IN-VIVO PHOTOAFFINITY-LABELING OF GIBBERELLIN-BINDING PROTEINS IN AVENA-FATUA ALEURONE

It is generally accepted that specific recognition between plant hormones and proteins involved in their biosynthesis, metabolism, transport and perception are of profound importance in the hormonal regulation of plant growth and development. The identification and detailed characterisation of hormone-binding proteins which perform these functions is an important component of research that aims to understand plant hormone action. In this report the development of photoaffinity reagents for gibberellin (GA)-binding proteins is reviewed, and their use as probes with which to identify and characterise GA-binding proteins in Avena fatua aleurone is described. In vivo GA-photoaffinity labelling of aleurone layers, using the new photoaffinity probe GA(4)-17-sulfoxyethyl-p-azido-[I-125] iodosalicylate, leads to the covalent attachment of this reagent to numerous aleurone polypeptides. Biologically active and inactive GAs used as competitors during in vivo GA-photoaffinity labelling help discriminate between specific and non-specific binding. The biologically active GA(4) and GA(4)-17-yl-1'-(1'-thia)propan-3'-ol-4-azido-5-iodosalicylate compete for photoaffinity labelling of a 60 kDa aleurone polypeptide, while the inactive GA(8) does not. These GA-photoaffinity labelling characteristics suggest that the 60 kDa aleurone polypeptide may interact specifically with active GAs. This is the first report to identify a specific GA-binding protein in aleurone. We suggest that this specific interaction observed in vivo, under conditions where the aleurone cells are responding to the GA-photoaffinity probe by expressing alpha-amylase genes, may be of significance in the perception and action of GA in this tissue.