The floral regulators GIGANTEA (GI), CONSTANS (CO), and FLOWERING LOCUS T (FT) play key roles in the photoperiodic flowering responses of the long-day plant Arabidopsis thaliana. The GI-CO-FT pathway is highly conserved in plants. Here, we demonstrate that the circadian clock proteins LATE ELONGATED HYPOCOTYL (LHY) and CIRCADIAN CLOCK-ASSOCIATED1 (CCA1) not only repressed the floral transition under short-day and long-day conditions but also accelerated flowering when the plants were grown under continuous light (LL). LHY and CCA1 accelerated flowering in LL by promoting FT expression through a genetic pathway that appears to be independent of the canonical photoperiodic pathway involving GI and CO proteins. A genetic screen revealed that the late-flowering phenotype of the lhy;cca1 double mutant under LL was suppressed through mutations in SHORT VEGETATIVE PHASE (SVP), a MADS box transcription factor. Yeast two-hybrid analysis demonstrated an interaction between SVP and FLOWERING LOCUS C, and genetic analysis indicated that these two proteins act as partially redundant repressors of flowering time. SVP protein accumulated in lhy;cca1 plants under LL. We propose a model in which LHY and CCA1 accelerate flowering in part by reducing the abundance of SVP and thereby antagonizing its capacity to repress FT expression under LL.

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LHY and CCA1 play key roles in circadian clock functions and photoperiodic flowering in Arabidopsis. Double loss of function of LHY and CCA1 genes (lhy cca1) accelerated flowering under long days or short days, but the lhy cca1 delayed flowering time under constant-light (LL) conditions. FCA encodes an RNA binding protein that plays key roles in the autonomous pathway. Loss of function of FCA increases mRNA level of a major floral repressor gene, FLC. A mutation in FLC gene partially suppressed the late flowering phenotype of the lhy cca1 in LL. Based on this result, we have proposed that FLC may be involved in this process. Increased level of FLC mRNA in fca is responsible for the delay of flowering and the late flowering phenotype of the fca is suppressed by vernalization. In this paper, we isolated an enhancer of the lateflowering phenotype of the lhy cca1 in LL based on natural variation of two Arabidopsis accessions, Columbia and Landsberg erecata. The enhancer was named ELLCL and mapped near the FLC. This result suggested that the gene responsive to ELLCL might be FLC and was consistent with our previous results. The late-flowering phenotype of lhy cca1 was insensitive to vernalization. By contrast, flowering time of the fca was accelerated by vernalization as reported. These results suggested that posttranslational, but not the transcriptional, regulation of FLC might be involved in this process. Key words: CCA1, circadian rhythms, FLC, LHY, natural variation.Plant Biotechnology 27, 455-461 (2010) Short CommunicationAbbreviations: CCA1, CIRCADIAN CLOCK ASSOCIATED 1; FLC, FLOWERING LOCUS C; LHY, LATE ELONGATED HYPOCOTYL; LD, long-days, LL, continuous light, SD, short-days; SVP, SHORT VEGETATIVE PHASE. This article can be found at http://www.jspcmb.jp/ the late-flowering phenotype of lhy cca1 under LL by both ethyl methanesulfonate (EMS) mutagenesis and genetic analysis. Based on our results, we proposed novel roles of the oscillator components LHY and CCA1 in the control of flowering time through the ELF3-SVP/FLC pathway in Arabidopsis (Mizoguchi and Yoshida 2009).Some mutations in key clock genes resulted in severe defects in clock function and differences between phenotypes of plants with these mutations under conditions of light/dark cycling compared with under LL. These circadian clock mutants include lhy cca1 (Fujiwara et al. 2008;Yoshida et al. 2009), elf3 cry2 (Natsui et al. 2010, and prr9 prr7 prr5 . Petiole length of prr9 prr7 prr5 was longer than that of wild-type plants under LD, but shorter under LL . Both lhy cca1 and elf3 cry2 produced fewer and more leaves than wild-type plants under SD and LL, respectively. Nevertheless, isolation of enhancers and suppressors of these mutants would be important to understand the precise molecular mechanism of inversion in the photoperiodic response type. Therefore, we performed mutagenesis of lhy cca1 seeds with EMS and screening of putative enhancers and suppressors of the clock mutant. Use of natural populations has several advantages for ...