Enhanced available methionine concentration associated with higher phaseolin levels in common bean seeds

The relationship between available methionine concentration and the levels of phaseolin - the major seed storage proteins of the common bean - was studied using three groups of genetic materials: First, the F2 progenies of interspecific crosses between P. vulgaris cultivars and aP. coccineus subsp. coccineus line (cv. 'Mexican Red Runner') having no detectable phaseolin; second, the F2 progenies and segregating F3 families of crosses between cultivated P. vulgaris lines and a Mexican wild bean accession (PI 325690-3) carrying a gene producing a reduction in phaseolin content; third, two inbred backcross populations: 'Sanilac'x'Bush Blue Lake 240' (population 2) and 'Sanilac'x'15R 148' (population 6). Total seed N levels were determined by micro-Kjeldahl, phaseolin levels by rocket immunoelectrophoresis and available methionine levels by the Streptococcus zymogenes bioassay. Our results indicate that in all the genetic materials studied, with the exception of population 6, higher phaseolin levels lead to increased available methionine concentration. Although phaseolin has a low methionine concentration, it is actually a major source of available methionine in common bean seeds, because it represents a large part of total seed nitrogen and because limited differences exist between the methionine concentrations of the different protein fractions. This contrasts with the situation in cereals such as maize, barley and sorghum, where increased levels of the major limiting amino acid (lysine) can be achieved through a decrease in the amounts of the main seed storage protein fraction (prolamines). In population 6, no relationship was observed between available methionine and phaseolin content. Other factors, such as additional methionine-rich polypeptides or the presence of tannins, might obscure the positive relationship between phaseolin and available methionine content in population 6.