TITLE:
Herbicide-Resistant Mutations in Acetolactate Synthase Can Reduce Feedback Inhibition and Lead to Accumulation of Branched-Chain Amino Acids
AUTHORS:
Masaki Endo, Tsutomu Shimizu, Tamaki Fujimori, Shuichi Yanagisawa, Seiichi Toki
KEYWORDS:
Rice; Acetolactate Synthase; Herbicide-Resistance; Branched-Chain Amino Acids
JOURNAL NAME:
Food and Nutrition Sciences,
Vol.4 No.5,
May
13,
2013
ABSTRACT: The branched-chain amino
acids (BCAAs) valine, leucine and isoleucine are essential amino acids that are
critical for animal growth and development. Animals need to obtain BCAAs from
their diet because they cannot synthesize them. Plants are the ultimate source
of these amino acids. Acetolactate synthase (ALS) is the
first common enzyme in the biosynthesis of BCAAs. The metabolic control of BCAA
biosynthesis involves allosteric regulation of ALS by the end-products of the
pathway, i.e., valine, leucine and
isoleucine. ALS holoenzyme seems to consist of two large catalytic subunits and two small
regulatory subunits. In a previous study, using homologous recombination
dependent gene targeting we created rice plants in which W548Land S627I mutations were induced into
the endogenous gene encoding the ALS catalytic subunit. These two amino acid
substitutions conferred hypertolerance to the ALS-inhibiting herbicide
bispyripac-sodium. In this study, we revealed that feedback regulation by
valine and leucine was reduced by these two amino acid substitutions.
Furthermore, in leaves and seeds of ALS mutants with W548Land/or S627I substitution, a 2- to 3-fold
increase in BCAAs was detected. Our results suggest that the ALS catalytic
subunit is also involved in feedback regulation of ALS, and that judicious modification of the regulatory
and catalytic subunits of ALS-coding genes by gene targeting can lead to the
efficient accumulation of BCAA in plants.