RADIP LIGHT-INDUCED PHOSPHORYLATION CHANGES IN MICROTUBULE RELATED PROTEINS IN ARABIDOPSIS

Rapid hypocotyl elongation allows buried seedlings to reach the surface, where light triggers de-etiolation and inhibits hypocotyl growth mainly by phytochromes A, B and cryptochromes 1, 2. Dynamic phosphorylation/dephosphorylation events provide a mechanism to rapidly transduce light signals. Only recently we have begun to uncover the earliest phospho-signaling responders to light. Here, we report a large-scale phosphoproteomic analysis and identify 20 proteins that change their phosphorylation pattern after 20 min of white light pulse compared to darkness. Microtubule-associated proteins (MAPs) were highly overrepresented in this group. Among them, we studied CIP7 (COP1-INTERACTING-PROTEIN-7), which presented microtubule (MT) localization, in contrast to what was previously described. Phosphorylated isoform in Serine 915 (Sp915) of CIP7 was detected in etiolated seedlings but undetectable after a light pulse in the presence of photoreceptors, while its expression decays with long light exposure. The short hypocotyl phenotype and rearrangement of MTs in etiolated cip7 mutants are complemented by CIP7-YFP and the phospho-mimetic CIP7S915D-YFP, but not the phospho-null CIP7S915A-YFP suggesting that Sp915CIP7 is the active isoform that promotes hypocotyl elongation thorough MT reorganisation in darkness. Our results reveal that the small repertory of proteins that changes the phosphorylation status after a rapid light signal is tightly focused on MAPs; suggesting that phospho-regulation of microtubule-base processes are early targets during de-etiolation. The evidence on Sp915CIP7 supports this idea.