Dormancy by communication shutdown: Trees become dormant in winter, with encapsulated buds protected against harsh conditions. Tylewicz et al. found that, as the days get shorter, communication channels between cells in aspen trees shut down. The blocked plasmodesmata sequester the dormant meristems from growth signals. Growth-promoting signals can be turned on and off relatively rapidly, but the closed plasmodesmata are not so nimble. Thus, despite the occasional sunny day, the trees stay dormant until spring. Science , this issue p. 212

In boreal and temperate ecosystems, temperature signal regulates the reactivation of growth (bud break) in perennials in the spring. Molecular basis of temperature-mediated control of bud break is poorly understood. Here we identify a genetic network mediating the control of bud break in hybrid aspen. The key components of this network are transcription factor SHORT VEGETATIVE PHASE-LIKE (SVL), closely related to Arabidopsis floral repressor SHORT VEGETATIVE PHASE, and its downstream target TCP18, a tree homolog of a branching regulator in Arabidopsis. SVL and TCP18 are downregulated by low temperature. Genetic evidence demonstrates their role as negative regulators of bud break. SVL mediates bud break by antagonistically acting on gibberellic acid (GA) and abscisic acid (ABA) pathways, which function as positive and negative regulators of bud break, respectively. Thus, our results reveal the mechanistic basis for temperature-cued seasonal control of a key phenological event in perennial plants.

Significance: Perennial plants display seasonal cycles of growth. For example, in the trees of boreal temperate forests, growth must cease prior to the advent of winter and cold hardiness must be acquired to survive extreme low temperature. Growth cessation and activation of transcriptional programs underlying adaptive responses associated with cold hardiness are photoperiodically controlled. We show that the evolutionarily conserved protein FD implicated in the control of flowering mediates photoperiodic control of seasonal growth in trees by forming a complex with FLOWERING LOCUS T (FT) protein. FD genes of hybrid aspen display neofunctionalization and, in contrast to Arabidopsis , have evolved functions that are independent of their interaction with FT, such as transcriptional control of the adaptive response and bud maturation pathways in trees.

Significance: In perennial plants such as long-lived trees growing in boreal and temperate forest, transition from summer to winter is associated with induction of growth cessation and bud set at the shoot apex. Where in the plant these seasonal shifts are perceived and how these are communicated to the shoot apex remain unresolved. We identify leaves as a site for perception of seasonal shifts and reveal that components of floral transition such as FLOWERING LOCUS T ( FT ) and plant hormone GA have been recruited to function as long-range signals to communicate seasonal changes perceived in leaves to the shoot apical meristem to control its activity to synchronize bud set with the change of seasons in perennials.