The plant longevity gene AHL15 delays leaf senescence by repressing ORESARA1 and cytokinin degradation

The Arabidopsis thaliana (Arabidopsis) AT-HOOK MOTIF NUCLEAR LOCALIZED 15 (AHL15) gene is associated with various longevity phenotypes and extends the life span of plants when overexpressed. In this study, we show that, in addition to previously described longevity phenotypes, constitutive overexpression of AHL15 in Arabidopsis delays leaf senescence, whereas ahl15 loss-of-function accelerates this process. Dexamethasone-induced nuclear localization of AHL15-GR during dark-triggered senescence results in a stay-green phenotype and represses the expression of several early senescence-associated genes. Among these, the ORESARA1 (ORE1) locus is directly bound by AHL15, suggesting a direct repressive effect of AHL15 on senescence. Furthermore, we demonstrate that AHL15 acts by directly repressing the expression of several CYTOKININ OXIDASE (CKX) genes involved in cytokinin inactivation, resulting in a delayed degradation of cytokinins during dark-induced senescence. Cytokinins are known to delay senescence, and together with the downregulation of ORE1 expression, this explains the repressive effect of AHL15 on senescence. SignificanceHere, we show that in addition to its previously reported effects on aging processes, the AT-HOOK MOTIF NUCLEAR LOCALIZED-family protein AHL15 also represses leaf senescence. Our results demonstrate that AHL15 delays the senescence program in two ways: by directly repressing the senescence master regulator ORESARA1, and by transcriptional repression of CYTOKININ OXIDASE genes, resulting in a delayed breakdown of the senescence-inhibiting hormone cytokinin, which together explain the strong stay-green phenotype of AHL15-overexpressing plants.