Grain development mutants of barley - alpha-amylase production during grain maturation and its relation to endogenous gibberellic acid content

Barley (Hordeum vulgare L. Himalaya) mutants with altered grain morphology were isolated to investigate whether defects in grain development, possibly involving gibberellins (GAs) and abscisic acid, would lead to altered patterns of alpha-amylase gene expression. Following treatment with sodium azide, 75 mutants, typically showing grain shriveling, were identified. At grain maturity 15 of the 75 mutants had higher alpha-amylase activities in shriveled grains compared with either phenotypically normal grains that developed on the same heterozygous plant or with grains of cv Himalaya. Studies of four of these mutants demonstrated increased levels of both high-and low-isoelectric point alpha-amylase isozymes midway through grain development. This category of mutant has been designated pga, for premature grain alpha-amylase. One such mutant (M326) showed an endosperm-determined inheritance pattern When crossed into a (GA-deficient) dwarfing background there was a 10-to 20-fold reduction in alpha-amylase activity, suggesting a requirement for CA biosynthesis. Endogenous GAs and abscisic acid were quantified by combined gas chromatography-specific ion monitoring in normal and mutant grains of heterozygous M326 plants during the period of alpha-amylase accumulation. Mutant grains had significantly higher (5.8-fold) levels of the bioactive GA(1) compared with normal grains but much lower (approximately 10-fold) levels of the 2 beta-hydroxylated (''inactive'') GAs, typical of developing barley grains (e.g. GA(8), GA(34), GA(48)). We propose that a reduced extent of Pp-hydroxylation in the mutant grains results in an increased level of GA(I), which is responsible for premature alpha-amylase gene expression.