VICILIN AND NAPIN STORAGE-PROTEIN GENE PROMOTERS ARE RESPONSIVE TO ABSCISIC-ACID IN DEVELOPING TRANSGENIC TOBACCO SEED BUT LOSE SENSITIVITYFOLLOWING PREMATURE DESICCATION

In transgenic tobacco (Nicotiana tabacum L.) seed, expression of chime ric beta-glucuronidase (GUS) genes containing the vicilin or napin sto rage-protein gene promoters is responsive to premature drying and decl ines upon rehydration (L. Jiang, W.L. Downing, C.L. Baszczynski, A.R. Kermode [1995] Plant Physiol 107: 1439-1449). Desiccation may cause ch anges in the content of or sensitivity to abscisic acid (ABA), partial ly or wholly removing the effects of this ''modulator'' of development al gene expression. We studied:the enhancement of GUS reporter enzyme activities in transgenic tobacco by exogenous ABA before and after dry ing. A racemic mixture of ABA at 10 mu M caused a 2- to 3-fold increas e in GUS activity in developing transgenic seed expressing chimeric ge nes containing the vicilin or napin gene promoters. However, when thes e seeds were prematurely dried prior to their culture on ABA medium, e nhancement of GUS activity was virtually abolished. Use of optically p ure ABAs revealed that the enhancement in GUS activity in developing f resh seed was due largely to the natural (+) form of ABA. Chimeric con structs 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 t he hormone following drying. A decline in ABA sensitivity may be an im portant factor in the cessation of storage-protein gene expression.