Starch degradation in leaves

Study of mutants of the model plant Arabidopsis is providing new information about the nature and regulation of starch degradation in leaves. The onlyoc-amylase currently predicted to be in the chloroplast is not necessary for the initial attack on the starch granule, and the enzyme(s) responsible for this attack remain unknown. A starch-water dikinase that phosphorylates glucose residues in amylopectin is necessary for starch degradation, and it seems likely that the enzyme(s) responsible for the initial attack require either the dikinase itself or phosphorylated regions of amylopectin for their activity. At least four chloroplastic enzymes could potentially debranch glucans released by the initial attack on the granule: the relative importance of these enzymes is not yet known. The degradation of linear glucans to monomers is catalysed by β-amylasesrather than starch phosphorylase. Plants lacking chloroplastic starch phosphorylase have normal rates of starch degradation. The fate of the maltose produced by β-amylolysis of linear glucans is being investigated through study of maltose-accumulating mutants. Other malto-oligosaccharides too short to be attacked by β-amylase are metabolised by disproportionating enzyme to produce longer chains susceptible to further attack. The process of starch degradation is subject to strong diurnal regulation. The nature of this regulation is not understood, but new approaches to the problem are suggested.