biostudies-literatureUnknown3(1)Kumimoto RWCircadian rhythms align biological processes with environmental cycles to enhance fitness. Eukaryotic circadian oscillators rely on transcriptional-translational feedback loops, although specific oscillator components vary among lineages. The plant oscillator consists of a uniquely complex network of transcriptional regulators. These clock genes regulate each other's expression and influence key developmental processes such as the transition from vegetative to reproductive growth. Here, we investigate the function of XAP5 CIRCADIAN TIMEKEEPER (XCT), a protein conserved across eukaryotes and previously implicated in RNA processing and circadian regulation in <i>Arabidopsis thaliana</i>. We show that XCT genetically and physically interacts with CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), an E3 Npj biological timing and sleep2https://www.ebi.ac.uk/biostudies/studies/S-EPMC12789028biostudies-literatureConserved eukaryotic factors XCT and COP1 work together to control circadian clock function and reproductive timing in plants.PMC12789028Marshall CMHarmer SLKumimoto RWZhang HfalseConserved eukaryotic factors XCT and COP1 work together to control circadian clock function and reproductive timing in plants.Circadian rhythms align biological processes with environmental cycles to enhance fitness. Eukaryotic circadian oscillators rely on transcriptional-translational feedback loops, although specific oscillator components vary among lineages. The plant oscillator consists of a uniquely complex network of transcriptional regulators. These clock genes regulate each other's expression and influence key developmental processes such as the transition from vegetative to reproductive growth. Here, we investigate the function of XAP5 CIRCADIAN TIMEKEEPER (XCT), a protein conserved across eukaryotes and previously implicated in RNA processing and circadian regulation in <i>Arabidopsis thaliana</i>. We show that XCT genetically and physically interacts with CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), an E3 2026-01-01T00:00:00Z20262026-07-16T12:54:30.195Z2026-07-10T03:09:14.254ZS-EPMC127890284152272010.1038/s44323-025-00057-x