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.