The UV-B photoreceptor UVR8 promotes photosynthetic efficiency in Arabidopsis thaliana exposed to elevated levels of UV-B
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The UV-B photoreceptor UVR8 regulates expression of genes in response to UV-B, some encoding chloroplast proteins, but the importance of UVR8 in maintaining photosynthetic competence is unknown. The maximum quantum yield of PSII (F v/F m) and the operating efficiency of PSII (Φ PSII) were measured in wild-type and uvr8 mutant Arabidopsis thaliana. The importance of specific UVR8-regulated genes in maintaining photosynthetic competence was examined using mutants. Both F v/F m and Φ PSII decreased when plants were exposed to elevated UV-B, in general more so in uvr8 mutant plants than wild-type. UV-B increased the level of psbD-BLRP (blue light responsive promoter) transcripts, encoding the PSII D2 protein. This increase was mediated by the UVR8-regulated chloroplast RNA polymerase sigma factor SIG5, but SIG5 was not required to maintain photosynthetic efficiency at elevated UV-B. Levels of the D1 protein of PSII decreased markedly when plants were exposed to elevated UV-B, but there was no significant difference between wild-type and uvr8 under conditions where the mutant showed increased photoinhibition. The results show that UVR8 promotes photosynthetic efficiency at elevated levels of UV-B. Loss of the DI polypeptide is probably important in causing photoinhibition, but does not entirely explain the reduced photosynthetic efficiency of the uvr8 mutant compared to wild-type.
KeywordsArabidopsis ELIP PSII SIG5 UV-B UVR8
G.I.J thanks the UK Biotechnology and Biological Sciences Research Council, The Leverhulme Trust and the University of Glasgow for supporting his research on UVR8. N.S. was supported by Satya Wacana Christian University, Indonesia and The Ministry of Education of Indonesia. J.J.W and N.D.P thank the UK Agricultural and Horticultural Development Board for supporting their research with a studentship to J.J.W. We are grateful to Drs Takashi Shina, Kan Tanaka and Carlo Soave for providing seeds of mutants and to Dr Bobby Brown for valuable comments on the RT-PCR experiments.