The senescence regulator S40 family members from Caragana intermedia and Arabidopsis thaliana inhibit leaf senescence via promoting cytokinins synthesis

Leaf senescence is regulated by both endogenous hormones and environmental stimuli in a programmed and concerted way. The members of the S40 family have been reported to play roles in leaf senescence. Here we report that overexpression of an S40 family member from Caragana intermedia, CiS40-11 , delayed the leaf senescence. Phylogenetic analysis revealed that the CiS40-11 protein had the highest identity with AtS40-5 and AtS40-6 of A. thaliana. CiS40-11 was highly expressed in leaves and was down-regulated after dark treatment. The subcellular localization analysis showed that CiS40-11 was a cytoplasm-nucleus dual-localized protein. Leaf senescence was delayed in CiS40-11 transgenic A. thaliana or by its transient expression in C. intermedia . Transcriptomic analysis and endogenous hormones assay revealed that CiS40-11 inhibited leaf senescence via promoting the biosynthesis of cytokinins, through blocking AtMYB2 expression in CiS40-11 overexpression lines. Furthermore, in the ats40-5a and ats40-6a mutants, AtMYB2 expression was increased and their leaves exhibited a premature senescence phenotype. Our results show that CiS40-11 (and its orthologs, AtS40-5 and AtS40-6) promoted cytokinin synthesis by inhibiting the expression of MYB2 and releasing its negative regulation on the expression of IPTs to inhabit leaf senescence. Highlight The senescence regulator S40 family members CiS40-11, AtS40-5 and AtS40-6 are induced by light and inhibition leaf senescence by promoting cytokinin synthesis.