Molecular mechanism of host specificity in legume-rhizobium symbiosis.

Rhizobium - legume symbiosis is a highly specific interaction between the two partners. Host specificity is evident at early stages of infection and results from multiple interactions involving signalling among bacteria and host plants. Host specific plant signals (flavanoids) convert the NodD protein to an active form and its binding with nod box initiates the transcription of inducible nod operons. Common nod genes (nodABC) code for an extracellular mitogenic Nod factor which is required for nodule organogenesis. Host specific genes (hsn) modify the Nod factor to induce root hair deformation on specific hosts. The structure of Nod factor controls host range distinction between species and biovars of rhizobia. Interactions of lectins and Exopolysaccharide/Lipopolysaccharide result in host specific attachment of Rhizobium and its subsequent invasion. Change in Expopolysaccharide structure by the transfer of hsn genes enables the Rhizobium to bind with heterologous host lectins. Conversely, changes in root lectins via gene manipulation enables the heterologous rhizobia to bind and initiate nodulation on heterologous hosts. Finally, host specific signals are required to initiate nitrogen fixation in nodules that are formed.