genetic studies revealed that PIN clusters depend on phosphoinositol pathways, cytoskeletal structures and specific cell-wall components as well as connections between the cell wall and the plasma membrane. This study identifies the role of different cellular processes and structures in polar cargo clustering and provides initial mechanistic insight into the maintenance of polarity in plants and other systems., (© 2020 The Authors New Phytologist © 2020 New Phytologist Trust.)

Parasitic plants form vascular connections with host plants for efficient material transport. The haustorium is the responsible organ for host invasion and subsequent vascular connection. After invasion of host tissues, vascular meristem-like cells emerge in the central region of the haustorium, differentiate into tracheary elements and establish a connection, known as a xylem bridge, between parasite and host xylem systems. Despite the importance of this parasitic connection, the regulatory mechanisms of xylem bridge formation are unknown. Here, we show the role of auxin and auxin transporters during the process of xylem bridge formation using an Orobanchaceae hemiparasitic plant, Phtheirospermum japonicum The auxin response marker DR5 has a similar expression pattern to tracheary element differentiation genes in haustoria. Auxin transport inhibitors alter tracheary element differentiation in haustoria, but biosynthesis inhibitors do not, demonstrating the importance of auxin transport during xylem bridge formation. The expression patterns and subcellular localization of PIN family auxin efflux carriers and AUX1/LAX influx carriers correlate with DR5 expression patterns. The cooperative action of auxin transporters is therefore responsible for controlling xylem vessel connections between parasite and host., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2020. Published by The Company of Biologists Ltd.)

Plants, like other multicellular organisms, survive through a delicate balance between growth and defense against pathogens. Salicylic acid (SA) is a major defense signal in plants, and the perception mechanism as well as downstream signaling activating the immune response are known. Here, we identify a parallel SA signaling that mediates growth attenuation. SA directly binds to A subunits of protein phosphatase 2A (PP2A), inhibiting activity of this complex. Among PP2A targets, the PIN2 auxin transporter is hyperphosphorylated in response to SA, leading to changed activity of this important growth regulator. Accordingly, auxin transport and auxin-mediated root development, including growth, gravitropic response, and lateral root organogenesis, are inhibited. This study reveals how SA, besides activating immunity, concomitantly attenuates growth through crosstalk with the auxin distribution network. Further analysis of this dual role of SA and characterization of additional SA-regulated PP2A targets will provide further insights into mechanisms maintaining a balance between growth and defense., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2019 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

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., (© 2019 The Authors. New Phytologist © 2019 New Phytologist Trust.)

Root tip is capable of sensing and adjusting its growth direction in response to gravity, a phenomenon known as root gravitropism. Previously, we have shown that negative gravitropic response of roots (NGR) is essential for the positive gravitropic response of roots. Here, we show that NGR, a plasma membrane protein specifically expressed in root columella and lateral root cap cells, controls the positive root gravitropic response by regulating auxin efflux carrier localization in columella cells and the direction of lateral auxin flow in response to gravity. Pharmacological and genetic studies show that the negative root gravitropic response of the ngr mutants depends on polar auxin transport in the root elongation zone. Cell biology studies further demonstrate that polar localization of the auxin efflux carrier PIN3 in root columella cells and asymmetric lateral auxin flow in the root tip in response to gravistimulation is reversed in the atngr1;2;3 triple mutant. Furthermore, simultaneous mutations of three PIN genes expressed in root columella cells impaired the negative root gravitropic response of the atngr1;2;3 triple mutant. Our work revealed a critical role of NGR in root gravitropic response and provided an insight of the early events and molecular basis of the positive root gravitropism., (© 2019 John Wiley & Sons Ltd.)

Plant cells sheath themselves in a complex lattice of polysaccharides, proteins and enzymes forming an integral matrix known as the cell wall. Cellulose microfibrils, the primary component of cell walls, are synthesized at the plasma membrane by CELLULOSE SYNTHASE A (CESA) proteins throughout cellular growth and are responsible for turgor-driven anisotropic expansion. Associations between hormone signaling and cell wall biosynthesis have long been suggested, but recently direct links have been found revealing hormones play key regulatory roles in cellulose biosynthesis. The radially swollen 1 (rsw1) allele of Arabidopsis thaliana CESA1 harbors a single amino acid change that renders the protein unstable at high temperatures. We used the conditional nature of rsw1 to investigate how auxin contributes to isotropic growth. We found that exogenous auxin treatment reduces isotropic swelling in rsw1 roots at the restrictive temperature of 30�C. We also discovered decreases in auxin influx between rsw1 and wild-type roots via confocal imaging of AUX1-YFP, even at the permissive temperature of 19�C. Moreover, rsw1 displayed mis-expression of auxin-responsive and CESA genes. Additionally, we found altered auxin maxima in rsw1 mutant roots at the onset of swelling using DII-VENUS and DR5:vYFP auxin reporters. Overall, we conclude disrupted cell wall biosynthesis perturbs auxin transport leading to altered auxin homeostasis impacting both anisotropic and isotropic growth that affects overall root morphology., (� The Author(s) 2019. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

