GIBBERELLIN-REPRESSIBLE GENE-EXPRESSION IN THE BARLEY ALEURONE LAYER

Gibberellins are noted for their ability to induce expression of genes , such as alpha-amylase, in the aleurone layers of cereals. However, a number of mRNA species in the mature imbibed aleurone cell of barley, such as a storage globulin (Heck et nl., Mol Gen Genet 239: 209-218 1 993), are simultaneously and specifically repressed by gibberellin. In a continuing effort to understand this effect, we report cloning and characterization of two additional cDNAs from barley designated pHvGS- 1 and pcHth3 that have high corresponding mRNA levels in the mature im bibed aleurone but are repressed 10-fold or more within 24 h of treatm ent with gibberellic acid (GA(3)). The extent of repression was concen tration dependent and maximally effective at 10(-6) M GA(3). Repressio n was also noted in the constitutive gibberellin response mutant, slen der, in the absence of exogenous GA(3). The antagonistic phytohormone, abscisic acid, had no effect or was weakly inductive of the steady-st ate levels of these mRNAs. During development of the seed, repressible mRNAs are present to different degrees in the maturing aleurone layer and embryo, but not in the starchy endosperm. Some repressible mRNA p ersists in the mature dry aleurone layer, but is degraded during imbib ition, replenished by de novo transcription, and maintained at high st eady-state levels until GA(3) is perceived. Preliminary investigation suggests that repression is at least partly due to destabilization of the mRNAs which have estimated half-lives of 12 h or greater in the ab sence of GA(3). pcHth3 encodes a member of the gamma-thionin gene fami ly located on chromosome 7. pHvGS-1 corresponds to a gene on chromosom e 3 of unknown function.