OsUVR8b, rather than OsUVR8a, plays a predominant role in rice UVR8-mediated UV-B response.

ABSTRACT

UV RESISTANCE LOCUS 8 (UVR8) has been identified in Arabidopsis thaliana as the receptor mediating responses to UV-B radiation. However, UVR8-mediated UV-B signaling pathways in rice, which possesses two proteins (UVR8a and UVR8b) with high identities to AtUVR8, remain largely unknown. Here, UVR8a/b were found to be predominantly expressed in rice leaves and leaf sheaths, while the levels of UVR8b transcript and UVR8b protein were both higher than those of UVR8a. Compared to wild-type (WT) plants, uvr8b and uvr8a uvr8b rice mutants exposed to UV-B showed reduced UV-B-induced growth inhibition and upregulation of CHS and HY5 transcripts alongside UV-B acclimation. However, uvr8a mutant was similar to WT plants and exhibited changes comparable with WT. Overexpressing OsUVR8a/b enhanced UV-B-induced growth inhibition and acclimation to UV-B. UV-B was able to enhance the interaction between E3 ubiquitin ligase OsCOP1 and OsUVR8a/b, whereas the interaction of the homologous protein of Arabidopsis REPRESSOR OF UV-B PHOTOMORPHOGENESIS2 (AtRUP2) in rice with OsUVR8a/b was independent of UV-B. Additionally, OsUVR8a/b proteins were also found in the nucleus and cytoplasm even in the absence of UV-B. The abundance of OsUVR8 monomer showed an invisible change in the leaves of rice seedlings transferred from white light to that supplemented with UV-B, even though UV-B was able to weaken the interactions between OsUVR8a and OsUVR8b homo and heterodimers. Therefore, both OsUVR8a and OsUVR8b, which have different localization and response patterns compared with AtUVR8, function in the response of rice to UV-B radiation, whereas OsUVR8b plays a predominant role in this process.