The RING-type E3 ligase ATL72 positively regulates leaf senescence by monoubiquitinating the phosphatase SSPP to impair its dephosphorylation activity in Arabidopsis

Posttranslational modifications (PTMs), such as ubiquitination and phosphorylation, regulate diverse cellular processes. Whereas individual contributions of PTMs to leaf senescence have been well documented, their crosstalk remains largely unexplored. In this study, we identified Arabidopsis To'xicos en Levadura 72 (ATL72), a RING-type E3 ligase, as a positive regulator of leaf senescence in Arabidopsis. ATL72 targets senescence-suppressed protein phosphatase (SSPP), which negatively regulates leaf senescence by dephosphorylating Arabidopsis thaliana (A. thaliana) senescence-associated receptor-like kinase (AtSARK). Expression patterns of ATL72, SSPP, and AtSARK overlap during senescence initiation in leaf development, suggesting that these proteins coordinate to regulate senescence. Our results show that the effect of ATL72 on leaf senescence is dependent on AtSARK. Furthermore, ATL72 monoubiquitinates SSPP; this interaction does not affect the stability of SSPP but significantly reduces its ability to dephosphorylate AtSARK. The SSPP-induced delay in leaf senescence can be effectively rescued by ATL72. We also identified the monoubiquitination sites on SSPP. Collectively, these findings provide critical insights into how complex networks of PTM crosstalk orchestrate the initiation and progression of leaf senescence.