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Main Conclusion: Mutation of OsSHR2 adversely impacted root and shoot growth and impaired plant response to N conditions, further reducing the yield per plant. Nitrogen (N) is a crucial factor that regulates the plant architecture. There is still a lack of research on it. In our study, it was observed that the knockout of the SHORTROOT 2 (OsSHR2) which was induced by N deficiency, can significantly affect the regulation of plant architecture response to N in rice. Under N deficiency, the mutation of OsSHR2 significantly reduced root growth, and impaired the sensitivity of the root meristem length to N deficiency. The mutants were found to have approximately a 15% reduction in plant height compared to wild type. But mutants showed a significant increase in tillering at post-heading stage, approximately 26% more than the wild type, particularly in high N conditions. In addition, due to reduced seed setting rate and 1000-grain weight, mutant yield was significantly decreased by approximately 33% under low N fertilizer supply. The mutation also changed the distribution of N between the vegetative and reproductive organs. Our findings suggest that the transcription factor OsSHR2 plays a regulatory role in the response of plant architecture and yield per plant to N in rice., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Main Conclusion: Cadmium stress induces WDR5a expression to promote NO accumulation to repress root meristem growth via suppressing auxin transport and synthesis in Arabidopsis. Nitric oxide (NO) synthase (NOS)-like activity plays a vital role in toxic cadmium (Cd)-induced NO production and inhibition of root meristem growth, while factor(s) regulating NOS-like activity and root meristem growth in plant response to Cd has not been identified yet. Here, we report that WD40 repeat 5a (WDR5a) functions in Cd-induced NOS-like activity, NO accumulation and root meristem growth suppression. We found that wdr5a-1 mutant root has increased root meristem growth with lower NOS-like activity and NO accumulation than wild type upon Cd exposure, and exogenous NO donors sodium nitroprusside or nitrosoglutathione can restore its reduced Cd sensitivity. In addition, Cd activates WDR5a expression in roots, and overexpressing WDR5a results in increased NO accumulation and suppressed root meristem growth similar to Cd-stressed wild-type roots, while scavenging NO or inhibiting NOS-like activity significantly reverts these effects of Cd. Furthermore, WDR5a acts in Cd-repressed auxin accumulation through reducing the levels of auxin efflux carriers PIN1/3/7 and biosynthetic enzyme TAA1, and reduced sensitivity of wdr5a-1 root meristem to Cd can be partially reverted by inhibiting TAA1 activity pharmaceutically or mutating TAA1 genetically. This study identified WDR5a as a key factor modulating NO accumulation and root meristem growth in plant response to Cd.

Main Conclusion: In this study, we show that ectopic expression of either HtNHX1 or HtNHX2, from Helianthus tuberosus plant (located at vacuolar and endosome membranes, respectively), in rice plants could enhance its tolerance to aluminum (Al 3+ ) stress and soil acidity. Plant sodium (potassium)/proton (Na + (K + )/H + antiporters of the NHX family have been extensively characterized as they are related to the enhancement of salt tolerance. However, no previous study has reported NHX transporter functions in plant tolerance to Al 3+ toxicity. In this study, we demonstrate their role as a component of the Al 3+ stress tolerance mechanism. We show that the ectopic expression of either HtNHX1 or HtNHX2 , from Helianthus tuberosus plant, in rice (located at vacuole and endosome, respectively) could also enhance rice tolerance to Al 3+ stress and soil acidity. Expression of either HtNHX1 or HtNHX2 reduced the inhibitory effect of Al 3+ on the rice root elongation rate; both genes were reported to be equally effective in improvement of stress conditions. Expression of HtNHX1 enhanced Al 3+ -trigged-secretion of citrate acids, rhizosphere acidification, and also reduced K + efflux from root tissues. In contrast, expression of HtNHX2 prevented Al 3+ -trigged-decrease of H + influx into root tissues. Al 3+ -induced damage of the cell wall extensibility at the root tips was impaired by either HtNHX1 or HtNHX2. Co-expression of HtNHX1 and HtNHX2 further improved rice growth, particularly under the Al 3+ stress conditions. The results demonstrate that HtNHX1 and HtNHX2 improved rice tolerance to Al 3+ via different mechanisms by altering the K + and H + fluxes and the cell wall structure.

