The sulfated peptide CLEL6 is a negative regulator of anthocyanin biosynthesis in Arabidopsis thaliana

Post-translationally modified peptides are now recognized as important regulators of plant stress responses. Here we identified the small sulfated CLE-LIKE6 (CLEL6) peptide as a negative regulator of stress-induced anthocyanin biosynthesis. The expression of CLEL6 and its negative effect on anthocyanin biosynthesis were strongly down-regulated by light. The function of CLEL6 depends on proteolytic processing of the CLEL6 precursor by the subtilisin-like serine proteinase 6.1 (SBT6.1), and on tyrosine sulfation by tyrosylprotein sulfotransferase (TPST). Loss of function mutants of either sbt6.1 or tpst showed significantly higher anthocyanin accumulation upon light stress. The overaccumulation phenotype of sbt6.1 and tpst was suppressed by application of mature CLEL6. Further confirming the role of CLEL6 as an inhibitor of anthocyanin biosynthesis, overexpression and external application of CLEL6 inhibited the expression of genes involved in anthocyanin biosynthesis in etiolated and light-stressed seedlings. Small post-translationally modified peptides are known to be perceived by leucine-rich-repeat receptor like kinases. Through a genetic approach, using a ROOT MERISTEM GROWTH FACTOR 1 INSENSITIVE (RGI) receptor quintuple mutant, we could show the essential function of the RGI receptor family in CLEL6 signaling. Our data indicate that CLEL6 inhibits anthocyanin biosynthesis through RGI receptors in dark-grown seedlings, and that this inhibition is released when CLEL6 expression is down-regulated upon transition to light. One sentence summaryThe formation of CLEL6 as a negative regulator of anthocyanin biosynthesis depends on proteolytic processing by SBT6.1, post-translational modification by TPST, and perception by RGI receptors.