Bud dormancy in Vitis vinifera: molecular regulation and response to temperature changes

Environmental conditions are paramount for optimal grapevine development and fruit ripening, and consequently for wine production. Temperature, water status and radiation are all influencing factors. On these premises, climate change is an obvious threat to viticulture as it is currently practiced, and adaptation measures are required to maintain productivity levels and wine tipicity. Due to higher average surface temperatures that accelerate bud phenological development, spring frost damage risk cannot be overlooked in the future in several areas of the world, making the identification of effective adaptive measures an issue of the present. Understanding the molecular mechanisms underlying cold acclimation and freezing tolerance acquisition, deacclimation and budbreak is essential to improve crop sustainability and directing breeding efforts. On this regard, the potential of wild Vitis varieties to bear favorable traits, starting from changing chilling requirements and budburst rates, needs to be considered. As a first step in elucidating grapevine dormancy regulation, cultivar Fleurtai and selection UD 31-103, characterized by different tolerance to winter minima, were studied. It is known that, paradoxically, grapevine hybrids most resistant to freezing temperatures are also the most vulnerable to spring frost damage due to higher responsiveness to warm temperatures. This evidence was confirmed by differential thermal analysis (DTA) carried out on Fleurtai and UD 31-103 buds during the 2019-2020 winter season, with cv. Fleurtai, characterized by a greater winter freezing tolerance compared to UD 31-103, being also the fastest to deacclimate. Carbohydrate metabolism was analyzed in both hybrids due to the relevance of soluble sugars as osmoprotectants and metabolic substrates in grapevine buds, and the role as signaling molecules in several developmental processes. Evidence on multiple sugar-related responses taking place inside buds was detected, with soluble sugars content appearing to be greatly influenced by warm spells during winter. The involvement of hexose transporter VvHT5 in the tolerance of physiological water reduction, connected to cold hardiness or resistance to freezing-induced dehydration stress in grapevine buds, was hypothesized. Moreover, the first evidence on VvMSA expression in buds, sole member of ASRs proteins in grapevine, was presented and a role in bud phenological advancement towards budbreak speculated. Lastly, two DEMETER-like DNA demethylases (DMLs) homologs were identified in Fleurtai and UD 31-103 and a role for DEMETER-dependent DNA demethylation in the regulation of dormancy-growth cycle wss suggested. To further inspect the transcriptomic landscape of dormancy release, two cultivars characterized by contrasting budbreak timing were considered, early-budbreak hybrid Sauvignon Nepis and late-budbreak Cabernet Sauvignon. Total RNA sequencing allowed to hypothesize that a plethora of developmental and reproductive processes are differentially activated in winter buds of the early-budbreak cultivar during dormancy, possibly anticipating the late-budbreak one due to an early chilling requirement fulfilment. In addition, new insight has been provided regarding the potential participation of specific transcription factors in dormancy release regulation, with possible genotype-specific roles. In detail, particular interest was raised by dormancy-associated MADS-box VvDAM3-SVP and positive regulator of budbreak VvEBB1, whose expression was significantly different in the two cultivars. Ongoing Whole Genome Bisulfite Sequencing analyses will provide information regarding possible VvDMLs targets during deacclimation and dormancy release. Lastly, a preliminary insight into early molecular responses of actively deacclimating bud tissues to spring frost occurrence was provided by the analysis of sever