Vernalization and the chilling requirement to exit bud dormancy: shared or separate regulation?

Abstract

Similarities have long been recognized between vernalization, the prolonged exposure to cold temperatures that promotes the floral transition in many plants, and the chilling requirement to release bud dormancy in woody plants of temperate climates. In both cases the extended chilling period occurring during winter is used to coordinate developmental events to the appropriate seasonal time. However, whether or not these processes share common regulatory components and molecular mechanisms remain largely unknown. Both gene function and association genetics studies in Populus are beginning to answer this question. In Populus, studies have revealed that orthologs of the antagonistic flowering time genes FT and CEN/TFL1 might have central roles in both processes. We review Populus seasonal shoot development related to dormancy release and the floral transition and evidence for FT/TFL1-mediated regulation of these processes to consider the question of regulatory overlap. In addition, we discuss the potential for and challenges to integrating functional and population genomics studies to uncover the regulatory mechanisms underpinning these processes in woody plant systems.

Keywords: FT; TFL1; adaptation; dormancy; vernalization.

FIGURE 1

Winter to spring shoot development and roles of FT1, FT2, and CEN1 in Populus. (A) Seasonal shoot phenology and FT1, FT2, and CEN1 expression associated with dormancy release and the initiation of flowering. Seasonal phenology and relative gene expression patterns are based on study of adult Populus deltoides in Mississippi, USA (Yuceer et al., 2003; Hsu et al., 2011) with the exception of the timing of dormancy release, which is an estimate based on the apparent correlation between increasing FT1 expression and dormancy release in controlled environment studies reported by Rinne et al. (2011). The blue dotted line indicates that the shoot apex sample used for expression studies included leaf primordia smaller than 1 mm (e.g., LPL in the dormant bud). Note that the diagrams are not to scale and the number of EPL in a bud can be several more than the depicted number. In addition, an inflorescence typically forms more than 100 flowers that develop within the inflorescence bud over the course of the growing season and after winter dormancy, anthesis occurs the following spring. (B) Conceptual model of how seasonal changes in levels of FT1, CEN1, and FT2 sequentially contribute to the regulation of dormancy release, meristem identity and growth. AM, axillary meristem; IM, inflorescence meristem; FM, floral meristem; VM, vegetative meristem; EPL, early preformed leaves; EPS, early preformed shoot; LPL, late preformed leaves; NL, neoformed leaves; Br, bracts.