Water stress enhances expression of an alpha-amylase gene in barley leaves.

The amylases of the second leaves of barley seedlings (Hordeum vulgare L. cv Betzes) were resolved into eight isozymes by isoelectric focusing, seven of which were beta-amylase and the other, alpha-amylase. The alpha-amylase had the same isoelectric point as one of the gibberellin-induced alpha-amylase isozymes in the aleurone layer. This and other enzyme characteristics indicated that the leaf isozyme corresponded to the type A aleurone alpha-amylase (low pI group). Crossing experiments indicated that leaf and type A aleurone isozymes resulted from expression of the same genes.In unwatered seedlings, leaf alpha-amylase increased as leaf water potential decreased and ABA increased. Water stress had no effect on beta-amylase. alpha-Amylase occurred uniformly along the length of the leaf but beta-amylase was concentrated in the basal half of the leaf. Cell fractionation studies indicated that none of the leaf alpha-amylase occurred inside chloroplasts.Leaf radiolabeling experiments followed by extraction of alpha-amylase by affinity chromatography and immunoprecipitation showed that increase of alpha-amylase activity involved synthesis of the enzyme. However, water stress caused no major change in total protein synthesis. Hybridization of a radiolabeled alpha-amylase-related cDNA clone to size fractionated RNA showed that water-stressed leaves contained much more alpha-amylase mRNA than unstressed plants. The results of these and other studies indicate that regulation of gene expression may be a component in water-stress induced metabolic changes.