Regulatory subunit of acetohydroxy-acid synthase. Involved in the feed-back inhibition by branched-chain amino acids but not in herbicide tolerance. Amino-acid biosynthesis; L-isoleucine biosynthesis; L-isoleucine from 2-oxobutanoate: step 1/4. Amino-acid biosynthesis; L-valine biosynthesis; L-valine from pyruvate: step 1/4. The acetolactate synthase complex contains both large catalytic subunits and small regulatory subunits. Expressed in roots in the vascular tissuem in cells around the quiescent center, in floral organs at the tips of young siliques and in the joint region between the silique and the pedicel. Barely detectable in mature leaves or siliques. Belongs to the acetolactate synthase small subunit family.