CSN5A and DEG1 Contribute to FT Stability in Temperature-Dependent Flowering

FLOWERING LOCUS T (FT) functions as a florigen and plays a central role in the floral transition. Although FT regulation has been extensively characterized at the transcriptional level, mechanisms controlling FT protein stability remain poorly understood. Here, we provide evidence for a temperature-responsive proteolytic process involving COP9 signalosome subunit 5A (CSN5A) and the serine protease DEGRADATION OF PERIPLASMIC PROTEINS 1 (DEG1) that contributes to flowering-time regulation. csn5a mutants flower early at low ambient temperature and exhibit increased accumulation of a myc-tagged FT protein, suggesting post-translational regulation. deg1 mutants also flower early under low-temperature conditions, and genetic analyses reveal additive flowering phenotypes in csn5a deg1 double mutants, supporting partially independent contributions of CSN5A and DEG1. Biochemical assays show that recombinant DEG1 promotes FT degradation in vitro, whereas serine protease inhibition attenuates FT turnover in vivo. Phloem-specific expression of CSN5A restores the flowering phenotype of csn5a mutants, consistent with a vascular domain of action relevant to FT biology. Together, these findings reveal a temperature-responsive post-translational layer of FT regulation that contributes to flowering-time control.