Molecular aspects of legume/rhizobia symbiosis: perspectives for the '90s

The exchange of molecular signals between legume plant and rhizobia has a key role in mutual recognition and in determining bacterium-host specificity. Rhizobial nod genes are induced by plant compounds such as flavonoids and, in the case of alfalfa/R. meliloti, also by betaines like trigonelline and stachydrine, abundant in root exudates. Different legumes secrete different flavonoids inducers and a single plant may synthesise different inducers at different stages of development. Some flavonoids can also cause growth enhancement of Rhizobium, thus becoming biocontrol substances with regulation potential for the dynamic of soil microbiota. Nod factors can elicit various responses in the host legumes by inducing root hair formation, deformation and branching, infection, thread formation and cortical cell division for development of nodule primordia. The role that receptors can play in the plant, along with corresponding receptor genes in the legume family with constant and variable domains, remains to be elecidated. Further expoitation on molecular aspects of legumes/rhizobia symbiosis are also discussed.