Boron mediates nitrogen starvation-induced leaf senescence by regulating ROS production and C/N balance in Brassica napus

Rapeseed (Brassica napus) is highly sensitive to nitrogen (N) and boron (B) deficiencies. However, little is known about the effect of N and B interaction on rapeseed growth and the underlying mechanism of B regulating N starvation-induced leaf senescence in Brassica napus. In this study, B deficiency accelerated the senescence of old leaves induced by N starvation stress in B. napus, while young leaves were more susceptible to B deficiency under N-sufficient condition. B deficiency enhanced SGR1-mediated chlorophyll degradation, exhibiting the chlorosis phenotype of old leaves. The excessive production of reactive oxygen species (ROS) in old leaves was regulated by the photooxidation damage and the up-regulation of RBOHDs under B deficiency. The senescence-associated genes SAG12s were strongly enhanced by B deficiency in response to the ROS over-accumulation. However, B-sufficient supply improved the antioxidant capacity to alleviate ROS toxicity under N starvation. Metabolic analysis showed that B deficiency improved N remobilization from source to sink and increased sugar accumulation in the old leaves, resulting in a significant increase in the C/N ratio. In addition, B deficiency further disrupted the balance of C and N pools through GABA (γ-aminobutyri acid) shunts, which negatively regulated the leaf senescence process. Our results provide a comprehensive model for balancing B and N application in rapeseed production to optimize plant growth.