Postdoctoral Research Fellow Academia Sinica Taipei, Taipei, Taiwan (Republic of China)
Body of Abstract: Translation is enhanced by light in de-etiolating seedlings. We previously showed that a pathway involving auxin, target of rapamycin, and ribosomal protein S6 (RPS6) regulates this enhancement. Light also triggers a cascade of phosphorylation of RPS6; however, the biological relevance of multi-phosphorylation of RPS6 in photomorphogenesis remains elusive. The expression of pRPS6A:RPS6A-FLAG in rps6a functionally complemented the developmental defect of the rps6a mutant and allowed the immunoprecipitation of RPS6A-FLAG proteins. Mass spectrometry was employed to identify and quantify the phosphorylation of individual serine or threonine residues of RPS6A in response to light signals. Among the seven C-terminal putative light-responsive phosphorylation sites, serine-229 (S229), S231, and S237 or S240 were the seed sites for sequential phosphorylation. The expression of phospho-null (PN) or phospho-mimic (PM) versions of RPS6A can only partially complement the rps6a phenotypes, including the reduced hypocotyl elongation of etiolated seedlings and light-induced cotyledon opening. Together with the reduced PSII activity, the accumulation of LHCB1 protein in PM plants, and the compromised light-enhanced translation in both PM and PN plants, our data suggest the dynamic phosphorylation of RPS6A enables the full function of RPS6A. Our study provides a comprehensive view of the contribution of the multi-phosphorylation of RPS6A in ensuring a full capacity of photosynthetic activity and the translation of photosynthetic mRNAs for successful de-etiolation.