MOLECULAR MECHANISMS OF NOD FACTOR DIVERSITY

The rhizobia-legume symbiosis is highly specific. Major host specifici ty determinants are the bacterial Nod factor signals that trigger the nodulation programme in a compatible host. Nod factors are lipo-chitoo ligosaccharides (LCOs) varying in the oligosaccharide chain length, th e nature of the fatty acids and substitutions on the oligosaccharide. The nod genotype of rhizobia, which forms the genetic basis for this s tructural variety, includes a set of nodulation genes encoding the enz ymes that synthesize LCOs. Allelic and non-allelic variation in these genes ensures the synthesis of different LCO structures by the differe nt rhizobia. The nod genotypes co-evolved with host plant divergence i n contrast to the rhizobia, which followed a different evolution. Hori zontal gene transfer probably played an important role during evolutio n of symbiosis. The nod genotypes are particularly well equipped for h orizontal gene transfer because of their location on transmissible pla smids and/or on 'symbiosis islands', which are symbiotic regions assoc iated with movable elements.