Overexpression of <scp>PSEUDO</scp>‐<scp>RESPONSE REGULATOR</scp> 5a Promotes Bud Break in <i>Populus</i>

Perennial trees adjust to external annual variations in light and temperature, synchronizing dormancy-growth phases through the circadian clock. In Arabidopsis, PSEUDO-RESPONSE REGULATOR (PRR) proteins have been identified as core components of the plant circadian oscillator, negatively regulating the circadian clock and affecting a wide array of biological processes, including seed germination, hypocotyl growth, flowering, and stress responses. However, the roles that PRR might play in perennial trees-affecting seasonal growth cycles-remain poorly understood. In this study, we first identified PtoPRR5a as a potential regulator of bud break in Populus. Second, the expression of PtoPRR5a was low at active apices during the growing season, while it kept increasing, induced by short photoperiod and chilling temperatures, during autumn, and peaked in winter. Third, the genetic evidence presented here demonstrates that overexpressing PtoPRR5a promotes early bud flush by upregulating the expression of PtoFT1 and PtoEBB3, two key activators of bud break. Fourth, our comparative transcriptomic analysis reveals that genes co-regulated by PRR5a and FT1 were enriched in pathways related to cell proliferation and defense, revealing a functional link between the PRR5a and FT1 pathways during bud break.