Starch breakdown: recent discoveries suggest distinct pathways and novel mechanisms
Samuel C. Zeeman A B , Thierry Delatte A , Gaëlle Messerli A , Martin Umhang A , Michaela Stettler A , Tabea Mettler A , Sebastian Streb A , Heike Reinhold A and Oliver Kötting AA Institute of Plant Sciences, ETH Zurich, Universitätstrasse 2, CH-8092 Zurich, Switzerland.
B Corresponding author. Email: szeeman@ethz.ch
C This paper originates from a presentation at the 8th International Congress of Plant Molecular Biology, Adelaide, Australia, August 2006.
Functional Plant Biology 34(6) 465-473 https://doi.org/10.1071/FP06313
Submitted: 28 November 2006 Accepted: 7 February 2007 Published: 1 June 2007
Abstract
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 α-amylase, limit dextrinase (debranching enzyme), β-amylase and α-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 α-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 α-amylase and limit-dextrinase play minor roles and are dispensable for starch breakdown in leaves. Current data also casts doubt on the involvement of α-glucosidase. In contrast, several lines of evidence point towards a major role for β-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.
Additional keywords: amylase, amylopectin, Arabidopsis thaliana L., carbohydrate metabolism, cereal endosperm, metabolic regulation.
Acknowledgements
We offer particular thanks to our close collaborators, Professor Alison M. Smith and Professor Steven M. Smith, and to our colleagues in the Arabidopsis Starch Metabolism Network (www.starchmetnet.org, accessed 17 March 2007). We also thank Martine Trevisan and Simona Eicke for excellent technical support, past and present, in our laboratory. Our research is funded by the Roche Research Foundation, the Swiss National Science Foundation (Nation Centre for Competence in Research – Plant Survival and Grant 3100–067312.01/1), and the EMBO Young Investigator Programme.
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