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Plant and Cell Physiology Advance Access published online on April 22, 2008
Plant and Cell Physiology, doi:10.1093/pcp/pcn066
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© The Author 2008. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Granule-bound starch synthase I is responsible for biosynthesis of extra-long unit-chains of amylopectin in rice.

Isao Hanashiro1, Kimiko Itoh2,3,*, Yuki Kuratomi1, Mina Yamazaki3, Toshinari Igarashi4,**, Jun-ichi Matsugasako1 and Yasuhito Takeda1

1Faculty of Agriculture, Kagoshima University, Kagoshima 890-0065 Japan
2Center for Transdisciiplinary Research Institute, Niigata University, Niigata 950-2181, Japan
3Institute of Science and Technology (Graduate School of Science and Technology), Niigata University, Niigata 950-2181, Japan
4Hokkaido Prefectural Kamikawa Agricultural Experiment Station, Hokkaido 078-0397 Japan

*Corresponding author: Kimiko Itoh, 8050 Ikarashi-2, Faculty of Agriculture, Niigata University, 950-2181 Japan, Tel.: +81-25-262-7522, Fax: +81-25-262-7522, E-mail: kimi{at}agr.niigata-u.ac.jp


  Abstract

A rice Wx gene encoding a granule-bound starch synthase I (GBSSI) was introduced into the null-mutant waxy (wx) rice (Plant Cell Physiol., 44: 473–480 (2003)) and its effect on endosperm starches was examined. Apparent amylose content was increased from undetectable amounts for the non-transgenic wx cultivars to 21.6–22.2% of starch weight for the transgenic lines. The increase was in part due to a significant amount of extra-long unit-chains (ELC) of amylopectin (7.5–8.4% of amylopectin weight), that were absent in the non-transgenic wx cultivars. Thus, actual amylose content was calculated to be 14.9–16.0% for the transgenic lines. Only slight differences were found in chain-length distribution for the chains other than ELC, indicating that the major effect of the Wx transgene on amylopectin structure was ELC formation. ELC isolated from debranched amylopectin exhibited structures distinct from amylose. Structures of amylose from the transgenic lines were slightly different from cv. Labelle (Wxa) in terms of higher degree of branching and size distribution. Amylose and ELC content of starches of the transgenic lines resulted in the elevation of pasting temperature, a 50% decrease in peak viscosity, a large decrease in breakdown, and an increase in setback. As yet undetermined factors other than the GBSSI activity are thought to be involved in the control of formation and/or the amount of ELC. Structural analysis of Wx gene suggested that the presence of a tyrosine residue at position 224 of GBSSI correlates with the formation of large amounts of ELC in cultivars carrying the Wxa.

Keywords: amylopectin - granule-bound starch synthase - extra-long chains - rice - starch - Wx

**Present address, Hokkaido Prefectural Dohnan Agricultural Experiment Station, Hokkaido 041-1201 Japan

(Received March 23, 2008; Accepted April 17, 2008)
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