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SUMMARY: Hydrogen sulfide (H2S) promotes plant tolerance against various environmental cues, and d‐cysteine desulfhydrase (DCD) is an enzymatic source of H2S to enhance abiotic stress resistance. However, the role of DCD‐mediated H2S production in root growth under abiotic stress remains to be further elucidated. Here, we report that DCD‐mediated H2S production alleviates osmotic stress‐mediated root growth inhibition by promoting auxin homeostasis. Osmotic stress up‐regulated DCD gene transcript and DCD protein levels and thus H2S production in roots. When subjected to osmotic stress, a dcd mutant showed more severe root growth inhibition, whereas the transgenic lines DCDox overexpressing DCD exhibited less sensitivity to osmotic stress in terms of longer root compared to the wild‐type. Moreover, osmotic stress inhibited root growth through repressing auxin signaling, whereas H2S treatment significantly alleviated osmotic stress‐mediated inhibition of auxin. Under osmotic stress, auxin accumulation was increased in DCDox but decreased in dcd mutant. H2S promoted auxin biosynthesis gene expression and auxin efflux carrier PIN‐FORMED 1 (PIN1) protein level under osmotic stress. Taken together, our results reveal that mannitol‐induced DCD and H2S in roots promote auxin homeostasis, contributing to alleviating the inhibition of root growth under osmotic stress. Significance Statement: Hydrogen sulfide (H2S) promotes plant tolerance against various environmental stress resistance. However, the role of H2S in root development under abiotic stress remains to be further elucidated. Here, we report that d‐cysteine desulfhydrase‐mediated H2S accumulation alleviates root growth inhibition under osmotic stress by promoting auxin homeostasis. [ABSTRACT FROM AUTHOR]

Summary: Osmotic stress influences root system architecture, and polar auxin transport (PAT) is well established to regulate root growth and development. However, how PAT responds to osmotic stress at the molecular level remains poorly understood. In this study, we explored whether and how the auxin efflux carrier PIN‐FORMED3 (PIN3) participates in osmotic stress‐induced root growth inhibition in Arabidopsis (Arabidopsis thaliana).We observed that osmotic stress induces a HD‐ZIP II transcription factor‐encoding gene HOMEODOMAIN ARABIDOPSIS THALIANA2 (HAT2) expression in roots. The hat2 loss‐of‐function mutant is less sensitive to osmotic stress in terms of root meristem growth. Consistent with this phenotype, whereas the auxin response is downregulated in wild‐type roots under osmotic stress, the inhibition of auxin response by osmotic stress was alleviated in hat2 roots. Conversely, transgenic lines overexpressing HAT2 (Pro35S::HAT2) had shorter roots and reduced auxin accumulation compared with wild‐type plants.PIN3 expression was significantly reduced in the Pro35S::HAT2 lines. We determined that osmotic stress‐mediated repression of PIN3 was alleviated in the hat2 mutant because HAT2 normally binds to the promoter of PIN3 and inhibits its expression.Taken together, our data revealed that osmotic stress inhibits root growth via HAT2, which regulates auxin activity by directly repressing PIN3 transcription. [ABSTRACT FROM AUTHOR]

Summary: Lateral roots (LRs), which form in the plant postembryonically, determine the architecture of the root system. While negative regulatory factors that inhibit LR formation and are counteracted by auxin exist in the pericycle, these factors have not been characterised.Here, we report that SHI‐RELATED SEQUENCE5 (SRS5) is an intrinsic negative regulator of LR formation and that auxin signalling abolishes this inhibitory effect of SRS5. Whereas LR primordia (LRPs) and LRs were fewer and less dense in SRS5ox and Pro35S:SRS5‐GFP plants than in the wild‐type, they were more abundant and denser in the srs5‐2 loss‐of‐function mutant. SRS5 inhibited LR formation by directly downregulating the expression of LATERAL ORGAN BOUNDARIES‐DOMAIN 16 (LBD16) and LBD29.Auxin repressed SRS5 expression. Auxin‐mediated repression of SRS5 expression was not observed in the arf7‐1 arf19‐1 double mutant, likely because ARF7 and ARF19 bind to the promoter of SRS5 and inhibit its expression in response to auxin.Taken together, our data reveal that SRS5 negatively regulates LR formation by repressing the expression of LBD16 and LBD29 and that auxin releases this inhibitory effect through ARF7 and ARF19. [ABSTRACT FROM AUTHOR]

