PhD Candidate University of Washington Seattle, Washington
Body of Abstract: Arabidopsis times flowering to coincide with optimal environmental conditions through the modulation of FLOWERING LOCUS T (FT) gene expression. Previously, FT expression responsible for flowering was thought to be induced in long-day evenings with the coincidence of light and stabilization of key circadian-clock proteins. Recent work has shown this single dusk-peak pattern of expression to be specific to laboratory conditions in which Arabidopsis are subject to artificial fluorescent lighting, with a red:far-red light ratio of >2.0. When grown instead under natural long-day conditions, with a red:far-red ratio of 1.0, FT instead shows a bimodal pattern of expression, with peaks generated in both mornings and evenings. Though the external coincidence model of FT induction explains the presence of the evening peak, the mechanistic underpinnings of the morning peak, and especially its apparent dependance on supplemental far-red exposure in long-days remained unexplored.
Through a series of circadian gating experiments manipulating the extent of timing of supplemental far-red light, we sought to reconcile the laboratory-limited interpretation of the single-peaked external coincidence model with the reality of dual-peaked FT induction in nature. We found that the induction of the morning FT peak was limited to natural (red:far-red ~1.0) long days and mediated primarily by phytochrome A. Furthermore, the extent of FT induction was dependent on not only the overall period of far-red irradiance, but also demonstrated a heightened sensitivity to far-red light at specific times of the day. These results shed light on crucial details of FT induction in Arabidopsis and ultimately, the timing of flowering in plants by suggesting an additional layer of control – the morning FT peak; determining its primary environmental stimulus – the period of timing of far-red exposure; and finally incorporating a long underacknowledged component of the Arabidopsis flowering pathway – phytochrome A – into our understanding of flowering.