Regulation of genes encoding β-D-glucan glucohydrolases in barley (Hordeum vulgare)

Expression patterns of barley beta -D-glucan glucohydrolase genes were monitored using cDNAs encoding isoenzymes ExoI and ExoII. The cDNAs were isolated from 5-day-old seedling libraries. The enzymes are encoded by a small gene family, in which marked differences in codon usage are evident. The cDNAs can be used as specific probes for two subfamilies of beta -D-glucan glucohydrolase genes. Genes of both subfamilies are transcribed in the scutellum of germinated grain, in elongating coleoptiles, and in young roots and leaves. Low levels of mRNA for the isoenzyme ExoI gene subfamily could be detected in aleurone layers of germinated grain. Most of the beta -D-glucan glucohydrolase activity can be extracted from tissues with dilute aqueous buffers. Enzyme activity is highest in young leaves and elongating coleoptiles, but is not well-correlated with mRNA levels. The expression patterns are consistent with proposed roles for beta -glucan glucohydrolases in the turnover or modification of cell-wall (1 --> 3,1 --> 4)-beta -D-glucans in elongating coleoptiles; and in young vegetative tissues.