Alternative splicing of FLOWERING LOCUS T1 fine-tunes bud dormancy release in hybrid poplar

Bud dormancy release, triggered by winter chilling, is a critical adaptive process that enables temperate and boreal perennial trees to resume growth in spring. Although recent studies have identified FLOWERING LOCUS T1 (FT1) as a key regulator of dormancy release in Populus, the molecular mechanisms underlying FT1 action remain unclear. Here, through comprehensive analysis of the alternative splicing (AS) landscape during dormancy release, we identified a dormancy release-dependent AS event in FT1 that generates 2 isoforms: FT1 and an exon-skipped variant FT1SE. Functional characterization demonstrated that FT1SE acts as a dominant-negative regulator: overexpression of FT1SE delays bud break, whereas silencing accelerates it. Exon skipping introduced a premature stop codon, removing the C-terminal Phosphatidylethanolamine-binding protein (PEBP) domain. Despite this truncation, FT1SE retained interaction with FLOWERING LOCUS D-LIKE1 (FDL1) and competitively interfered with the formation of the FT1-FDL1 activation complex. We further showed that FT1SE production results from a CT-to-AG substitution at the splice acceptor site. Phylogenetic analysis indicated that this nucleotide variant is unique to species within the Populus section and is broadly distributed across cold-adapted habitats, suggesting that FT1 AS represents an evolutionary innovation enabling dormancy adaptation to freezing environments.