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Gibberellins (GAs) regulate numerous developmental processes in grapevine (Vitis vinifera) such as rachis elongation, fruit set, and fruitlet abscission. The ability of GA to promote berry enlargement has led to its indispensable use in the sternospermocarpic ('seedless') table grape industry worldwide. However, apart from VvGAI1 (VvDELLA1), which regulates internode elongation and fruitfulness, but not berry size of seeded cultivars, little was known about GA signalling in grapevine. We have identified and characterized two additional DELLAs (VvDELLA2 and VvDELLA3), two GA receptors (VvGID1a and VvGID1b), and two GA-specific F-box proteins (VvSLY1a and VvSLY1b), in cv. Thompson seedless. With the exception of VvDELLA3-VvGID1b, all VvDELLAs interacted with the VvGID1s in a GA-dependent manner in yeast two-hybrid assays. Additionally, expression of these grape genes in corresponding Arabidopsis mutants confirmed their functions in planta. Spatiotemporal analysis of VvDELLAs showed that both VvDELLA1 and VvDELLA2 are abundant in most tissues, except in developing fruit where VvDELLA2 is uniquely expressed at high levels, suggesting a key role in fruit development. Our results further suggest that differential organ responses to exogenous GA depend on the levels of VvDELLA proteins and endogenous bioactive GAs. Understanding this interaction will allow better manipulation of GA signalling in grapevine., (© The Author 2015. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

In warm-winter regions, induction of dormancy release by hydrogen cyanamide (HC) is mandatory for commercial table grape production. Induction of respiratory stress by HC leads to dormancy release via an uncharacterized biochemical cascade that could reveal the mechanism underlying this phenomenon. Previous studies proposed a central role for abscisic acid (ABA) in the repression of bud meristem activity, and suggested its removal as a critical step in the HC-induced cascade. In the current study, support for these assumptions was sought. The data show that ABA indeed inhibits dormancy release in grape (Vitis vinifera) buds and attenuates the advancing effect of HC. However, HC-dependent recovery was detected, and was affected by dormancy status. HC reduced VvXERICO and VvNCED transcript levels and induced levels of VvABA8'OH homologues. Regulation of these central players in ABA metabolism correlated with decreased ABA and increased ABA catabolite levels in HC-treated buds. Interestingly, an inhibitor of ethylene signalling attenuated these effects of HC on ABA metabolism. HC also modulated the expression of ABA signalling regulators, in a manner that supports a decreased ABA level and response. Taken together, the data support HC-induced removal of ABA-mediated repression via regulation of ABA metabolism and signalling. Expression profiling during the natural dormancy cycle revealed that at maximal dormancy, the HC-regulated VvNCED1 transcript level starts to drop. In parallel, levels of VvA8H-CYP707A4 transcript and ABA catabolites increase sharply. This may provide initial support for the involvement of ABA metabolism also in the execution of natural dormancy., (© The Author 2015. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

Root formation is dependent on meristematic activity and is influenced by nitrogen supply. We have previously shown that ubiquitin ligase, EL5, in rice (Oryza sativa) is involved in the maintenance of root meristematic viability. When mutant EL5 protein is overexpressed to dominantly inhibit the endogenous EL5 function in rice, primordial and meristematic necrosis ia observed. Here, we analysed the cause of root cell death in transgenic rice plants (mEL5) overexpressing EL5V162A, which encodes a partly inactive ubiquitin ligase. The mEL5 mutants showed increased sensitivity to nitrogen that was reflected in the inhibition of root formation. Treatment of mEL5 with nitrate or nitrite caused meristematic cell death accompanied by browning. Transcriptome profiling of whole roots exhibited overlaps between nitrite-responsive genes in non-transgenic (NT) rice plants and genes with altered basal expression levels in mEL5. Phytohormone profiling of whole roots revealed that nitrite treatment increased cytokinin levels, but mEL5 constitutively contained more cytokinin than NT plants and showed increased sensitivity to exogenous cytokinin. More superoxide was detected in mEL5 roots after treatment with nitrite or cytokinin, and treatment with an inhibitor of superoxide production prevented mEL5 roots from both nitrite- and cytokinin-induced meristematic cell death. These results indicate a nitrogen-triggered pathway that leads to changes in root formation through the production of cytokinin and superoxide, on which EL5 acts to prevent meristematic cell death., (© The Author 2014. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

The shade avoidance syndrome (SAS) refers to a set of plant responses initiated after perception by the phytochromes of light enriched in far-red colour reflected from or filtered by neighbouring plants. These varied responses are aimed at anticipating eventual shading from potential competitor vegetation. In Arabidopsis thaliana, the most obvious SAS response at the seedling stage is the increase in hypocotyl elongation. Here, we describe how plant proximity perception rapidly and temporally alters the levels of not only auxins but also active brassinosteroids and gibberellins. At the same time, shade alters the seedling sensitivity to hormones. Plant proximity perception also involves dramatic changes in gene expression that rapidly result in a new balance between positive and negative factors in a network of interacting basic helix-loop-helix proteins, such as HFR1, PAR1, and BIM and BEE factors. Here, it was shown that several of these factors act as auxin- and BR-responsiveness modulators, which ultimately control the intensity or degree of hypocotyl elongation. It was deduced that, as a consequence of the plant proximity-dependent new, dynamic, and local balance between hormone synthesis and sensitivity (mechanistically resulting from a restructured network of SAS regulators), SAS responses are unleashed and hypocotyls elongate., (© The Author 2014. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

Critical responses to developmental or environmental stimuli are mediated by different transcription factors, including members of the ERF, bZIP, MYB, MYC, and WRKY families. Of these, MYB genes play roles in many developmental processes. The overexpression of one MYB gene, MYBH, significantly increased hypocotyl elongation in Arabidopsis thaliana plants grown in the light, and the expression of this gene increased markedly in the dark. The MYBH protein contains a conserved motif, R/KLFGV, which was implicated in transcriptional repression. Interestingly, the gibberellin biosynthesis inhibitor paclobutrazol blocked the increase in hypocotyl elongation in seedlings that overexpressed MYBH. Moreover, the function of MYBH was dependent on phytochrome-interacting factor (PIF) proteins. Taken together, these results suggest that hypocotyl elongation is regulated by a delicate and efficient mechanism in which MYBH expression is triggered by challenging environmental conditions such as darkness, leading to an increase in PIF accumulation and subsequent enhanced auxin biosynthesis. These results indicate that MYBH is one of the molecular components that regulate hypocotyl elongation in response to darkness.