ABSCISIC ACID-INSENSITIVE 5 (ABI5) Orchestrates Seasonal Growth Cessation and Wood Formation Inhibition in Populus tomentosa

Perennial trees in temperate regions precisely coordinate the timing of seasonal growth cessation and dormancy with environmental cues, primarily photoperiod. While the roles of abscisic acid (ABA) in dormancy regulation are well-established, its function in growth cessation remains less defined. ABSCISIC ACID-INSENSITIVE 5 (ABI5) is a basic leucine zipper transcription factor that plays a central role in ABA-mediated development and abiotic stress responses, yet its roles in photoperiodic regulation of growth cessation and its coordination with radial stem growth remain unknown. Here, we demonstrate that in poplar (Populus tomentosa) trees, exogenous ABA application exacerbated short-day (SD)-induced growth inhibition, accelerated bud set, and strongly suppressed secondary xylem formation. We identified a Populus ABI5 homolog, PtoABI5, whose expression is induced by both ABA and SDs. Overexpression of PtoABI5 phenocopied and enhanced SD responses, leading to premature growth cessation and a pronounced inhibition of cambial division and wood formation under SDs. Conversely, PtoABI5 suppressed the expression of the GA biosynthesis gene, while it enhanced the expression of GA catabolic genes. Exogenous GA application partially rescued both the apical growth defects and the impaired secondary xylem development in PtoABI5-overexpressing plants. Our findings establish PtoABI5 as a central integrator, linking ABA and GA signaling pathways to coordinately arrest shoot apical growth and seasonal wood formation, thereby fine-tuning the seasonal growth cycle in perennial trees.