Casein kinase 1 AELs promote senescence by enhancing ethylene biosynthesis through phosphorylating WRKY22 transcription factor

Leaf senescence is a complex process regulated by developmental and environmental factors, and plays a pivotal role in the development and life cycle of higher plants. Casein kinase 1 (CK1) is a highly conserved serine/threonine protein kinase in eukaryotes and functions in various cellular processes including cell proliferation, light signaling and hormone effects of plants. However, the biological function of CK1 in plant senescence remains unclear. Through systemic genetic and biochemical studies, we here characterized the function of Arabidopsis EL1-like (AEL), a CK1, in promoting leaf senescence by stimulating ethylene biosynthesis through phosphorylating transcription factor WRKY22. Seedlings lacking or overexpressing AELs presented delayed or accelerated leaf senescence, respectively. AELs interact with and phosphorylate WRKY22 at Thr57, Thr60 and Ser69 residues to enhance whose transactivation activity. Being consistent, increased or suppressed phosphorylation of WRKY22 resulted in the promoted or delayed leaf senescence. WRKY22 directly binds to promoter region and stimulates the transcription of 1-amino-cyclopropane-1-carboxylate synthase 7 gene to promote ethylene level and hence leaf senescence. Our studies demonstrated the crucial role of AEL-mediated phosphorylation in regulating ethylene biosynthesis and promoting leaf senescence by enhancing WRKY22 transactivation activity, which helps to elucidate the fine-controlled ethylene biosynthesis and regulatory network of leaf senescence.