Phytochrome mediates the external light signal to repress <i>FT</i> orthologs in photoperiodic flowering of rice

TLDR

Phytochromes mediate photoperiodic control of flowering in the short‑day rice plant Oryza sativa. The study aimed to elucidate how phytochrome signals interact with circadian clocks to enable day‑length recognition in rice, and to propose a model for this photoperiodic response. The authors examined phytochrome–circadian interactions in rice mutants, identified downstream floral switch genes (rice FT orthologs), and suggested that SE1 transcriptional activity is posttranscriptionally regulated in coordination with Pfr phytochromes. They found that circadian phase remains unchanged in a phytochrome‑deficient early‑flowering mutant under both SD and LD, that SE1 acts as a circadian output with diurnal expression unaffected by phytochrome loss, and that phytochrome signals repress FT ortholog expression while SE1 can both promote and suppress FT expression depending on day length.

Abstract

Phytochromes confer the photoperiodic control of flowering in rice ( Oryza sativa ), a short-day plant. To better understand the molecular mechanisms of day-length recognition, we examined the interaction between phytochrome signals and circadian clocks in photoperiodic-flowering mutants of rice. Monitoring behaviors of circadian clocks revealed that phase setting of circadian clocks is not affected either under short-day (SD) or under long-day (LD) conditions in a phytochrome-deficient mutant that shows an early-flowering phenotype with no photoperiodic response. Non-24-hr-light/dark-cycle experiments revealed that a rice counterpart gene of Arabidopsis CONSTANS ( CO ), named PHOTOPERIOD SENSITIVITY 1 ( Heading date 1 ) [ SE1 ( Hd1 )], functions as an output of circadian clocks. In addition, the phytochrome deficiency does not affect the diurnal mRNA expression of SE1 upon floral transition. Downstream floral switch genes were further identified with rice orthologs of Arabidopsis FLOWERING LOCUS T ( FT ). Our RT-PCR data indicate that phytochrome signals repress mRNA expression of FT orthologs, whereas SE1 can function to promote and suppress mRNA expression of the FT orthologs under SD and LD, respectively. This SE1 transcriptional activity may be posttranscriptionally regulated and may depend on the coincidence with Pfr phytochromes. We propose a model to explain how a short-day plant recognizes the day length in photoperiodic flowering.