In Arabidopsis, embryonic development follows a stereotypical pattern of cell division. Although many factors have been reported to participate in establishment of the proper embryonic pattern, the molecular mechanisms underlying pattern formation are unclear. In this study we showed that RAF22 and RAF28, two RAF-like mitogen-activated protein kinase kinase kinases (MAPKKKs) in Arabidopsis, are involved in the regulation of embryogenesis. The double knockout mutant of RAF22 and RAF28 was embryo lethal. A large proportion of the raf22 -/- raf28 +/- mutant embryos exhibited various defects, including disordered proembryo cell divisions, disruption of the bilaterally symmetrical structure, abnormally formative divisions of hypophysis and exaggerated suspensor growth. Whereas the kinase active form of RAF22 could complement these embryonic aberrant phenotypes, the kinase inactive form could not. The restrictive expression of the basal cell fate marker WOX8 in the abnormally dividing suspensor cells and the apical cell linage marker WOX2 in the abnormal proembryos indicated that apical and basal cell fates were unchanged in the abnormal embryos. The polar distribution of the auxin maxima and the PIN1 and PIN7 auxin transporters was markedly altered in the abnormal embryos. Our results suggest that RAF22 and RAF28 are important components of embryogenesis and that auxin polar transport may be involved in this regulation., (© 2018 The Authors The Plant Journal © 2018 John Wiley & Sons Ltd.)

The vascular system develops in response to auxin flow as continuous strands of conducting tissues arranged in regular spatial patterns. However, a mechanism governing their regular and repetitive formation remains to be fully elucidated. A model system for studying the vascular pattern formation is the process of leaf vascularization in Arabidopsis. In this paper, we present current knowledge of important factors and their interactions in this process. Additionally, we propose the sequence of events leading to the emergence of continuous vascular strands and point to significant problems that need to be resolved in the future to gain a better understanding of the regulation of the vascular pattern development.

Organ size homeostasis, compensatory growth to replace lost tissue, requires constant measurement of size and adjustment of growth rates. Morphogen gradients control organ and tissue sizes by regulating stem cell activity, cell differentiation, and removal in animals [1-3]. In plants, control of tissue size is of specific importance in root caps to protect the growing root tip from mechanical damage [4]. New root cap tissue is formed by the columella and lateral root-cap-epidermal stem cells, whose activity is regulated through non-dividing niche-like cells, the quiescent center (QC) [4, 5]. Columella daughter cells in contact with the QC retain the potency to divide, while derivatives oriented toward the mature cap undergo differentiation. The outermost columella layers are sequentially separated from the root body, involving remodeling of cell walls [6]. Factors regulating the balance between cell division, elongation, and separation to keep root cap size constant are currently unknown [4]. Here, we report that stem cell proliferation induced cell separation at the periphery of the root cap, resulting in tissue size homeostasis. An auxin response gradient with a maximum in the QC and a minimum in the detaching layer was established prior to the onset of cell separation. In agreement with a mathematical model, tissue size was positively regulated by the amount of auxin released from the source. Auxin transporters localized non-polarly to plasma membranes of the inner cap, partly isolating separating layers from the auxin source. Together, these results are in support of an auxin gradient measuring and regulating tissue size., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Plant-parasitic root-knot nematodes induce the formation of giant cells within the plant root, and it has been recognized that auxin accumulates in these feeding sites. Here, we studied the role of the auxin transport system governed by AUX1/LAX3 influx proteins and different PIN efflux proteins during feeding site development in Arabidopsis thaliana roots. Data generated via promoter-reporter line and protein localization analyses evoke a model in which auxin is being imported at the basipetal side of the feeding site by the concerted action of the influx proteins AUX1 and LAX3, and the efflux protein PIN3. Mutants in auxin influx proteins AUX1 and LAX3 bear significantly fewer and smaller galls, revealing that auxin import into the feeding sites is needed for their development and expansion. The feeding site development in auxin export (PIN) mutants was only slightly hampered. Expression of some PINs appears to be suppressed in galls, probably to prevent auxin drainage. Nevertheless, a functional PIN4 gene seems to be a prerequisite for proper nematode development and gall expansion, most likely by removing excessive auxin to stabilize the hormone level in the feeding site. Our data also indicate a role of local auxin peaks in nematode attraction towards the root., (© The Author 2016. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