Main Conclusion: Mild Cd stress-activated diphenyleneiodonium-sensitive superoxide production is utilized in root morphogenic responses, while severe Cd stress-induced robust rotenone-sensitive superoxide generation may lead to cell and root death. In barley, even a few minute exposure of roots to Cd concentration higher than 10 µM evoked a strong superoxide generation in the root transition zone. This superoxide generation was strongly inhibited by the inhibition of mitochondrial electron flow into complex III in the presence of the mitochondrial complex I inhibitor rotenone. Similarly, the superoxide generation induced by antimycin A, an inhibitor of mitochondrial complex III, was considerably reduced by rotenone, suggesting the involvement of complex III also in the severe Cd stress-induced superoxide generation. This severe Cd stress-induced superoxide generation was followed by an extensive cell death in this part of the root tip, which similar to the superoxide generation, was eliminated by rotenone co-treatment. In turn, mild Cd stress-induced diphenyleneiodonium (DPI)-sensitive superoxide generation was observed only in the post-stressed roots, suggesting that it is not directly associated with Cd toxicity. Diphenyleneiodonium, an inhibitor of NADPH oxidase, markedly inhibited the mild Cd stress-induced radial expansion of root apex, indicating that enhanced DPI-sensitive superoxide production is required for rapid isotropic cell growth. Severe Cd stress, probably through the inhibition of complex III, caused a rapid and robust superoxide generation leading to cell and/or root death. By contrast, mild Cd stress did not evoke oxidative stress, and the enhanced DPI-sensitive superoxide generation is utilized in adaptive morphogenic responses.

Cadmium uptake, tissue localization and structural changes induced at cellular level are essential to understand Cd tolerance in plants. In this study we have exposed plants of Pteris vittata to different concentrations of CdCl2 (0, 30, 60, 100 μM) to evaluate the tolerance of the fern to cadmium. Cadmium content determination and its histochemical localization showed that P. vittata not only takes up, but also transports and accumulates cadmium in the aboveground tissues, delocalizing it mainly in the less bioactive tissues of the frond, the trichomes and the scales. Cadmium tolerance in P. vittata was strictly related to morphogenic response induced by the metal itself in the root system. Adaptive response regarded changes of the root apex size, the developmental pattern of root hairs, the differentiation of xylem elements and endodermal suberin lamellae. All the considered parameters suggest that, in our experimental conditions, 60 μM of Cd may represent the highest concentration that P. vittata can tolerate; indeed this Cd level even improves the absorbance features of the root and allows good transport and accumulation of the metal in the fronds. The results of this study can provide useful information for phytoremediation strategies of soils contaminated by Cd, exploiting the established ability of P. vittata to transport, delocalize in the aboveground biomass and accumulate polluting metals.

Exposure of roots to low Cd concentration induced morphogenic responses including the inhibition of root growth and the radial swelling of root tip. High Cd concentrations within a few minutes caused a robust induction of superoxide generation leading to the cell death and root growth arrest. This toxic superoxide generation blocked the development of low Cd concentration-activated morphogenic responses. While the morphogenic responses of roots to low Cd concentration are induced very rapidly and probably due to the interaction of Cd with the apoplast of root tissue, high Cd concentration-induced superoxide production required the entry of Cd into the symplast. Auxin signaling is involved in the activation of Cd-induced morphogenic defence responses but not in the Cd-induced toxic superoxide generation. These results suggest that oxidative stress is not a primary cause for the Cd-induced morphogenic responses such as growth reduction and radial cell expansion in barley root tips.

Grapevine bud fruitfulness is determined by the differentiation of uncommitted meristem (UCM) into either tendril or inflorescence. Since tendril and inflorescence differentiation have long been considered sequential steps in inflorescence development, factors that control the progression of floral meristem development may regulate the final outcome of UCM differentiation, and thus affect fruitfulness. A comparison of the expression profiles of the master regulators of floral meristem identity (FMI) during development of fruitful and non-fruitful buds along the same cane allowed associating the expression of a homolog of terminal flower 1 (TFL1, a negative regulator of FMI) to fruitful buds, and the expression of positive FMI regulators to non-fruitful buds. Combined with (a) cytokinin-induced upregulation of VvTFL1A expression in cultured tendrils, which accompanied cytokinin-derived tendril transformation into branched, inflorescence-like structures, (b) positive regulation of VvTFL1A expression by cytokinin, which was demonstrated in transgenic embryonic culture expressing GUS reporter under the control of VvTFL1A promoter, and (c) a significantly higher level of active cytokinins in fruitful positions, the data may support the assumption of cytokinin-regulated VvTFL1A activity's involvement in the control of inflorescence development. Such activity may delay acquisition of FMI and allow an extended branching period for the UCM, resulting in the differentiation of inflorescence primordia.

