Abstract
The molecular mechanism regulating dormancy release in grapevine buds is as yet unclear. It has been hypothesized that (i) abscisic acid (ABA) represses bud-meristem activity; (ii) perturbation of respiration induces an interplay between ethylene and ABA metabolism, which leads to removal of repression; and (iii) gibberellin (GA)-mediated growth is resumed. The first two hypothesis have been formally supported. The current study examines the third hypothesis regarding the potential involvement of GA in dormancy release. We found that during natural dormancy induction, levels of VvGA3ox, VvGA20ox, and VvGASA2 transcripts and of GA1 were decreased. However, during dormancy release, expression of these genes was enhanced, accompanied by decreased expression of the bud-expressed GA-deactivating VvGA2ox. Despite indications for its positive role during natural dormancy release, GA application had inhibitory effects on bud break. Hydrogen cyanamide up-regulated VvGA2ox and down-regulated VvGA3ox and VvGA20ox expression, reduced GA1 levels, and partially rescued the negative effect of GA. GA had an inhibitory effect only when applied simultaneously with bud-forcing initiation. Given these results, we hypothesize that during initial activation of the dormant bud meristem, the level of GA must be restricted, but after meristem activation an increase in its level serves to enhance primordia regrowth.
Highlights
The molecular mechanism regulating dormancy release in grapevine buds in response to either natural or artificial stimuli is as yet unclear, and this limits its manipulation to optimize grape production (Or, 2009)
The transcript profiles of GA metabolism genes recorded in the buds throughout the natural dormancy cycle generally agree with the changes in the level of endogenous active GA and the GA-responsive VvGASA2 transcript, with the assumptions of the proposed cascade that leads to dormancy release (Ophir et al, 2009), and with a recent study in Arabidopsis seeds (Footitt et al, 2011)
The period of dormancy induction was accompanied by a gradual decrease in transcript levels for the bud-expressed VvGA3ox and VvGA20ox genes, and their levels were lowest during deepest dormancy
Summary
The molecular mechanism regulating dormancy release in grapevine buds in response to either natural or artificial stimuli is as yet unclear, and this limits its manipulation to optimize grape production (Or, 2009). We have previously proposed a working model that describes the biochemical pathways that are involved in artificially induced bud dormancy release (Ophir et al, 2009). According to this model, perturbation of cytochrome pathway activity in the mitochondria leads to respiratory stress. Perturbation of cytochrome pathway activity in the mitochondria leads to respiratory stress In response to this energy crisis, anaerobic respiration is up-regulated, which mimics hypoxia and thereby affects interplay between ethylene and abscisic acid (ABA) metabolism, which in turn leads to gibberellin (GA)mediated growth resumption (Ophir et al, 2009). The potential involvement of GA in the cascade that leads to grapevine bud dormancy release, which has not yet been addressed, is the theme of the current study
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