BKK1/BAK7 and TCP21 form a positive feedback loop to facilitate submergence tolerance in plants

Submergence can be a deleterious threat to plant development and the agricultural production of Brassica napus (B. napus), an important worldwide oil crop. However, the molecular mechanisms underlying submergence tolerance in B. napus remain elusive. Here, through a genome-wide association study, we identify a SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE, BKK1/BAK7, which is indispensable for plant survival after submergence stress. Submergence rapidly activates the expression of BAK7 and induces the accumulation of the kinase part of BAK7 in the nucleus. BAK7 interacts with and phosphorylates class I TCP transcription factor TCP21, stabilizing it. TCP21, along with several other class I TCPs, modulates the transcriptomic reprogramming after submergence to enhance plant tolerance to submergence. Moreover, overexpression of TCP21 rescues the submergence-intolerant phenotype of bak1-3 bkk1-1 mutant. Interestingly, TCP21 directly binds to the promoter regions of BAK7 to activate its expression after submergence. This study provides insights into the molecular mechanism of plant adaptation to submergence stress and offers promising solutions for breeding submergence-resistant B. napus accessions. To illuminate bak1-3 bkk1-1, tcp21 and tcpq sensitive phenotype under submergence treatment, we performed gene expression profilling analysis using data obtained from RNA-seq under mock and submergence treatment.