Rapeseed WRKY57-BOR2 Module Mediates Cellular Boron Homeostasis to Confer Low-Boron Adaptation

Boron (B) is a vital micronutrient for plant growth and development. However, the regulatory mechanism underlying cellular B homeostasis, particularly under low B, remain poorly understood. Here, we identified a WRKY transcription factor, BnaC2.WRKY57 as a key regulator of cellular B homeostasis in rapeseed (Brassica napus). Under low-B stress, BnaC2.WRKY57 exhibited dynamically upregulated expression in mature leaves while downregulated in rapidly expanding leaves, thereby functioning as a 'molecular switch' to fine-tune cellular B partitioning to cell walls. Loss-of-function mutants of BnaC2.WRKY57 showed enhanced low-B tolerance, whereas its overexpression lines displayed hypersensitivity, manifested by leaf curling and impaired root elongation. Mechanistically, BnaC2.WRKY57 directly repressed the transcript level of the B transporter BnaC4.BOR2, thereby reducing B sequestration in the cell wall and facilitating B remobilization. This transcriptional regulation fine-tuned B allocation, ensuring efficient utilization under limited B supply. Our findings reveal the BnaC2.WRKY57-BnaC4.BOR2 module as a critical adaptive strategy for low-B stress, providing novel genetic targets for improving B-use efficiency in crops.