HOS15 together with PWR-HDA9 positively regulates dark-induced senescence in Arabidopsis

Senescence is a conserved phenomenon in all living organisms, including plants. The initiation and progression of leaf senescence can be triggered by natural internal factors or induced by external stress conditions. Over the past few decades, several transcriptional regulators, histone deacetylases/acetyltransferases (HDACs/HATs), signaling transduction pathway components, hormonal regulators, and other proteins have been extensively studied and reported to play a role in regulating leaf senescence. However, a deeper molecular understanding of their mechanisms is needed. We recently reported that a WD40-repeat domain protein, HOS15, regulates aging- and dark-induced senescence. Loss-of-function HOS15 mutant plants exhibited a late senescence phenotype with greater chlorophyll content accumulation. The transcript levels of senescence-related (SAG12, SAG29, and ORE1) genes were downregulated in hos15-2 plants compared with those in wild-type (WT) plants, whereas photosynthesis-related (CAB1 and RBCS1A) genes were upregulated. Our studies also revealed that HOS15 works together with PWR-HDA9 complex to associate with the promoters and negatively regulates the expression levels of the senescence negative regulators NPX1, APG9, and WRKY57. Moreover, hos15-2 plants increased H3 acetylation levels, similar to those of hda9 and pwr plants compared to those of WT plants. In addition, the H3 acetylation level was reduced in the dark-induced senescent leaves in WT plants, but not in the hos15-2 plants, which suggests that dark-reduced H3 acetylation requires functional HOS15. Taken together, we conclude that HOS15 together with the PWR-HDA9 complex epigenetically regulates aging- and dark-induced senescence through a common set of genes in Arabidopsis.