Plant development mediated by the phytohormone auxin depends on tightly controlled cellular auxin levels at its target tissue that are largely established by intercellular and intracellular auxin transport mediated by PIN auxin transporters. Among the eight members of the Arabidopsis PIN family, PIN6 is the least characterized candidate. In this study we generated functional, fluorescent protein-tagged PIN6 proteins and performed comprehensive analysis of their subcellular localization and also performed a detailed functional characterization of PIN6 and its developmental roles. The localization study of PIN6 revealed a dual localization at the plasma membrane (PM) and endoplasmic reticulum (ER). Transport and metabolic profiling assays in cultured cells and Arabidopsis strongly suggest that PIN6 mediates both auxin transport across the PM and intracellular auxin homeostasis, including the regulation of free auxin and auxin conjugates levels. As evidenced by the loss- and gain-of-function analysis, the complex function of PIN6 in auxin transport and homeostasis is required for auxin distribution during lateral and adventitious root organogenesis and for progression of these developmental processes. These results illustrate a unique position of PIN6 within the family of PIN auxin transporters and further add complexity to the developmentally crucial process of auxin transport., (© 2016 The Authors. New Phytologist © 2016 New Phytologist Trust.)

The plant hormone auxin and its directional transport are known to play a crucial role in defining the embryonic axis and subsequent development of the body plan. Although the role of PIN auxin efflux transporters has been clearly assigned during embryonic shoot and root specification, the role of the auxin influx carriers AUX1 and LIKE-AUX1 (LAX) proteins is not well established. Here, we used chemical and genetic tools on Brassica napus microspore-derived embryos and Arabidopsis thaliana zygotic embryos, and demonstrate that AUX1, LAX1 and LAX2 are required for both shoot and root pole formation, in concert with PIN efflux carriers. Furthermore, we uncovered a positive-feedback loop between MONOPTEROS (ARF5)-dependent auxin signalling and auxin transport. This MONOPTEROS-dependent transcriptional regulation of auxin influx (AUX1, LAX1 and LAX2) and auxin efflux (PIN1 and PIN4) carriers by MONOPTEROS helps to maintain proper auxin transport to the root tip. These results indicate that auxin-dependent cell specification during embryo development requires balanced auxin transport involving both influx and efflux mechanisms, and that this transport is maintained by a positive transcriptional feedback on auxin signalling., (© 2015. Published by The Company of Biologists Ltd.)

L-Cysteine plays a prominent role in sulfur metabolism of plants. However, its role in root development is largely unknown. Here, we report that L-cysteine reduces primary root growth in a dosage-dependent manner. Elevating cellular L-cysteine level by exposing Arabidopsis thaliana seedlings to high L-cysteine, buthionine sulphoximine, or O-acetylserine leads to altered auxin maximum in root tips, the expression of quiescent center cell marker as well as the decrease of the auxin carriers PIN1, PIN2, PIN3, and PIN7 of primary roots. We also show that high L-cysteine significantly reduces the protein level of two sets of stem cell specific transcription factors PLETHORA1/2 and SCR/SHR. However, L-cysteine does not downregulate the transcript level of PINs, PLTs, or SCR/SHR, suggesting that an uncharacterized post-transcriptional mechanism may regulate the accumulation of PIN, PLT, and SCR/SHR proteins and auxin transport in the root tips. These results suggest that endogenous L-cysteine level acts to maintain root stem cell niche by regulating basal- and auxin-induced expression of PLT1/2 and SCR/SHR. L-Cysteine may serve as a link between sulfate assimilation and auxin in regulating root growth., (© 2014 Institute of Botany, Chinese Academy of Sciences.)

