Abstract
The common bean (Phaseolus vulgaris) 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, P. vulgaris accumulates large amounts of the γ-glutamyl dipeptide of S-methylcysteine, and lower levels of free S-methylcysteine and S-methylhomoglutathione. Past results suggest two distinct metabolite pools. Free S-methylcysteine levels are high at the beginning of seed development and decline at mid-maturation, while there is a biphasic accumulation of γ-glutamyl-S-methylcysteine, at early cotyledon and maturation stages. A possible model involves the formation of S-methylcysteine by cysteine synthase from O-acetylserine and methanethiol, whereas the majority of γ-glutamyl-S-methylcysteine may arise from S-methylhomoglutathione. Metabolite profiling during development and in genotypes differing in total S-methylcysteine accumulation showed that γ-glutamyl-S-methylcysteine accounts for most of the total S-methylcysteine in mature seed. Profiling of transcripts for candidate biosynthetic genes indicated that BSAS4;1 expression is correlated with both the developmental timing and levels of free S-methylcysteine accumulated, while homoglutathione synthetase (hGS) expression was correlated with the levels of γ-glutamyl-S-methylcysteine. Analysis of S-methylated phytochelatins by liquid chromatography and high resolution tandem mass spectrometry revealed only small amounts of homophytochelatin-2 with a single S-methylcysteine. The mitochondrial localization of phytochelatin synthase 2—predominant in seed, determined by confocal microscopy of a fusion with the yellow fluorescent protein—and its spatial separation from S-methylhomoglutathione may explain the lack of significant accumulation of S-methylated phytochelatins.
Highlights
The common bean (Phaseolus vulgaris L.) is an important source of protein, dietary fiber, complex carbohydrates, vitamins, minerals and phenolic compounds [1,2,3,4,5]
To obtain more information on the accumulation of the different forms of S-methylcysteine and its precursors, free amino acids were profiled at seven developmental stages in BAT93 seed by high pressure liquid chromatography (HPLC) after derivatization with 3-mercaptopropionic acid and O-phthalaldehyde (Table 1)
Stages IV–early cotyledon to VI–early maturation are characterized by the presence of storage protein transcripts, while storage proteins accumulate from stages V–mid-cotyledon to VII–mid-maturation [42]
Summary
The common bean (Phaseolus vulgaris L.) is an important source of protein, dietary fiber, complex carbohydrates, vitamins, minerals and phenolic compounds [1,2,3,4,5]. Its nutritional quality is restricted by a low concentration of the essential sulfur amino acids, methionine and cysteine. Major seed proteins present in the common bean, including the 7S globulin phaseolin and lectin phytohaemagglutinin, have a low concentration of methionine and cysteine. The common bean accumulates non-protein sulfur amino acid derivatives such as γ–glutamyl-S-methylcysteine [13,14], S-methylcysteine [15,16,17] and S-methylhomoglutathione [18]. Two genetically related lines of the common bean, SARC1 and SMARC1N-PN1 differ in their composition of major storage proteins [19]. SMARC1N-PN1 has an increased concentration of methionine and cysteine, by 10 and 70%, respectively This increase occurs largely at the expense of total S-methylcysteine measured after acid hydrolysis, which is reduced by 70% [20]
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