Reduction of wheat endosperm starch phosphate content alters grain and growth characteristics

Abstract

Starch is composed of linked glucan residues arranged in branched long chains. The nature of these chains means that starch is a highly insoluble carbohydrate storage form. However, the ability of starch to be degraded depends upon solubilisation of the exposed starch granule surfaces. In order for this to occur, glucan, water dikinase (GWD), and its sister enzyme, phosphoglucan, water dikinase (PWD), act to add net-negatively charged phosphate residues to the C6 and C3 carbons of glucose residues, respectively. This increases peripheral starch granule solubility and other physicochemical properties such as gelatinisation temperature and viscosity. Reduction of GWD in wheat grain endosperm via RNAi has produced grain with these altered physicochemical properties but has also produced unexpected phenotypic modification of grain and plant growth characteristics. This includes increased plant growth and yield, and increased grain size. This work investigates these changes and aims to provide some explanation for the pleiotropic alterations. Physiological measurements of grain characteristics are performed, as are biochemical analysis of starch quantity and properties. Starch degradation enzyme quantity and starch degradation rates are measured, as are soluble sugars. Germination and growth is characterised, and hormone quantification of grain, embryo and young shoots are also performed. Here it is reported that reduced wheat endosperm starch phosphate content, produced by RNAi repression of GWD in this tissue only, has resulted in increased grain and embryo size, altered starch branching patterns, and reduced grain hardness. Starches exhibit increased branching rates but no change in starch proportion of grain weight. Germination is delayed in these lines, but coleoptile growth potential is increased. Starch in vivo degradation rates are unchanged and starch hydrolytic enzyme activity is lower post germination that in controls. Hormone levels are modified in some tissue types, but no significant changes in soluble sugar evolution are found. A hypothetical mechanism of action is suggested to explain these changes involving modification of starch and oligosaccharide capacity to alter a central regulatory system, the sucrose non-fermenting related kinase regulatory complex. While circumstantial evidence is available to support in some way this suggestion, further detailed work is required to provide sufficient evidence of its involvement. Show more