Auxin, as a vital plant hormone, regulates almost every aspect of plant growth and development. We previously identified a dominant mutant, adp1-D, displaying loss of apical dominance. We also demonstrated that down-regulation of local auxin biosynthesis in adp1-D was responsible for the bushy phenotype of this mutant. Consistent with the reduction of local auxin biosynthesis, we recently discovered that protein abundance of PIN1, PIN3, and PIN7 was reduced in adp1-D without accompanying transcription level changes. Additionally, subcellular analysis revealed that over-expression of ADP1 inhibited endocytosis of PIN proteins. Taken together, we conclude that ADP1 regulates plant architecture through the fine-tuning of local auxin biosynthesis and through post-transcriptional regulation of auxin transporters.

Roots play important roles in plant survival and productivity as they not only anchor the plants in the soil but are also the primary organ for the uptake of nutrients from the outside. The growth and development of roots depend on the specification and maintenance of the root meristem. Here, we report a previously unknown role of TIME FOR COFFEE (TIC) in controlling root meristem size in Arabidopsis. The results showed that loss of function of TIC reduced root meristem length and cell number by decreasing the competence of meristematic cells to divide. This was due to the repressed expression of PIN genes for decreased acropetal auxin transport in tic-2, leading to low auxin accumulation in the roots responsible for reduced root meristem, which was verified by exogenous application of indole-3-acetic acid. Downregulated expression of PLETHORA1 (PLT1) and PLT2, key transcription factors in mediating the patterning of the root stem cell niche, was also assayed in tic-2. Similar results were obtained with tic-2 and wild-type plants at either dawn or dusk. We also suggested that the MYC2-mediated jasmonic acid signalling pathway may not be involved in the regulation of TIC in controlling the root meristem. Taken together, these results suggest that TIC functions in an auxin-PLTs loop for maintenance of post-embryonic root meristem.

Plant ATP-binding cassette (ABC) transporters consist of largest family members among many other membrane transporters and have been implicated in various functions such as detoxification, disease resistance and transport of diverse substrates. Of the ABC-B/multi-drug resistance/P-glycoprotein (ABCB/MDR/PGP) subfamily, at least five members have been reported to mediate cellular transport of auxin or auxin derivatives. Although single mutant phenotypes of these genes are milder than PIN-FORMED (PIN) mutants, those ABCBs significantly contribute for the directional auxin movement in the tissue-level auxin-transporting assay. Uniformly localized ABCB proteins in the plasma membrane (PM) are generaly found in different plant species and stably retained regardless of internal and external signals. This implies that these ABCB proteins may play as basal auxin transporters.

In a recent study, we demonstrated that although the auxin efflux carrier PIN-FORMED (PIN) proteins, such as PIN3 and PIN7, are required for the pulse-induced first positive phototropism in etiolated Arabidopsis hypocotyls, they are not necessary for the continuous-light-induced second positive phototropism when the seedlings are grown on the surface of agar medium, which causes the hypocotyls to separate from the agar surface. Previous reports have shown that hypocotyl phototropism is slightly impaired in pin3 single mutants when they are grown along the surface of agar medium, where the hypocotyls always contact the agar, producing some friction. To clarify the possible involvement of PIN3 and PIN7 in continuous-light-induced phototropism, we investigated hypocotyl phototropism in the pin3 pin7 double mutant grown along the surface of agar medium. Intriguingly, the phototropic curvature was slightly impaired in the double mutant when the phototropic stimulus was presented on the adaxial side of the hook, but was not impaired when the phototropic stimulus was presented on the abaxial side of the hook. These results indicate that PIN proteins are required for continuous-light-induced second positive phototropism, depending on the direction of the light stimulus, when the seedlings are in contact with agar medium.

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]

How multiple growth programs coordinate during development is a fundamental question in biology. During plant stem development, radial growth is continuously adjusted in response to longitudinal-growth-derived weight increase to guarantee stability. 1,2,3 Here, we demonstrate that weight-stimulated stem radial growth depends on the auxin efflux carrier PIN3, which, upon weight increase, expands its cellular localization from the lower to the lateral sides of xylem parenchyma, phloem, procambium, and starch sheath cells, imposing a radial auxin flux that results in radial growth. Using the protein synthesis inhibitor cycloheximide (CHX) or the fluorescent endocytic tracer FM4-64, we reveal that this expansion of the PIN3 cellular localization domain occurs because weight increase breaks the balance between PIN3 biosynthesis and removal, favoring PIN3 biosynthesis. Experimentation using brefeldin A (BFA) treatments or arg1 and arl2 mutants further supports this conclusion. Analyses of CRISPR-Cas9 lines for Populus PIN3 orthologs reveals that PIN3 dependence of weight-induced radial growth is conserved at least in these woody species. Altogether, our work sheds new light on how longitudinal and radial growth coordinate during stem development. • Weight-induced stem radial growth is dependent on PIN3 activity • In Arabidopsis , PIN3 expands its cellular localization domain upon weight increase • PIN3 dependence of weight-induced stem radial growth is conserved in Populus In plant stems, longitudinal and radial growth coordinate so that the radial growth provides enough stability for the longitudinal growth to expand. Carrió-Seguí et al. demonstrate that this coordination depends on the phytohormone auxin transporter PIN3, which, upon weight increase, generates an auxin radial flux that triggers radial growth. [ABSTRACT FROM AUTHOR]

