Molecular Genetic Analysis of Flowering in Arabidopsis and Tomato
Authors: Wagner D. Ry , Lifschitz Eliezer , Kay Steve A. 
Published: December 2021

The primary objectives for the US lab included: the characterization of ELF3 transcription and translation; the creation and characterization of various transgenic lines that misexpress ELF3; defining genetic pathways related to ELF3 function regulating floral initiation in Arabidopsis; and the identification of genes that either interact with or are regulated by ELF3. Light quality, photoperiod, and temperature often act as important and, for some species, essential environmental cues for the initiation of flowering. However, there is relatively little information on the molecular mechanisms that directly regulate the developmental pathway from the reception of the inductive light signals to the onset of flowering and the initiation of floral meristems. The ELF3 gene was identified as possibly having a role in light-mediated floral regulation since elj3 mutants not only flower early, but exhibit light-dependent circadian defects. We began investigating ELF3's role in light signalling and flowering by cloning the ELF3 gene. ELF3 is a novel gene only present in plant species; however, there is an ELF3 homolog within Arabidopsis. The Arabidopsis elj3 mutation causes arrhythmic circadian output in continuous light; however, we show conclusively normal circadian function with no alteration of period length in elj3 mutants in dark conditions and that the light-dependent arrhythmia observed in elj3 mutants is pleiotropic on multiple outputs regardless of phase. Plants overexpressing ELF3 have an increased period length in constant light and flower late in long-days; furthermore, etiolated ELF3-overexpressing seedlings exhibit a decreased acute CAB2 response after a red light pulse, whereas the null mutant is hypersensitive to acute induction. This finding suggests that ELF3 negatively regulates light input to both the clock and its outputs. To determine whether ELF3's action is phase dependent, we examined clock resetting by light pulses and constructed phase response curves. Absence of ELF3 activity causes a significant alteration of the phase response curve during the subjective night, and overexpression of ELF3 results in decreased sensitivity to the resetting stimulus, suggesting that ELF3 antagonizes light input to the clock during the night. Indeed, the ELF3 protein interacts with the photoreceptor PHYB in the yeast two-hybrid assay and in vitro. The phase ofELF3 function correlates with its peak expression levels of transcript and protein in the subjective night. ELF3 action, therefore, represents a mechanism by which the oscillator modulates light resetting. Furthermore, flowering time is dependent upon proper expression ofELF3. Scientifically, we've made a big leap in the understanding of the circadian system and how it is coupled so tightly with light reception in terms of period length and clock resetting. Agriculturally, understanding more about the way in which the clock perceives and relays temporal information to pathways such as those involved in the floral transition can lead to increased crop yields by enabling plants to be grown in suboptimal conditions.