Phytochrome B stabilizes the KNOX transcription factor BP/KNAT1 to promote light-initiated seed germination in Arabidopsis thaliana

Seed germination represents a critical step in the life cycle of plants. The far-red/red light photoreceptor phytochrome B (phyB) plays a dominant role in promoting germination by modulating gibberellin (GA) and abscisic acid (ABA) metabolism, although the underlying mechanism remains poorly understood. In this study, we identified BREVIPEDICELLUS (BP)/KNAT1, a KNOX transcription factor that acts downstream of phyB and activates light-initiated seed germination in Arabidopsis thaliana. BP undergoes ubiquitination and is degraded through the 26S proteasome pathway. phyB directly interacts with BP and stabilizes BP protein by decreasing its ubiquitination in imbibed seeds. A genome-wide transcriptomic analysis revealed that BP and phyB co-regulate the expression of genes associated with ABA biosynthesis, signal transduction, seed dormancy, and cell wall organization. BP suppresses the expression of two key ABA biosynthetic genes, NCED6 and NCED9, thus lowering ABA levels in imbibed seeds under phyB-activated conditions. BP directly binds to NCED6 and NCED9 and represses their expression by increasing the levels of H3K27me3, a repressive histone modification marker. Genetic analysis demonstrated that NCED6/NCED9 acts epistatic to BP in phyB-dependent germination. Overall, this study reveals a transcriptional module consisting of phyB-BP-NCED6/9, which transfers light signals that inhibit ABA biosynthesis, thereby promoting light-induced germination.