Parasitic plants form vascular connections with host plants for efficient material transport. The haustorium is the responsible organ for host invasion and subsequent vascular connection. After invasion of host tissues, vascular meristem-like cells emerge in the central region of the haustorium, differentiate into tracheary elements and establish a connection, known as a xylem bridge, between parasite and host xylem systems. Despite the importance of this parasitic connection, the regulatory mechanisms of xylem bridge formation are unknown. Here, we show the role of auxin and auxin transporters during the process of xylem bridge formation using an Orobanchaceae hemiparasitic plant, Phtheirospermum japonicum. The auxin response marker DR5 has a similar expression pattern to tracheary element differentiation genes in haustoria. Auxin transport inhibitors alter tracheary element differentiation in haustoria, but biosynthesis inhibitors do not, demonstrating the importance of auxin transport during xylem bridge formation. The expression patterns and subcellular localization of PIN family auxin efflux carriers and AUX1/ LAX influx carriers correlate with DR5 expression patterns. The cooperative action of auxin transporters is therefore responsible for controlling xylem vessel connections between parasite and host. [ABSTRACT FROM AUTHOR]

Co-ordination of auxin and cytokinin activities determines root meristem size during post-embryonic development. Calcineurin B-like proteins (CBLs) and their interacting protein kinases (CIPKs) constitute signaling modules that relay calcium signals. Here we report that CIPK25 is involved in regulating the root meristem size. Arabidopsis plants lacking CIPK25 expression displayed a short root phenotype and a slower root growth rate with fewer meristem cells. This phenotype was rescued by restoration of CIPK25 expression. CIPK25 interacted with CBL4 and -5, and displayed strong gene expression in the flower and root, except in the cell proliferation domain in the root apical meristem. Its expression in the root was positively and negatively regulated by auxin and cytokinin, respectively. The cipk25 T-DNA insertion line was compromised in auxin transport and auxin-responsive promoter activity. The cipk25 mutant line showed altered expression of auxin efflux carriers (PIN1 and PIN2) and an Aux/IAA family gene SHY2. Decreased PIN1 and PIN2 expression in the cipk25 mutant line was completely restored when combined with a SHY2 loss-of-function mutation, resulting in recovery of root growth. SHY2 and PIN1 expression was partially regulated by cytokinin even in the absence of CIPK25, suggesting a CIPK25-independent cytokinin signaling pathway(s). Our results revealed that CIPK25 plays an important role in the co-ordination of auxin and cytokinin signaling in root meristem development. [ABSTRACT FROM AUTHOR]

Plant growth flexibly adapts to environmental conditions, implying cross‐talk between environmental signalling and developmental regulation. Here, we show that the PIN auxin efflux carrier family possesses three highly conserved putative mitogen‐activated protein kinase (MAPK) sites adjacent to the phosphorylation sites of the well‐characterised AGC kinase PINOID, which regulates the polar localisation of PINs and directional auxin transport, thereby underpinning organ growth. The conserved sites of PIN1 are phosphorylated in vitro by two environmentally activated MAPKs, MPK4 and MPK6. In contrast to AGC kinases, MAPK‐mediated phosphorylation of PIN1 at adjacent sites leads to a partial loss of the plasma membrane localisation of PIN1. MAPK‐mediated modulation of PIN trafficking may participate in environmental adjustment of plant growth. [ABSTRACT FROM AUTHOR]

