Dual role of tree florigen activation complex componentFDin photoperiodic growth control and adaptive response pathways

A complex consisting of evolutionarily conserved FD, FLOWERING LOCUS T (FT) proteins is a regulator of floral transition. Intriguingly,FTorthologs are also implicated in developmental transitions distinct from flowering, such as photoperiodic control of bulbing in onions, potato tuberization, and growth cessation in trees. However, whether an FT–FD complex participates in these transitions and, if so, its mode of action, are unknown. We identified two closely relatedFDhomologs,FD-like 1(FDL1) andFD-like 2(FDL2), in the model tree hybrid aspen. Using gain of function and RNAi-suppressedFDL1andFDL2transgenic plants, we show thatFDL1andFDL2have distinct functions and a complex consisting of FT and FDL1 mediates in photoperiodic control of seasonal growth. The downstream target of the FT–FD complex in photoperiodic control of growth isLike AP1(LAP1), a tree ortholog of the floral meristem identity geneAPETALA1. Intriguingly,FDL1also participates in the transcriptional control of adaptive response and bud maturation pathways, independent of its interaction with FT, presumably via interaction with ABSCISIC ACID INSENSITIVE 3 (ABI3) transcription factor, a component of abscisic acid (ABA) signaling. Our data reveal that in contrast to its primary role in flowering,FDhas dual roles in the photoperiodic control of seasonal growth and stress tolerance in trees. Thus, the functions of FT and FD have diversified during evolution, andFDhomologs have acquired roles that are independent of their interaction with FT.