Reduced activity of Arabidopsis thaliana HMT2, a methionine biosynthetic enzyme, increases seed methionine content.

In the S-methylmethionine cycle of plants, homocysteine methyltransferase (HMT) catalyzes the formation of two molecules of methionine from homocysteine and S-methylmethionine, and methionine methyltransferase (MMT) catalyzes the formation of methionine from S-methylmethionine using S-adenosylmethionine as a methyl group donor. Somewhat surprisingly, two independently isolated knockdown mutations of HMT2 (At3g63250), one of three Arabidopsis thaliana genes encoding homocysteine methyltransferase, increased free methionine abundance in seeds. Crosses and flower stalk grafting experiments demonstrate that the maternal genotype at the top of the flower stalk determines the seed S-methylmethionine and methionine phenotype of hmt2 mutants. Uptake, transport and inter-conversion of [(13)C]S-methylmethionine and [(13)C]methionine in hmt2, mmt and wild-type plants show that S-methylmethionine is a non-essential intermediate in the movement of methionine from vegetative tissue to the seeds. Together, these results support a model whereby elevated S-methylmethionine in hmt2 vegetative tissue is transported to seeds and either directly or indirectly results in the biosynthesis of additional methionine. Manipulation of the S-methylmethionine cycle may provide a new approach for improving the nutritional value of major grain crops such as rice, as methionine is a limiting essential amino acid for mammalian diets.