A POSSIBLE TYROSINE PHOSPHORYLATION OF PHYTOCHROME

Red/far-red light signal transduction by the phytochrome family of pho toreceptors regulates plant growth and development. We investigated th e possibility that tyrosine kinases and/or phosphatases are involved i n phytochrome-mediated signal transduction using crude extracts of oat seedlings that are grown in the dark. We found that a 124 kDa protein was tyrosine-phosphorylated as determined by Western blotting with a phosphotyrosine-specific monoclonal antibody. The 124 kDa protein was recognized by the anti-phosphotyrosine antibody in anti-phytochrome A immunoprecipitates. The level of anti-phosphotyrosine antibody binding to the 124 kDa protein(s) in phytochrome immunoprecipitates that had been treated with red light prior to immunoprecipitation decreased rel ative to dark controls. These results suggest that either phytochrome from dark-grown seedlings is tyrosine phosphorylated or that it co-imm unoprecipitates with a phosphotyrosine-containing protein of the same molecular weight. The implications of these results in the regulation of (a) the putative Ser/Thr kinase activity of the photoreceptor and ( b) the binding of signaling molecules, such as phospholipase C to phyt ochrome, are discussed.