Differences in respiration between dormant and non-dormant buds suggest the involvement of ABA in the development of endodormancy in grapevines

Abstract

Grapevine buds (Vitis vinifera L) enter endodormancy (ED) after perceiving the short-day (SD) photoperiod signal and undergo metabolic changes that allow them to survive the winter temperatures. In the present study, we observed an inverse relationship between the depth of ED and the respiration rate of grapevine buds. Moreover, the respiration of dormant and non-dormant buds differed in response to temperature and glucose, two stimuli that normally increase respiration in plant tissues. While respiration in non-dormant buds rose sharply in response to both stimuli, respiration in dormant buds was only slightly affected. This suggests that a metabolic inhibitor is present. Here, we propose that the plant hormone abscisic acid (ABA) could be this inhibitor. ABA inhibits respiration in non-dormant buds and represses the expression of respiratory genes, such as ALTERNATIVE NADH DEHYDROGENASE (VaND1, VvaND2), CYTOCHROME OXIDASE (VvCOX6) and CYTOCHROME C (VvCYTC), and induces the expression of VvSnRK1, a gene encoding a member of a highly conserved family of protein kinases that act as energy sensors and regulate gene expression in response to energy depletion. In addition to inducing ED the SD-photoperiod up-regulated the expression of VvNCED, a gene that encodes a key enzyme in ABA synthesis. Taken together, these results suggest that ABA through the mediation of VvSnRK1, could play a key role in the regulation of the metabolic changes accompanying the entry into ED of grapevine buds.

Introduction

The growth of woody-perennial plants in temperate zones is characterized by alternating periods of growth and dormancy in response to seasonal fluctuations in environmental conditions, such as temperatures and photoperiod (Rohde and Bhalereao, 2007). Although there are many definitions of bud-dormancy, today it is considered a transient state of the meristem. During bud dormancy, the meristem suspends its activity and becomes insensitive to growth-promoting signals for a period of time before growth resumes (Rohde and Bhalereao, 2007). According to Lang et al. (1987), the lack of activity in the meristem may be due to factors located outside the bud, known as paradormancy (PD) and/or factors located within the bud, known as endodormancy (ED) or true dormancy. At the end of the growing season, the transition from PD to ED allows buds to survive winter temperatures (Morrison, 1991). In V. vinifera and other members of the Vitaceae family, the meristem of the latent bud differentiates into all of the basic elements of the shoots, including uncommitted primordium, which develop into tendrils or inflorescence (Butrosse, 1974, Mullins et al., 1992). However, all of these developmental events are interrupted by the entrance of the latent bud into ED. At the end of ED, buds resume shoot growth associated with flower formation and development (Morrison, 1991). In contrast to poplar and other tree species, Vitis does not set terminal buds in response to the SD-photoperiod. Upon reaching a critical day length (CDL), other hallmark phenotypic responses such as periderm development, growth cessation and bud dormancy are induced (Fennell and Hoover, 1991, Wake and Fennell, 2000, Grant et al., 2013). The outgrowth of the latent bud is initially prevented by PD signals from the apex (He et al., 2012). However, these latent buds maintain cell division and differentiation activities until the SD-photoperiod signal is perceived and the onset of ED is triggered (Kühn et al., 2009, Grant et al., 2013). Recent works has studied the transcriptional response of grapevine buds to chilling (Mathiason et al., 2009), photoperiod (Sreekantan et al., 2010) and dormancy breaking treatments (Halaly et al., 2008, Pérez et al., 2009). In these studies, candidate genes with dual roles in flowering and dormancy have been identified (Mathiason et al., 2009, Sreekantan et al., 2010) and a role has been proposed for oxidative stress as part of the dormancy release mechanism (Halay et al., 2008; Pérez et al., 2009). Recently, Díaz-Riquelme et al. (2012) analyzed transcriptomic variation during latent grape bud development and demonstrated that major transcriptional changes were associated with para/endodormancy, endo/ecodormancy and ecodormancy/bud-break transitions. Despite these findings, the primary metabolic and biochemical changes that reduce meristem sensitivity to growth-promoting signals during ED in latent buds remain unknown.

We propose that ABA, through the mediation of the SnRK1-type kinases, could act as a potential regulator of the metabolic switch from high to low activity associated with the entry to ED in grapevine buds.