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.