T-DNA tagging with a promoterless .beta.-glucuronidase (GUS) gene generated a transgenic Nicotiana tabacum plant that expressed GUS activity only in developing seed coats. Cloning and deletion analysis of the GUS fusion revealed that the promoter responsible for seed coat specificity was located in the plant DNA proximal to the GUS gene. Analysis of the region demonstrated that the seed coat-specificity of GUS expression in this transgenic plant resulted from T-DNA insertion next to a cryptic promoter. This promotor is useful in controlling the expression of genes to the developing seed coat in plant seeds.