Artifacts in the localization of GUS activity in anothers of petunia transformed with a CaMV 35S‐GUS construct

Summary Blue staining of pollen in transgenic petunia plants containing a β‐glucuronidase gene under the control of a cauliflower mosaic virus 35S promoter, is an artifact caused by the diffusion of reaction products from sites in the another and their dimerization into indigo in pollen. The CaMV 35S promoter is active in various tissues of the another. When pollens from Tradescantia and snapdragon are mixed with transgenic petunia anothers during staining they stain blue, whereas they are colorless when stained alone. In assays for promoter activity in pollen it is essential that pollen is isolated from the another or another section prior to staining for GUS activity, to prevent diffusion artifacts and false staining of pollen.