The FLOWERING LOCUS T gene expression is controlled by high‐irradiance response and external coincidence mechanism in long days in Arabidopsis

Abstract

In natural long days, the florigen gene FLOWERING LOCUS T (FT) shows a bimodal expression pattern with morning and dusk peaks in Arabidopsis. This pattern differs from the one observed in the laboratory, and little is known about underlying mechanisms. A red : farred (R : FR) ratio difference between sunlight and fluorescent light causes this FT pattern mismatch.We showed that bimodal FT expression patterns were induced in a day longer than 14 h with sunlight R : FR (= c. 1) conditions. By circadian gating experiments, we found that cumulative exposure of R : FR-adjusted light (R : FR ratio was adjusted to 1 with FR supplement) spanning from the afternoon to the next morning required full induction of FT in the morning. Conversely, only 2 h of R : FR adjustment in the late afternoon was sufficient for FT induction at dusk.We identified that phytochrome A (phyA) is required for the morning FT expression in response to the R : FR adjustment on the previous day. As a part of this mechanism, we showed that PHYTOCHROME-INTERACTING FACTOR 7 contributes to FT regulation.Our results suggest that phyA-mediated high-irradiance response and the external coincidence mechanism contribute to morning FT induction under natural long-day conditions.

Low temperature‐mediated repression and far‐red light‐mediated induction determine morning FLOWERING LOCUS T expression levels

“…Our findings proposed the existence of an inhibitory complex consisting of ZTL and TOEs that modulates FT expression during the morning in natural LDs (Figure 5). In LD + FR or far‐red light‐enriched conditions, FT transcript levels are much higher throughout the day compared to LDs, with the highest difference observed in the morning (Kim et al, 2008; Wollenberg et al, 2008; Song et al, 2018; Lee et al, 2023). As the levels of morning FT expression under natural LDs are still much higher than those in LDs, even though daily temperature oscillations reduce FT expression levels (Song et al, 2018), we hypothesized that a far‐red light‐mediated mechanism(s) might strongly counteract the activities of FT repressors, including ZTL and TOEs.…”

“…We then investigated the effect of R:FR = 1.0 on ZTL‐induced FT regulation. Under LD + FR conditions, FT mRNA levels in the WT are upregulated throughout the day, with two distinct peaks, resulting in accelerated flowering compared to LD conditions (Song et al, 2018; Lee et al, 2023). Notably, FT mRNA abundance during the daytime under LD + FR conditions was much higher in the ztl‐4 mutant than the WT, with the greatest difference observed in the morning and no apparent change in the evening (Figure 4A).…”

“…The phyA‐mediated high‐irradiance response (HIR) is important for the intensity of the morning FT peak under LD + FR conditions (Lee et al, 2023). Moreover, morning FT induction is absent in the gi‐2 mutant under LD + FR + temp conditions (Song et al, 2018).…”

“…The effect of R:FR = 1.0 on the generation of the morning FT peak is specific to a LD photoperiod, as bimodal expression patterns of FT are observed for day lengths longer than 14 h (Song et al, 2018; Lee et al, 2023). Similarly, in the summer annual accession Columbia (Col‐0), a prominent increase in morning FT induction is seen during the spring months of April and May (Song et al, 2018).…”

“…We also discovered that a R:FR of 1.0 (in sunlight) and daily temperature oscillations are critical environmental factors contributing to this bimodal expression pattern (Song et al, 2018). Moreover, the far‐red light photoreceptor phyA is essential for inducing the morning peak of FT expression (Song et al, 2018; Lee et al, 2023). These findings, along with the observation that red light inhibits FT induction (Valverde et al, 2004), suggest that the current model of floral induction pathways elucidated in laboratory settings may be insufficient to fully recapitulate the mechanisms of seasonal flowering in nature.…”