Starch breakdown: recent discoveries suggest distinct pathways and novel mechanisms

The aim of this article is to provide an overview of current models of starch breakdown in leaves. We summarise the results of our recent work focusing on Arabidopsis, relating them to other work in the field. Early biochemical studies of starch containing tissues identified numerous enzymes capable of participating in starch degradation. In the non-living endosperms of germinated cereal seeds, starch breakdown proceeds by the combined actions of alpha-amylase, limit dextrinase (debranching enzyme), beta-amylase and alpha-glucosidase. The activities of these enzymes and the regulation of some of the respective genes on germination have been extensively studied. In living plant cells, additional enzymes are present, such as alpha-glucan phosphorylase and disproportionating enzyme, and the major pathway of starch breakdown appears to differ from that in the cereal endosperm in some important aspects. For example, reverse-genetic studies of Arabidopsis show that alpha-amylase and limit-dextrinase play minor roles and are dispensable for starch breakdown in leaves. Current data also casts doubt on the involvement of alpha-glucosidase. In contrast, several lines of evidence point towards a major role for beta-amylase in leaves, which functions together with disproportionating enzyme and isoamylase (debranching enzyme) to produce maltose and glucose. Furthermore, the characterisation of Arabidopsis mutants with elevated leaf starch has contributed to the discovery of previously unknown proteins and metabolic steps in the pathway. In particular, it is now apparent that glucan phosphorylation is required for normal rates of starch mobilisation to occur, although a detailed understanding of this step is still lacking. We use this review to give a background to some of the classical genetic mutants that have contributed to our current knowledge.