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

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.