Is GABA-shunt functional in endodormant grapevine buds under respiratory stress?

Abstract

It has been suggested that a respiratory stress is part of the mechanism through which the dormancy-breaking compounds, hydrogen cyanamide (HC) and sodium azide, induce the release of buds from the endodormancy (ED) in grapevines. The accumulation of metabolites like succinate, alanine (Ala) and γ-amino butyric acid (GABA), together with the activation of the GABA-shunt pathway, is a general feature of plants in response to oxygen deprivation and to respiratory stress. Unexpectedly, in a previous study, we found that GABA applied exogenously to grapevine buds, down-regulated the expression of most genes encoding for antioxidant enzymes, suggesting that its accumulation under respiratory stress conditions could be deleterious for the bud. In order to analyze whether GABA accumulates under respiratory stress conditions in grapevine buds, we analysed in this study, the effect of hypoxia, the respiration inhibitor KCN and the dormancy breaker compound HC, on the level of GABA, and on the expression levels of the GABA-shunt genes (VvGAD, VvGABA-T, VvSSADH). Additionally, genes from the Ala fermentative pathway (VvAlaAT, VvAspAT) were also analysed. The results revealed that although the three treatments mentioned above, up-regulated the expression of VvGAD1, the content of GABA remained constant, while Ala content increased. The lack of GABA accumulation under respiratory stress is an important physiological fact in grapevine buds, since it avoids the down-regulation of antioxidant genes, and promotes the incorporation of succinate into the TCA cycle, a fact that would be important in the release of buds from the ED.