BSAS4;1 - key candidate in developing common bean as an excellent source of protein.

With the increasing food insecurity in the populated world, the number of people affected by chronic undernourishment is also increasing. Protein energy malnutrition alone is causing 6 million deaths annually. Despite being a good source of protein and dietary fibre, quality of bean protein is questioned because of sub optimal levels of essential sulfur amino acids: methionine and cysteine. Levels of cysteine and methionine in developing seeds have an inverse relationship with the non-proteinogenic sulfur amino acid S-methyl-cysteine (SMC). One of the strategies to improve protein quality can be to redirect the sulfur from SMC to the cysteine pool. Elucidation of unknown biochemical pathway of SMC synthesis with 13C and 15N labelled serine and cysteine revealed serine as the precursor of SMC biosynthesis in common bean. Feeding common bean developing seeds with 13C and 15N lebelled methionine also suggested role of methionine in SMC biosynthesis. In cytoplasm, methanethiol released during hydrolysis of methionine might be condensed with O-acetylserine to form SMC. BSAS4;1, a -cyanoalanine synthase family member plays a key role in this reaction. BSAS4;1, a cysteine synthase, utilizes O-acetylserine as a carbon backbone donor and methionine as a methyl donor to synthesize SMC. According to the Canadian Food Inspection Agency, the label “Good source of protein” can be used for beans and other legumes. In future silencing of SMC pathway candidate genes or development of TILLING lines with high cysteine and methionine levels may change bean from a good source to an “excellent source of protein”. Hence our findings would provide a helping hand to overcome the food insecurity in the growing world.