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Plant Cell Advance Online Publication
Published on December 12, 2008; 10.1105/tpc.108.063487

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Received September 30, 2008
Returned for revision November 17, 2008
Accepted November 24, 2008

Starch Granule Biosynthesis in Arabidopsis Is Abolished by Removal of All Debranching Enzymes but Restored by the Subsequent Removal of an Endoamylase

Sebastian Streb 1, Thierry Delatte 1, Martin Umhang 1, Simona Eicke 1, Martine Schorderet 2, Didier Reinhardt 2, and Samuel C. Zeeman 1*

1 Institute of Plant Sciences, ETH Zurich, CH-8092 Zurich, Switzerland
2 Department of Plant Biology, University of Fribourg, CH-1700 Fribourg, Switzerland

* To whom correspondence should be addressed. E-mail: szeeman{at}ethz.ch.

Several studies have suggested that debranching enzymes (DBEs) are involved in the biosynthesis of amylopectin, the major constituent of starch granules. Our systematic analysis of all DBE mutants of Arabidopsis thaliana demonstrates that when any DBE activity remains, starch granules are still synthesized, albeit with altered amylopectin structure. Quadruple mutants lacking all four DBE proteins (Isoamylase1 [ISA1], ISA2, and ISA3, and Limit-Dextrinase) are devoid of starch granules and instead accumulate highly branched glucans, distinct from amylopectin and from previously described phytoglycogen. A fraction of these glucans are present as discrete, insoluble, nanometer-scale particles, but the structure and properties of this material are radically altered compared with wild-type amylopectin. Superficially, these data support the hypothesis that debranching is required for amylopectin synthesis. However, our analyses show that soluble glucans in the quadruple DBE mutant are degraded by {alpha}- and {beta}-amylases during periods of net accumulation, giving rise to maltose and branched malto-oligosaccharides. The additional loss of the chloroplastic {alpha}-amylase AMY3 partially reverts the phenotype of the quadruple DBE mutant, restoring starch granule biosynthesis. We propose that DBEs function in normal amylopectin synthesis by promoting amylopectin crystallization but conclude that they are not mandatory for starch granule synthesis.




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[Abstract] [Full Text] [PDF]


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