The photoperiodic response is one of the adaptation mechanisms to seasonal changes of lengths of day and night. The circadian clock plays pivotal roles in this process. In Arabidopsis, LHY, CCA1, ELF3, and other clock proteins play major roles in maintaining circadian rhythms. lhy;cca1 double mutants with severe defects in circadian rhythms showed accelerated flowering under short days (SDs), but delayed flowering under continuous light (LL). The protein level of the floral repressor SVP increased in lhy;cca1 mutants under LL, and the late-flowering phenotype of lhy;cca1 mutants was partially suppressed by svp, flc, or elf3. ELF3 interacted with both CCA1 and SVP, and elf3 suppressed the SVP accumulation in lhy;cca1 under LL. These results suggest that the unique mechanism of the inversion of the flowering response of lhy;cca1 under LL may involve both the ELF3-SVP/FLC-dependent and -independent pathways. In this work, elf3-1 seeds were mutagenized with heavy-ion beams and used to identify mutation(s) that delayed flowering under LL but not long days (LDs) or SDs even without ELF3. In this screening, seven candidate lines named suppressor of elf3 1 (self1), sel3, sel5, sel7, sel14, sel15, and sel20 were identified. Genetic analysis indicated that sel20 was a new deletion allele of a mutation in the blue light receptor, CRY2. A late-flowering phenotype and decrease of FT expression in the elf3;sel20 double mutant was obvious under LL but not under SDs or LDs. These results indicated that the late-flowering phenotype in the double mutant elf3;sel20 as well as in lhy;cca1 was affected by the presence of darkness. The results suggest that CRY2 may play more essential roles in the acceleration of flowering under LL than LDs or SDs.

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Many environmental and endogenous factors affect flowering time of plants. In Arabidopsis, there are three major pathways for the control of flowering time; the photoperiod or long-day (LD) pathway, autonomous/vernalization pathway and gibberellic acid (GA) pathway. The flowering regulation under most common photoperiods in Arabidopsis involves some floral activators such as CONSTANS (CO) and GIGANTEA (GI) and a circadian clock protein, EARLY FLOWERING 3 (ELF3). In this work, we examined the effect of ultra-short photoperiod (USP) on flowering time of three accessions (Ler, Col and Ws). All the wild-type plants tested showed remarkable delay of flowering under 3 h or less photoperiods, but different sensitivity to the USP was found in these accessions. Late flowering phenotype of plants with mutations in genes involved in the three major pathways was enhanced under the USP. Expression of FLOWERING LOCUS T (FT) encoding a florigen decreased under the USP. A mutation in the ELF3 gene in Ler largely suppressed the delay of flowering under the USP. These results suggest that floral regulation pathway under the USP may be independent of the three well-characterized pathways. ELF3 may play a key role in the USP pathway.

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