DELLA protein RGL2 integrates with FLOWERING LOCUS T to mediate growth and seasonal dormancy in poplar

Seasonal growth is a survival mechanism evolved by perennial trees in the process of adaptation to different latitudinal and seasonal climate changes. Gibberellins (GAs) participate in the induction of dormancy and bud break in trees. DELLA proteins may modulate GA signaling, but their role in regulating seasonal growth of poplar has not been elucidated yet. In this study, we identified a key gene encoding a DELLA protein, PtoRGL2, which was mainly expressed in the apical buds of poplar, and was regulated by photoperiod and low temperature, showing a typical diurnal expression rhythm. Functional analysis revealed that overexpression of PtoRGL2 and its variants lacking different functional domains (PtoRGL2ΔDELLA and PtoRGL2ΔGRAS) regulates the growth and xylem development of transgenic poplar. PtoRGL2 promoted short-day-induced bud dormancy and suppressed cold-mediated bud break, via negative regulation of FLOWERING LOCUS T1 (FT1) and FT2 expression, respectively. Additionally, PtoRGL2 may directly interact with FT1 and FT2, and its GRAS domain is essential for this interaction and inhibition of activity. These findings contribute to the molecular understanding of both GA and FT pathways, and how they jointly regulate the seasonal growth in poplar.