An indeterminate developmental program allows plant organs to grow continuously by maintaining functional meristems over time. The molecular mechanisms involved in the maintenance of the root apical meristem are not completely understood. We have identified a new Arabidopsis thaliana mutant named moots koom 1 (mko1) that showed complete root apical meristem exhaustion of the primary root by 9 days post-germination. MKO1 is essential for maintenance of root cell proliferation. In the mutant, cell division is uncoupled from cell growth in the region corresponding to the root apical meristem. We established the sequence of cellular events that lead to meristem exhaustion in this mutant. Interestingly, the SCR and WOX5 promoters were active in the mko1 quiescent center at all developmental stages. However, during meristem exhaustion, the mutant root tip showed defects in starch accumulation in the columella and changes in auxin response pattern. Therefore, contrary to many described mutants, the determinate growth in mko1 seedlings does not appear to be a consequence of incorrect establishment or affected maintenance of the quiescent center but rather of cell proliferation defects both in stem cell niche and in the rest of the apical meristem. Our results support a model whereby the MKO1 gene plays an important role in the maintenance of the root apical meristem proliferative capacity and indeterminate root growth, which apparently acts independently of the SCR/SHR and WOX5 regulatory pathways.

Sprouting of potatoes during storage, due to tuber dormancy release, is associated with weight loss and softening. Sprout-preventing chemicals, such as chlorpropham (CIPC), can negatively impact the environment and human health. Monthly thermal fogging with mint (Mentha spicata L.) essential oil (MEO) inhibited sprouting in eight potato cultivars during large-volume 6-month storage: the tubers remained firm with 38% lower weight loss after 140 days of storage. The sprout-inhibitory action may be nullified: treated tubers washed with water resumed sprouting within days, with reduced apical dominance. MEO application caused local necrosis of the bud meristem, and a few weeks later, axillary bud (AX) growth was induced in the same sprouting eye. MEO components analysis showed that 73% of its content is the monoterpene R-carvone. Tubers treated with synthetic R-carvone in equivalent dose, 4.5 microl l(-1), showed an inhibitory effect similar to that of MEO. Surprisingly, 0.5 microl l(-1) of MEO or synthetic R-carvone catalyzed AX sprouting in the tuber. To the best of our knowledge, this is the first report of an essential oil vapor inducing early sprouting of potato tubers. R-carvone caused visible damage to the meristem membrane at sprout-inhibiting, but not sprout-inducing doses, suggesting different underlying mechanisms. After 5 days' exposure to R-carvone, its derivatives transcarveol and neo-dihydrocarveol were found in buds of tubers treated with the inhibitory dose, suggesting biodegradation. These experiments demonstrate the potential of MEO vapor as an environmentally friendly alternative to CIPC in stored potatoes and as a research tool for the control of sprouting in plants.

The effect of cadmium (Cd) on the expression and activity of NADPH oxidase, peroxidase and oxalate oxidase as well as on the expression of aquaporins and dehydrins was studied in barley root tip. The root tip represented intact apical part of the barley root containing the root cap, meristems and elongation zone. Except stress induced by Cd, barley root tips were analysed after their exposure to phytotoxic concentration of mercury (Hg)-, hydrogen peroxide (H2O2)- or polyethylene glycol (PEG)-induced water stress in order to compare the Cd-induced changes with changes induced by these other stress factors. Cd, Hg, H2O2 and with some exceptions also PEG treatments caused similar alterations in the gene expression of reactive oxygen species (ROS)-generating and water deficiency-related genes, and in the activity of ROS-generating enzymes. These evidences support our opinion that ROS accumulation and water imbalance are the common symptoms of these stress factors and that the elevated production of H2O2 plays, probably as a signal molecule, a key role in the induction of plant responses to abiotic stresses in barley root tip. On the other hand, H2O2 at permanent high concentration is probably the main toxic factor during stress conditions.

