Role of rhizobial biosynthetic pathways of amino acids, nucleotide bases and vitamins in symbiosis.

Rhizobia require the availability of 20 amino acids for the establishment of effective symbiosis with legumes. Some of these amino acids are synthesized by rhizobium, whereas the remaining are supplied by the host plant. The supply from plant appears to be plant-type specific. Alfalfa provides arginine, cysteine, isoleucine, valine and tryptophan, and cowpea and soybean provide histidine. The production of ornithine and anthranilic acid, the intermediates in the biosynthetic pathways of arginine and tryptophan, respectively, seems to be essential for effective symbiosis of Sinorhizobium meliloti with alfalfa. The expression of ilvC gene of S. meliloti is required for induction of nodules on the roots of alfalfa plants. An undiminished metabolic flow through the rhizobial pathways for the synthesis of purines and pyrimidines and the synthesis of biotin, nicotinic acid, riboflavin and thiamine by rhizobium appear to be requirements for normal symbiosis. To the best of our knowledge, this is the first review article on the role of rhizobial biosynthetic pathways of amino acids, nucleotide bases and vitamins in rhizobium-legume symbiosis. The scientific developments of about 35 years in this field have been reviewed.