In situ detection of expression of the gus reporter gene transgenic plants: Ten years of blue genes

Among the methods now available to localize the sites of gene expression in plant materials, reporter genes based on the gus (uidA) gene of Escherichia coli, which encodes a beta-glucuronidase (E.C. 3.2.1.31; GUS), have been the most widely used during the last ten years. The apparent simplicity of the histochemical GUS assay has been a major factor in the increase in articles using gus genes. However, over the last four years, there have been occasional reports expressing doubts concerning the specificity of the observed localizations, based on discrepancies between results obtained with GUS histochemistry and immunocytochemistry and/or in situ hybridization. This brief review compares the results obtained with immunocytochemistry with those obtained with various GUS substrates for histochemical studies. Certain sources of artefact are discussed, as are the limits that should be imposed on interpretation of GUS histochemistry results al the organ, tissue and cell levels.