Summary: Although nitric oxide (NO) is known to regulate root growth, the factor(s) modulating NO during this process have not yet been elucidated. Here, we identified Arabidopsis WD40‐REPEAT 5a (WDR5a) as a novel factor that functions in root growth by modulating NO accumulation. The wdr5a‐1 mutant accumulated less NO and produced longer roots than the wild type, whereas the WDR5a overexpression lines had the opposite phenotype. The role of NO was further supported by our observation that the NO donor sodium nitroprusside (SNP) and the NO scavenger 2‐(4‐carboxyphenyl)‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide (cPTIO) rescued the root meristem growth phenotypes of the wdr5a‐1 and WDR5a overexpression lines, respectively. The regulation of root growth by WDR5a was found to involve auxin because the auxin levels were similar in SNP‐treated wdr5a‐1 and wild‐type roots, but higher in untreated wdr5a‐1 roots than in wild‐type roots. In addition, the wdr5a‐1 mutant had higher production and activity levels of the auxin biosynthetic enzyme TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS1 (TAA1), in contrast to its reduced expression and activity in the WDR5a overexpression lines, and the increased root meristem growth in wdr5a‐1 was suppressed by treatment with l‐kynurenine, which inhibits TAA1, as well as by mutating TAA1. WDR5a therefore functions in root meristem growth by maintaining NO homeostasis, and thus TAA1‐mediated auxin biosynthesis. [ABSTRACT FROM AUTHOR]

Increased fatty acid β-oxidation is essential for early postgerminative growth in seedlings, but high levels of H2O2 produced by β-oxidation can induce oxidative stress. Whether and how catalase (CAT) functions in fine-tuning H2O2 homeostasis during seedling growth remain unclear. Here, we report that CAT2 functions in early seedling growth. Compared to the wild type, the cat2-1 mutant, with elevated H2O2 levels, exhibited reduced root elongation on sucrose (Suc)-free medium, mimicking soils without exogenous sugar supply. Treatment with the H2O2 scavenger potassium iodide rescued the mutant phenotype of cat2-1. In contrast to the wild type, the cat2-1 mutant was insensitive to the CAT inhibitor 3-amino-1,2,4-triazole in terms of root elongation when grown on Suc-free medium, suggesting that CAT2 modulates early seedling growth by altering H2O2 accumulation. Furthermore, like cat2-1, the acyl-CoA oxidase (ACX) double mutant acx2-1 acx3-6 showed repressed root elongation, suggesting that CAT2 functions in early seedling growth by regulating ACX activity, as this activity was inhibited in cat2-1. Indeed, decreased ACX activity and short root of cat2-1 seedlings grown on Suc-free medium were rescued by overexpressing ACX3. Together, these findings suggest that CAT2 functions in early seedling growth by scavenging H2O2 and stimulating ACX2/3 activity. [ABSTRACT FROM AUTHOR]

Nitric oxide (NO) generation by NO synthase (NOS) in guard cells plays a vital role in stomatal closure for adaptive plant response to drought stress. However, the mechanism underlying the regulation of NOS activity in plants is unclear. Here, by screening yeast deletion mutants with decreased NO accumulation and NOS-like activity when subjected to H2O2 stress, we identified TUP1 as a novel regulator of NOS-like activity in yeast. Arabidopsis WD40-REPEAT 5a ( WDR5a), a homolog of yeast TUP1, complemented H2O2-induced NO accumulation of a yeast mutant Δ tup1, suggesting the conserved role of WDR5a in regulating NO accumulation and NOS-like activity. This note was further confirmed by using an Arabidopsis RNAi line wdr5a-1 and two T-DNA insertion mutants of WDR5a with reduced WDR5a expression, in which both H2O2-induced NO accumulation and stomatal closure were repressed. This was because H2O2-induced NOS-like activity was inhibited in the mutants compared with that of the wild type. Furthermore, these wdr5a mutants were more sensitive to drought stress as they had reduced stomatal closure and decreased expression of drought-related genes. Together, our results revealed that WDR5a functions as a novel factor to modulate NOS-like activity for changes of NO accumulation and stomatal closure in drought stress tolerance. [ABSTRACT FROM AUTHOR]

