Growth competence develops independently of cell division: uncoupling dormancy and cell cycle in grapevine buds

Background and Aim Winter chilling has long been considered the primary driver of bud dormancy release in woody perennials. However, the cellular mechanisms underlying dormancy transitions remain poorly understood. This study aimed to investigate the relationship between dormancy depth, cell cycle activity, and cellular ultrastructure in grapevine buds across the dormancy cycle. Methods Bud dormancy progression in single node explants of Vitis vinifera cv. Cabernet Sauvignon was monitored from early autumn dormancy to the end of winter. Dormancy depth was quantified as the time to 50% bud burst under forcing conditions. Cell cycle status was assessed using quantitative flow cytometry, while the cellular ultrastructure of the shoot apical meristem was examined by transmission electron microscopy. Key Results Dormancy depth declined dramatically from over 280 days in early autumn to ca 50 days in late autumn, despite negligible chilling exposure during this period. Following chilling exposure in winter, the depth of dormancy declined to ca 20 days. Strikingly, flow cytometry revealed that the majority of cells remained arrested in the G1 phase of mitosis throughout this period, regardless of dormancy depth, supported by ultrastructural analysis of the shoot apical meristem. Starch grains were abundant in both deeply dormant pre-winter buds (no chilling) and winter buds (with chilling), indicating that starch dynamics may be regulated independently of both dormancy status and chilling exposure. These findings demonstrate temporal uncoupling of dormancy release from cell cycle reactivation, challenging assumptions that dormancy release is directly linked to both chilling and cell cycle activation. Conclusion Our results establish that in grapevine, the transition to growth competence occurs independently of detectable cell division and can precede significant winter chilling accumulation. This work provides new insights into the cellular basis of dormancy regulation in woody perennials and establishes a methodological framework for dissecting this relationship in other species.