| Circadian clocks, present in nearly every type of eukaryote, evolved multiple times to allow for prediction of rhythmic environmental changes. These clocks are internal time keepers, consisting of molecular feedback loops, which confer a near 24 hour rhythm to many physiological outputs in order to coordinate growth and development with the external environment. In order to better understand the molecular constituents of the plant circadian clock, we mutagenized and screened Arabidopsis plants for altered circadian rhythms and isolated ten distinct mutant lines. These lines have defined five new alleles of genes known to be involved in the circadian clock: three new alleles of ZTL, one new allele of LHY, and one new allele of GI. We show GI acts in blue light signaling to the clock, has biochemically separable roles in clock and flowering pathways, and may function in parallel to TOC 1 within the core oscillator. Additionally, we have isolated XCT, a novel gene involved in light input to the clock and photomorphogenesis, which displays opposite phenotypic traits depending on light quality. XCT acts upstream of many light regulated processes, coordinating red and blue light input signals to these pathways. XCT may serve a role in red light signaling in parallel to PHYB, one of the major red light signaling components, and may act within the CRY2 mediated blue light signaling pathway. Additionally, XCT may function within the ELF3/ELF4 feedback arm of the core oscillator. XCT contains a XAP5 domain, a conserved sequence of unknown function present in XCT homologues in a broad range of eukaryotes. This sequence similarity suggests XCT function or interacting protein partners may also be conserved across varied taxa. |
