Common Bean ( Phaseolus vulgaris L.) Accumulates Most S -Methylcysteine as Its γ-Glutamyl Dipeptide

The common bean (<i>Phaseolus vulgaris</i>) constitutes an excellent source of vegetable dietary protein. However, there are sub-optimal levels of the essential amino acids, methionine and cysteine. On the other hand, <i>P. vulgaris</i> accumulates large amounts of the &#947;-glutamyl dipeptide of <i>S</i>-methylcysteine, and lower levels of free <i>S</i>-methylcysteine and <i>S</i>-methylhomoglutathione. Past results suggest two distinct metabolite pools. Free <i>S</i>-methylcysteine levels are high at the beginning of seed development and decline at mid-maturation, while there is a biphasic accumulation of &#947;-glutamyl-<i>S</i>-methylcysteine, at early cotyledon and maturation stages. A possible model involves the formation of <i>S</i>-methylcysteine by cysteine synthase from <i>O</i>-acetylserine and methanethiol, whereas the majority of &#947;-glutamyl-<i>S</i>-methylcysteine may arise from <i>S</i>-methylhomoglutathione. Metabolite profiling during development and in genotypes differing in total <i>S</i>-methylcysteine accumulation showed that &#947;-glutamyl-<i>S</i>-methylcysteine accounts for most of the total <i>S</i>-methylcysteine in mature seed. Profiling of transcripts for candidate biosynthetic genes indicated that <i>BSAS4;1</i> expression is correlated with both the developmental timing and levels of free <i>S</i>-methylcysteine accumulated, while homoglutathione synthetase (<i>hGS</i>) expression was correlated with the levels of &#947;-glutamyl-<i>S</i>-methylcysteine. Analysis of <i>S</i>-methylated phytochelatins by liquid chromatography and high resolution tandem mass spectrometry revealed only small amounts of homophytochelatin-2 with a single <i>S</i>-methylcysteine. The mitochondrial localization of phytochelatin synthase 2&#8212;predominant in seed, determined by confocal microscopy of a fusion with the yellow fluorescent protein&#8212;and its spatial separation from <i>S</i>-methylhomoglutathione may explain the lack of significant accumulation of <i>S</i>-methylated phytochelatins.

Abstract licence: CC BY

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