Homogeneous low phosphorus availability was reported to regulate root architecture in Arabidopsis via auxin, but the roles of auxin in root architecture plasticity to heterogeneous P availability remain unclear. In this study, we employed auxin biosynthesis-, transport- and signalling-related mutants. Firstly, we found that in contrast to low P (LP) content in the whole medium, primary root (PR) growth of Arabidopsis was partially rescued in the medium divided into two parts: upper with LP and lower with high P (HP) content or in the reverse arrangement. The down part LP was more effective to arrest PR growth as well as to decrease density of lateral roots (DLR) than the upper LP, and effects were dependent on polar auxin transport. Secondly, we verified that auxin receptor TIR1 was involved in the responses of PR growth and lateral root (LR) development to P supply and loss of function of TIR1 inhibited LR development. Thirdly, effects of heterogeneous P on LRD in the upper part of PR was dependent on PIN2 and PIN4, and in the down part on PIN3 and PIN4, whereas density of total LRs was dependent on auxin transporters PIN2 and PIN7. Finally, heterogeneous P availability altered the accumulation of auxin in PR tip and the expression of auxin biosynthesisrelated genes TAA1, YUC1, YUC2, and YUC4. Taken together, we provided evidences for the involvement of auxin in root architecture plasticity in response to heterogeneous phosphorus availability in Arabidopsis. [ABSTRACT FROM AUTHOR]

During gravitropism, the potential energy of gravity is converted into a biochemical signal. How this transfer occurs remains one of the most exciting mysteries in plant cell biology. New experiments are filling in pieces of the puzzle. In this review, we introduce gravitropism and give an overview of what we know about gravity sensing in roots of vascular plants, with special highlight on recent papers. When plant roots are reoriented sideways, amyloplast resedimentation in the columella cells is a key initial step in gravity sensing. This process somehow leads to cytoplasmic alkalinization of these cells followed by relocalization of auxin efflux carriers (PINs). This changes auxin flow throughout the root, generating a lateral gradient of auxin across the cap that upon transmission to the elongation zone leads to differential cell elongation and gravibending. We will present the evidence for and against the following players having a role in transferring the signal from the amyloplast sedimentation into the auxin signaling cascade: mechanosensitive ion channels, actin, calcium ions, inositol trisphosphate, receptors/ligands, ARGI/ARL2, spermine, and the TOC complex. We also outline auxin transport and signaling during gravitropism. [ABSTRACT FROM AUTHOR]

Gravitropism is a process that allows plant organs to guide their growth relative to the gravity vector. It requires them to sense changes in their orientation and generate a biochemical signal that they transmit to the tissues that drive organ curvature. Trafficking between the plasma membrane and endosomal compartments is important for all of these phases of the gravitropic response. The sedimentation of starch-filled organelles called amyloplasts plays a key role in sensing reorientation, and vacuolar integrity is required for amyloplast sedimentation in shoots. Other proteins associated with the vesicle trafficking pathway contribute to early gravity signal transduction independently of amyloplast sedimentation in both roots and hypocotyls. Phosphatidylinositol signaling, which starts at the plasma membrane and later affects the localization of auxin efflux facilitators, is a likely second messenger in the signal transduction phase of gravitropism. Finally, membrane-localized auxin influx and efflux facilitators contribute to a differential auxin gradient across the gravistimulated organs, which directs root curvature. [ABSTRACT FROM AUTHOR]

Abstract: We identified 12 PIN genes in rice which encode efflux carrier proteins of phytohormone auxin. Phytogenetic analysis showed that 11 genes were categorized into branches with Arabidopsis PIN genes and one had no close homologue in Arabidopsis. RT-PCR analysis showed expression of 10 PIN genes in various organs depending on genes. No expression of OsPIN3b was detected in the organs examined. OsPIN1b and OsPIN1d were very similar both in sequence and in expression pattern, and in the shoot apex, OsPIN1b/1d was expressed in regions where vascular tissues were developing in young leaf. OsPIN2, OsPIN3a and OsPIN5a were expressed in the entire shoot apex with slight preference in an outermost cell layer. These results indicate that rice has several distinct members of the PIN genes which have various expression patterns, and suggest that polar auxin transport is mediated by different combinations of the PIN genes in each organ in rice. [Copyright &y& Elsevier]

Studies the regulation of PIN genes by brassinolide (BL). Use of BL to treat seven-day-old light-grown Arabidopsis brassinosteroid-deficient mutants grown in 1/2 Murashige and Skoog medium; Suggestion that BL selectively regulates the PIN gene in a complex manner; Decrease in the abundance of PIN4 and PIN7 transcripts.