Glucose functions as a hormone-like signalling molecule that modulates plant growth and development in Arabidopsis thaliana. However, the role of glucose in root elongation remains elusive. Our study demonstrates that high concentrations of glucose reduce the size of the root meristem zone by repressing PIN1 accumulation and thereby reducing auxin levels. In addition, we verified the involvement of ABA INSENSITIVE 5 ( ABI5) in this process by showing that abi5-1 is less sensitive to glucose than the wild type, whereas glucose induces ABI5 expression and the inducible overexpression of ABI5 reduces the size of the root meristem zone. Furthermore, the inducible overexpression of ABI5 in PIN1:: PIN1- GFP plants reduces the level of PIN1- GFP, but glucose reduces the level of PIN1- GFP to a lesser extent in abi5-1 PIN1:: PIN1- GFP plants than in the PIN1:: PIN1- GFP control, suggesting that ABI5 is involved in glucose-regulated PIN1 accumulation . Taken together, our data suggest that ABI5 functions in the glucose-mediated inhibition of the root meristem zone by repressing PIN1 accumulation, thus leading to reduced auxin levels in roots. [ABSTRACT FROM AUTHOR]

Auxin and H2O2 play vital roles in plant development and environmental responses; however, it is unclear whether and how H2O2 modulates auxin levels. Here, we investigate this question using cat2-1 mutant, which exhibits reduced catalase activity and accumulates high levels of H2O2 under photorespiratory conditions. At a light intensity of 150 μmol m−2 s−1, the mutant exhibited up-curled leaves that have increased H2O2 contents and decreased auxin levels. At low light intensities (30 μmol m−2 s−1), the leaves of the mutant were normal, but exhibited reduced H2O2 contents and elevated auxin levels. These findings suggest that H2O2 modulates auxin levels. When auxin was directly applied to cat2-1 leaves, the up-curled leaves curled downwards. In addition, transformation of cat2-1 plants with pCAT2:iaa M, which increases auxin levels, rescued the hyponastic leaf phenotype. Using qRT- PCR, we demonstrated that the transcription of auxin synthesis-related genes and of genes that regulate leaf curvature is suppressed in cat2-1. Furthermore, application of glutathione rescued the up-curled leaves of cat2-1 and increased auxin levels, but did not change H2O2 levels. Thus, the hyponastic leaves of cat2-1 reveal crosstalk between H2O2 and auxin signalling that is mediated by changes in glutathione redox status. [ABSTRACT FROM AUTHOR]

Root negative phototropism is an important response in plants. Although blue light is known to mediate this response, the cellular and molecular mechanisms underlying root negative phototropism remain unclear. Here, we report that the auxin efflux carrier PIN- FORMED ( PIN) 3 is involved in asymmetric auxin distribution and root negative phototropism. Unilateral blue-light illumination polarized PIN3 to the outer lateral membrane of columella cells at the illuminated root side, and increased auxin activity at the illuminated side of roots, where auxin promotes growth and causes roots bending away from the light source. Furthermore, root negative phototropic response and blue-light-induced PIN3 polarization were modulated by a brefeldin A-sensitive, GNOM-dependent, trafficking pathway and by phot1-regulated PINOID ( PID)/ PROTEIN PHOSPHATASE 2A ( PP2A) activity. Our results indicate that blue-light-induced PIN3 polarization is needed for asymmetric auxin distribution during root negative phototropic response. [ABSTRACT FROM AUTHOR]

Abstract: The kinase activity of a Ca2+/calmodulin (CaM)-binding serine/threonine protein kinase from rice (Oryza sativa) (OsCBK) has been reported to be unaffected by OsCaM1 binding. In this study, we examined whether other rice CaMs can stimulate OsCBK. It was observed that OsCaM61 stimulated OsCBK in a Ca2+-dependent manner. In addition, Ala111, Gly123 and Ser127 were identified as critical residues for OsCBK activation. Mutational study and fluorescent spectroscopy analysis indicated that CaM-binding affinity does not correlate with the kinase activity and that these key amino-acids in OsCaM61 play a vital role in suitable changes of OsCBK conformation for kinase activation. [Copyright &y& Elsevier]

Interaction between oxyhaemoglobin and peroxynitrite was studied using stopped-flow rapid-scan spectrophotometry. The influence of pH, peroxynitrite concentration and temperature on the pseudo-first-order rate constants was studied and the activation energy calculated. The kinetic curve for the oxyhaemoglobin-peroxynitrite reaction showed that a fast reaction occurred in the initial seconds, followed by a slow process of decrease in absorbance. The biphasic reaction kinetics of oxyhaemoglobin with peroxynitrite or hydrogen peroxide demonstrated the existence of an intermediary species. For the first time a rapid-scan stopped-flow spectrophotometry study is presented, yielding spectral and kinetic data of the reaction. Copyright © 2005 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]