BKK1/BAK7 and TCP21 Form a Positive Feedback Loop to Facilitate Submergence Tolerance in Plants

Submergence poses 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 crucial for plant survival after submergence stress. Submergence rapidly activates the expression of BAK7 and induces the accumulation of its kinase domain in the nucleus. BAK7 interacts with and phosphorylates class I TEOSINTE BRANCHED1, CYCLOIDEA, PROLIFERATING CELL FACTOR 1 and 2 (TCP) transcription factor TCP21, thereby stabilizing it and modulating transcriptomic reprogramming after submergence. Moreover, overexpression of TCP21 rescues the submergence-intolerant phenotype of the 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.