Kana Miyata

Circadian Clock Proteins LHY and CCA1 Regulate SVP Protein Accumulation to Control Flowering in Arabidopsis

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.

Possible role of EARLY FLOWERING 3 (ELF3) in clock‐dependent floral regulation by SHORT VEGETATIVE PHASE (SVP) in Arabidopsis thaliana

Circadian clock proteins play key roles in adaptations of plants to diurnal environmental conditions. The photoperiodic flowering response is one of the mechanisms of adaptation to seasonal changes in the lengths of day and night. Double mutations in two clock genes, late elongated hypocotyl (LHY) and circadian clock associated 1 (CCA1), accelerated flowering under short days (SDs) but delayed flowering under continuous light (LL) in Arabidopsis thaliana. The mechanism underlying the late flowering of lhy;cca1 mutants under LL was investigated here. Late flowering of plants with overexpression of short vegetative phase (SVP) was much more pronounced under SDs and enhanced by constans 2 (co-2) under long days (LDs), suggesting that SVP and CO act independently in the photoperiodic flowering pathway. However, how SVP and flowering locus C (FLC) mediated the effects of LHY/CCA1 and thus influenced flowering time was not completely clear. A mutant line lhy;cca1 in the Landsberg erecta (Ler) background was established, ethyl methanesulfonate (EMS)-mutagenized and used to screen suppressors of late flowering of lhy;cca1 under LL. Mutations in the clock gene early flowering 3 (ELF3) were identified as suppressors. Overexpression and loss-of-function of ELF3 influenced SVP protein accumulation. Therefore, we propose that, as well as the classical GIGANTEA (GI)-CO pathway, LHY/CCA1 regulates a pathway negatively controlling flowering locus T (FT), possibly via ELF3-SVP/FLC.

Chapter 5 Roles of Circadian Clock and Histone Methylation in the Control of Floral Repressors

Abstract The flowering of Arabidopsis is controlled by several signaling pathways that converge on a small set of floral activator genes (e.g., FT, SOC1 , and LFY ) that function as pathway integrators. Both floral activators and repressors play key roles in controlling flowering time. Temporal balance between floral repressor and promoter activity both daily and seasonally is crucial in helping plants determine when to flower. This review summarizes recent progress on understanding interactions between floral repressors and activators. The possible roles of the circadian clock and histone methylation in the control of floral repressors are mainly discussed.

Suppression of late-flowering and semi-dwarf phenotypes in the Arabidopsis clock mutant lhy-12;cca1-101 by phyB under continuous light.

Photoperiodic flowering in Arabidopsis is controlled not only by floral activators such as GI, CO, and FT, but also by repressors such as SVP and FLC. Double mutations in LHY and CCA1 (lhy;cca1) accelerated flowering under short days, mainly by the GI-CO dependent pathway. In contrast, lhy;cca1 showed delayed flowering under continuous light (LL), probably due to the GI-CO independent pathway. This late-flowering phenotype was suppressed by svp, flc, and elf3. However, how SVP, FLC, and ELF3 mediate LHY/CCA1 and flowering time is not fully understood. We found that lhy;cca1 exhibited short hypocotyls and petioles under LL, but the molecular mechanism for these effects has not been elucidated. To address these questions, we performed a screen for mutations that suppress either or both of the lhy;cca1 phenotypes under LL, using two different approaches. We identified two novel mutations, a dominant (del1) and a recessive (phyB-2511) allele of phyB. The flowering times of single mutants of three phyB alleles, hy3-1, del1, and phyB-2511, are almost the same and earlier than those of wild-type plants. A similar level of acceleration of flowering time was observed in all three phyB mutants tested when combined with the late-flowering mutations co-2 and SVPox. However, the effect of phyB-2511 on lhy;cca1 was different from those by hy3-1 or del1. svp-3 did not strongly enhance the early-flowering phenotypes of phyB-2511 or del1. These results suggest that light signaling via PhyB may affect factors downstream of the clock proteins, controlling flowering time and organ elongation. phyB mutations with different levels of effects on lhy;cca1-dependent late flowering would be useful to determine a specific role for PHYB in the flowering pathway controlled by lhy;cca1 under LL.

Roles of Circadian Clock in Developmental Controls and Stress Responses in Arabidopsis: Exploring a Link for Three Components of Clock Function in Arabidopsis

Genetic analysis of the early flowering 3 (elf3) mutant of Arabidopsis thaliana indicates that ELF3 plays key roles in the regulation of plant morphology, flowering time and stress response, all of which are controlled by circadian clock. Although ELF3 appears to have multiple functions and has been shown to interact physically with the photoreceptor phyB, its ability to regulate several distinct signalling pathways has not been elucidated. This lack of information is attributable in part to the uniqueness of the ELF3 gene, which encodes a novel nuclear protein with no significant sequence similarity to any characterized protein in the existing public databases. Further, little is known about direct protein–protein interactions of ELF3, or about mutations that suppress elf3, phenotypes. Therefore, it is difficult to hypothesize about potential factors downstream of ELF3. In this chapter, we summarize recent progress on the characterization of ELF3 and discuss potential roles of ELF3 in plants. Several reports have demonstrated that a circadian clock affects stress responses in Arabidopsis and that DREB1A/CBF3 mediates between the clock and cold-inducible gene expression. Therefore, possible roles of clock genes such as ELF3, PRRs, LHY and CCA1 in the environmental stress responses of Arabidopsis are also discussed.