Plant genes induced in the Rhizobium-legume symbiosis

Rhizobium, Bradyrhizobium and Azorhizobium can elicit the formation of N2-fixing nodules on the roots or stems of their leguminous host plants. The nodule formation involves several developmental steps determined by different sets of genes from both partners, the gene expression being temporally and spatially coordinated. The plant proteins that are specifically synthesised during the formation and function of the nodule are called nodulins. The nodulins that are expressed before the onset of N2 fixation are termed early nodulins. These proteins are probably involved in the infection process as well as in nodule morphogenesis rather than in nodule function. The nodulins expressed just before or during N2 fixation are termed late nodulins and they participate in the function of the nodule by creating the physiological conditions required for nitrogen fixation, ammonium assimilation and transport. In this review we will describe nodulins, nodulin genes and the relationship between nodulin gene expression and nodule development. The study of nodulin gene expression may provide insight into root-nodule development and the mechanism of communication between bacteria and host plant.