Adventitious roots (ARs) are induced by auxins. Jasmonic acid (JA) and methyl jasmonate (MeJA) are also plant growth regulators with many effects on development, but their role on ARs needs investigation. To this aim, we analyzed AR formation in tobacco thin cell layers (TCLs) cultured with 0.01-10 microM MeJA, either under root-inductive conditions, i.e., on medium containing 10 microM indole-3-butyric acid (IBA) and 0.1 microM kinetin, or without hormones. The explants were excised from the cultivars Samsun, Xanthii and Petite Havana, and from genotypes with altered AR-forming ability in response to auxin, namely the non-rooting rac mutant and the over-rooting Agrobacterium rhizogenes rolB transgenic line. Results show that NtRNR1 (G1/S) and Ntcyc29 (G2/M) gene activity, cell proliferation and meristemoid formation were stimulated in hormone-cultured TCLs by submicromolar MeJA concentrations. The meristemoids developed either into ARs and xylogenic nodules, or into xylogenic nodules only (rac TCLs). MeJA-induced meristemoid over-production characterized rolB TCLs. No rooting or xylogenesis occurred under hormone-free conditions, independently of MeJA and genotype. Endogenous JA progressively (days 1-4) increased in hormone-cultured TCLs in the absence of MeJA. JA levels were enhanced by 0.1 microM MeJA, on both days 1 and 4. Endogenous IBA was the only auxin detected, both in the free form and as IBA-glucose. Free IBA increased up to day 2, remaining constant thereafter (day 4). Its level was enhanced by 0.1 microM MeJA only on day 1, while IBA conjugation was not affected by MeJA. Taken together, these results show that an interplay between jasmonates and auxins regulates AR formation and xylogenesis in tobacco TCLs.

Laser ablation coupled with inductively coupled plasma-mass spectrometry was used to find Cu and Zn concentration in surface tissue along a longitudinal developmental gradient with meristem, rapidly elongating tissue, and nongrowing tissue in a model system of seedling roots of Cucumis sativus L. (cucumber). Tissue metal accumulation was determined for roots of seedlings growing on cellulosic germination paper treated with nutrient solution (controls), and also treated with concentrations of Zn (40 ppm) and Cu (10 ppm) that reduced growth. Cu content of all roots is highest at the apex and falls sharply to lower values by 2 mm from the root tip. In contrast, at moderate Zn availability (0.07 ppm), Zn content rises from the apex to 2 mm then falls throughout the remainder of the growth zone. At high external Zn the spatial pattern resembles that of Cu. Cucumber root growth zones accumulate more of each metal with higher external availability. Metal deposition rates were calculated using a continuity equation with data on local metal content and growth velocity. Deposition rates of both metals are generally highest in the rapidly elongating region, 1.5-3.5 mm, even where metal concentration is decreasing with position and root age and even when the accumulation is inhibitory to growth.

The detection of genes having similar expression profiles following the application of different stimuli that trigger bud break may constitute potent tools for the identification of pathways with a central role in dormancy release. We compared the effects of heat shock (HS) and hydrogen cyanamide (HC) and demonstrated that HS leads to earlier and higher bud-break levels. Changes in transcript levels of catalase, alcohol dehydrogenase and pyruvate decarboxylase were induced following both treatments. However, timing and extent of changes in transcript level differed. Changes occurred earlier in HS-treated buds and were more intense in HC-treated buds. The changes in transcript levels after both treatments were temporary. The rapid and short-lasting changes in gene expression following HS treatment correlated with the faster and higher level of bud-break that this treatment exerted. This correlation may propose that the reported molecular events are mechanistically involved in dormancy release. To test the hypothesis that temporary oxidative stress is part of the mechanism inducing dormancy release, we analyzed the effect of HS and HC treatments on the expression of ascorbate peroxidase, glutathione reductase, thioredoxin h, glutathione S-transferase and sucrose synthase genes and found that they were induced by both treatments in a similar pattern. Taken together, these findings propose that similar cellular processes might be triggered by different stimuli that lead to dormancy release, and are consistent with the hypothesis that temporary oxidative stress and respiratory stress might be part of the mechanism that leads to bud break.

Sugars, the main growth substrates of plants, act as physiological signals in the complex regulatory network of sugar metabolism. To investigate the function of different glycolytic steps in sugar sensing and signaling we compared the effects of carbon starvation with those of glucose, glycerol and dihydroxyacetone on carbon metabolism, proteolysis, and protease expression in excised maize (Zea mays L.) root tips. Respiration, soluble proteins, protein turnover and proteolytic activities were monitored as a function of time, along with in vitro and in vivo analysis of a variety of metabolites (sugars, amino and organic acids, phosphoesters, adenine nucleotides...) using (13)C, (31)P and (1)H NMR spectroscopy. Our results indicate that, in maize root tips, endopeptidase activities and protease expression are induced in response to a decrease in carbon supply to the upper part of the glycolytic pathway, i.e. at the hexokinase step. Proteolysis would be controlled downstream glycolysis, probably at the level of the respiratory substrate supply to mitochondria.

A single application of cytokinin benzyladenine causes a threefold increase in the frequency of plasmodesmata in the vegetative shoot apical meristem (SAM) of Sinapis alba plants. This increase is observed 20 h after application within all cell layers (L1, L2, L3) as well as at the interfaces between these layers. Evidence is presented indicating that cytokinin promotes mainly the formation of new secondary plasmodesmata. A similar increase in the frequency of secondary plasmodesmata was observed in the Sinapis SAM during the floral transition induced by a single long day, suggesting that this effect of the long day is mediated by cytokinin.

