Analyses of isoamylase gene activity in wild-type barley indicate its involvement in starch synthesis

The notion of debranching enzyme activity as a participant in starch synthe sis is gaining acceptance. Inconsistent reports from mutant analyses implic ate either isoamylase or pullulanase as a determinant in amylopectin format ion and whether wild-type plants utilize one or the other, or both, of thes e debranching enzymes in starch synthesis is unclear. Recent results on the su1 mutant in maize suggest that both forms of debranching enzymes might b e involved in amylopectin formation. We wished to find out if isoamylase ta kes part in starch synthesis by comparing isoamylase gene activity under th ree conditions: (1) during starch accumulation in developing sink tissues; (2) during starch degradation in germinating seeds; (3) in ectopic expressi on after applying sucrose, a starch precursor. We isolated the gene for bar ley isoamylase, iso1, and analysed its expression and regulation in germina ting seeds, developing endosperm and vegetative tissues, and compared the i soamylase gene expression in sink tissues from three different species. Our results indicate that isoamylase gene activity is involved in starch synth esis in wild-type plants and is modulated by sucrose.