First published online May 20, 2005; 10.1104/pp.105.060319
Plant Physiology 138:663-674 (2005)
© 2005 American Society of Plant Biologists
BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES
Mutations Affecting Starch Synthase III in Arabidopsis Alter Leaf Starch Structure and Increase the Rate of Starch Synthesis1
Xiaoli Zhang,
Alan M. Myers and
Martha G. James*
Department of Biochemistry, Biophysics, and Molecular Biology, Iowa State University, Ames, Iowa 50011
The role of starch synthase (SS) III (SSIII) in the synthesis of transient starch in Arabidopsis (Arabidopsis thaliana) was investigated by characterizing the effects of two insertion mutations at the AtSS3 gene locus. Both mutations, termed Atss3-1 and Atss3-2, condition complete loss of SSIII activity and prevent normal gene expression at both the mRNA and protein levels. The mutations cause a starch excess phenotype in leaves during the light period of the growth cycle due to an apparent increase in the rate of starch synthesis. In addition, both mutations alter the physical structure of leaf starch. Significant increases were noted in the mutants in the frequency of linear chains in amylopectin with a degree of polymerization greater than approximately 60, and relatively small changes were observed in chains of degree of polymerization 4 to 50. Furthermore, starch in the Atss3-1 and Atss3-2 mutants has a higher phosphate content, approximately two times that of wild-type leaf starch. Total SS activity is increased in both Atss3 mutants and a specific SS activity appears to be up-regulated. The data indicate that, in addition to its expected direct role in starch assembly, SSIII also has a negative regulatory function in the biosynthesis of transient starch in Arabidopsis.
1 This work was supported by an award from the National Science Foundation (grant no. DBI 0209789 to A.M.M. and M.G.J.).
Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.060319.
* Corresponding author; e-mail mgjames{at}iastate.edu; fax 5152940453.
Received January 31, 2005;
returned for revision April 1, 2005;
accepted April 5, 2005.
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