The regeneration of difficile lily protoplasts isolated from suspension cells of the Oriental hybrid lily ( Lilium L.) cultivars Casablanca, Siberia and Acapulco was achieved by using the nurse-culture method. The divided protoplasts grew into colonies with nurse cells that have no regeneration ability, and developed to visible calli on a medium containing picloram. Many plantlets were formed on the calli after transfer of the proliferated calli to hormone-free medium. We were able to transplant the plantlets to soil in pots without acclimatization, and the plantlets grew in a greenhouse until flowering 2 years later.

Oxygen fluxes into and from root cells of Vitis rupestris (flooding sensitive), V. riparia (flooding tolerant) and V. vinifera (medium tolerance to flooding) were measured under different levels of O2 availability using a recently developed polarographic O2-selective, vibrating-microelectrode system. The system enables fluxes to be measured with a spatial resolution of 2-3 microm and a temporal resolution of 10 s. No difference in root porosity was found among the genotypes when grown for 30 days in an aerated solution. Under normoxic conditions, O2 influx was characterised by two distinct peaks, one in the division zone and the other in the elongation zone of the roots. This pattern was found in all three species studied, although the fluxes showed a different magnitude. The peak in the elongation zone coincided with maximum relative elemental growth rates. When the energetics of the cell was disturbed by cyanide, both growth and oxygen O2 influxes ceased at the same time. Under hypoxic conditions, V. riparia plants showed a precise strategy directed toward the maintenance of enough O2 for the respiratory needs of mitosis in the apical meristem of the roots. Thus, whereas in the division zone of V. rupestris and V. vinifera, at bulk O2 concentrations of 0.094 mol x m(-3), the O2 influx was reduced by 70.5 and 38.5%, respectively, for V. riparia no variation in the O2 influx was detected down to bulk O2 concentrations of 0.078 mol x m(-3). Moreover, in accordance with the different tolerances of the plants, the Vitis genotypes were found to differ in their radial O2 loss from the adventitious roots when in an O2-free environment. The results are discussed in terms of possible mechanisms of response to anoxia in Vitis species with different tolerances to flooding.

Under long hydroxyurea treatments, evidence was obtained for the sequential activation of four checkpoints located between the onset of S phase and mitosis in Allium cepa L. root meristems. Biparametric flow cytometry (Br-DNA/total DNA) showed that cells initially accumulated at early S phase but, after a delay, they resumed replication and paused again at mid S phase. Cells not only overrode this second replication block but also any G2 checkpoint they encountered. Thus, a late mitotic wave was produced in the presence of hydroxyurea. The wave was formed by cells that had apparently completed their replication (normal mitoses), while others displayed anaphases/telophases with less than the expected DNA content and with chromosomal breaks (aberrant mitoses). The presence of aberrant mitoses is direct evidence for the undue override of the two G2 checkpoints responsible for surveillance of completion of DNA synthesis and repair, respectively. Caffeine selectively abrogated the G2 block produced by the checkpoint that controls post-replication DNA repair, as it advanced the entry of cells into an aberrant mitosis. However, caffeine proved not to be the universal checkpoint-evading agent as postulated. Caffeine did not modify the spontaneous override of the replication checkpoints. Moreover, it seems to enforce the checkpoint that controls the completion of DNA synthesis, as the appearance of the late wave of normal mitoses produced in the presence of hydroxyurea was prevented by the use of caffeine.

The effects of trigonelline (TRG) on the cell cycle in root meristems of Lactuca sativa L. were examined in the knowledge that TRG is a cell cycle regulator that causes cell arrest in G2, and prevents ligation of replicons in S-phase. The hypothesis was tested that continuous exposure to TRG would perturb DNA replication which, in turn, would lengthen the cell cycle and impair root elongation. Using DNA fibre autoradiography, mean replicon size was 31 and 13 microm in the TRG (3 mM) and control treatments, respectively. Trigonelline also resulted in a lengthening of both S-phase and the cell cycle and a decrease in primary root elongation. Hence, replicon inactivation was responsible for the protracted S-phase. Trigonelline treatment also resulted in a 1.6-fold increase in fork rate (13.8 microm h(-1)) compared with the control (8.4 m h(-1)). The faster fork rate in the larger replicons is in accord with the highly significant positive relationship already established between fork rate and replicon size for various unrelated higher plants.