GmBBX13 and GmBBX4a/b antagonistically regulate GmCML19 transcription to fine-tune leaf senescence in soybean.

Leaf senescence is a critical developmental process that fundamentally determines crop yield and quality. Here, we report that the B-box CONTAINING PROTEIN 13 (GmBBX13) negatively regulates leaf senescence in soybean. Overexpression of GmBBX13 in soybean significantly delays leaf senescence onset. GmBBX13 physically interacts with two closely related B-box proteins, GmBBX4a and GmBBX4b. Loss of GmBBX4a and GmBBX4b function markedly delays leaf senescence, indicating that GmBBX4a/b serve as positive regulators of senescence. Transcriptomic profiling reveals that GmBBX13 and GmBBX4a/b antagonistically co-regulate approximately 4,700 target genes. Specifically, GmBBX13 represses, whereas GmBBX4a/b activate, the expression of GmCML19, a gene encoding CALMODULIN-LIKE PROTEIN 19, whose product promotes leaf senescence. Collectively, our study reveals a molecular mechanism whereby GmBBX13 and GmBBX4a/b constitute antagonistic regulatory modules that fine-tune the timing of leaf senescence in soybean, offering strategies to improve yield and extend the grain-filling period.