OsBAK2/OsSERK2 Expression is Repressed by OsBZR1 to Modulate Brassinosteroid Response and Grain Length in Rice.

Brassinosteroids (BRs) are a class of polyhydroxylated steroidal phytohormones and essential for plant growth and development. Rice BRASSINOSTEROID-INSENSITIVE1 (BRI1)-ASSOCIATED RECEPTOR KINASES (OsBAKs) are plasma membrane (PM)-localized receptor kinases, belonging to the subfamily of leucine-rich repeat (LRR) receptor kinases. In Arabidopsis, BRs induce the formation of BRI1-BAK1 heterodimer complex and transmit the signal cascade to the BRASSINAZOLE RESISTANT1/bri1-EMS-SUPPRESSOR1 (BZR1/BES1) to regulate BR signaling. In rice, we found that OsBZR1 binds directly to the promoter of OsBAK2, not OsBAK1, and represses the expression of OsBAK2 to form a BR feedback inhibition loop. Additionally, the phosphorylation of OsBZR1 by OsGSK3 reduced its binding activity to OsBAK2 promoter. The osbak2 displays a typical BR deficient phenotype and negatively modulates the accumulation of OsBZR1. Interestingly, the grain length of osbak2 mutant was increased whereas the cr-osbak2/cr-osbzr1 double mutant restored the reduced grain length of cr-osbzr1 mutant, implying that the increased grain length of osbak2 may be due to the rice SERKs-dependent pathway. Our study reveals a novel mechanism of OsBAK2 and OsBZR1 engaging in a negative feedback loop to maintain rice BR homeostasis, facilitating to a deeper understanding of BR signaling network and grain length regulation in rice.