UV‐B Irradiation Does Not Promote Flowering in Arabidopsis Despite Increased FT Expression

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Various environmental factors control the plant flowering time. However, the specific effects of ultraviolet (UV)‐B radiation on flowering remain unclear. UV‐B irradiation delays flowering in Arabidopsis during short‐day (SD) photoperiods. In contrast, UV‐B irradiation causes a variety of flowering phenotypes during long‐day (LD) photoperiods, including unchanged, delayed, and accelerated flowering. We hypothesized that variations in UV‐B intensity are responsible for the phenotypic changes under LD photoperiods. Therefore, in this study, Arabidopsis plants were exposed to two distinct UV‐B intensities: a low UV‐B intensity that activates UVR8‐dependent pathways and high UV‐B intensity that activates both UVR8‐dependent and ‐independent pathways. Under LD photoperiods, neither the wild‐type (WT) nor the uvr8 mutant showed any change in flowering time at either UV‐B irradiation intensity. Under the SD photoperiod, UV‐B irradiation delayed WT flowering. The expression of flowering locus T ( FT ) increased after UV‐B irradiation under the LD photoperiod in a UVR8‐dependent manner. However, despite the increased expression of FT , expression levels of floral meristem identity genes in shoot apical meristem (SAM) were not increased by UV‐B irradiation. As UV‐B irradiation is known to suppress flowering in SAM in a UVR8‐dependent manner, increase in FT expression induced by UV‐B irradiation possibly antagonized the suppressive effect of UV‐B irradiation. Overall, these results suggest that flowering phenotypes do not change with UV‐B intensity but with the balance between the inhibitory and promotive effects of UVR8 activated by UV‐B irradiation.