HOS15 Modulates Circadian Rhythm Through Transcriptional and Post-translational Regulation

Abstract

Plants synchronize physiological processes with environmental cycles through circadian clock, which generates biological rhythms in multiple cellular responses such as developmental processes and stress responses. Here, we demonstrate that HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES 15 (HOS15), a known regulator of stress responses and flowering time regulation, contributes to circadian rhythm control. Loss of HOS15 led to a shortened circadian period and increased amplitude. Treatment with a proteasome inhibitor MG132 or a histone acetyltransferase inhibitor MB3 restored the shortened period of hos15-2 to WT levels. Especially, MG132 treatment enhanced the amplitude in WT plants, whereas hos15-2 mutant were unresponsive. In hos15-2, the abundance of an evening-phased clock protein ZEITLUPE (ZTL) increased. HOS15 physically interacted with ZTL and formed a complex in vivo. Transcriptomic analyses further revealed that HOS15 represses the expression of evening-expressed clock genes, including GIGANTEA (GI) and EARLY FLOWERING 3 (ELF3). These findings expand the role of HOS15 from an environmental response regulator to a modulator of circadian rhythm via both post-translational and transcriptional mechanisms, providing new insights into its pivotal role in fine-tuning clock precision. © The Author(s), under exclusive licence to Korean Society of Plant Biologist 2025.