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

It is generally accepted that specific recognition between plant hormo nes 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 chara cterisation of hormone-binding proteins which perform these functions is an important component of research that aims to understand plant ho rmone action. In this report the development of photoaffinity reagents for gibberellin (GA)-binding proteins is reviewed, and their use as p robes with which to identify and characterise GA-binding proteins in A vena fatua aleurone is described. In vivo GA-photoaffinity labelling o f aleurone layers, using the new photoaffinity probe GA(4)-17-sulfoxye thyl-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 -1'-(1'-thia)propan-3'-ol-4-azido-5 -iodosalicylate compete for photoaffinity labelling of a 60 kDa aleuro ne polypeptide, while the inactive GA(8) does not. These GA-photoaffin ity labelling characteristics suggest that the 60 kDa aleurone polypep tide may interact specifically with active GAs. This is the first repo rt to identify a specific GA-binding protein in aleurone. We suggest t hat this specific interaction observed in vivo, under conditions where the aleurone cells are responding to the GA-photoaffinity probe by ex pressing alpha-amylase genes, may be of significance in the perception and action of GA in this tissue.