In transgenic tobacco (Nicotiana tabacum L.) seed, expression of chimeric [beta]-glucuronidase (GUS) genes containing the vicilin or napin storage-protein gene promoters is responsive to premature drying and declines upon rehydration (L. Jiang, W.L. Downing, C.L. Baszczynski, A.R. Kermode [1995] Plant Physiol 107: 1439-1449). Desiccation may cause changes in the content of or sensitivity to abscisic acid (ABA), partially or wholly removing the effects of this "modulator" of developmental gene expression. We studied the enhancement of GUS reporter enzyme activities in transgenic tobacco by exogenous ABA before and after drying. A racemic mixture of ABA at 10 [mu]M caused a 2- to 3-fold increase in GUS activity in developing transgenic seed expressing chimeric genes containing the vicilin or napin gene promoters. However, when these seeds were prematurely dried prior to their culture on ABA medium, enhancement of GUS activity was virtually abolished. Use of optically pure ABAs revealed that the enhancement in GUS activity in developing fresh seed was due largely to the natural (+) form of ABA. Chimeric constructs containing a viral 35S promoter did not respond to ABA whether or not premature drying was applied. Thus, vicilin and napin chimeric genes, initially sensitive to ABA, become relatively insensitive to the hormone following drying. A decline in ABA sensitivity may be an important factor in the cessation of storage-protein gene expression.