Defense responses during the legume- rhizobium symbiosis: induction and suppression (review)

Development of the legume- Rhizobium symbiosis when bacteria infect cells of legume plants has many similarities with pathogenic infection development. Symbiotic partners exchange with signal molecules, which have similarities with phytoalexins and elicitors, producing during pathogenic infection. Both processes are accompanied by the induction of defense reactions of the host plant; however, induction of these reactions during symbiosis is under strong regulation that allows forming a symbiotic nodule. Number of nodules is also under control, due to the fact that their formation is energy-intensive process and the most infections are aborted in the early stages. Studies of molecular-genetic mechanisms of development of the legume- Rhizobium symbiosis and pathogenic infection revealed that during evolution the legumes adapted receptors accommodated originally for mycorrhizal infection perception to establish symbiosis with rhizobia, and similar receptors are used for perception of pathogens and induction of defense response. It was also shown that bacteria exploit III and IV types of secretion systems (T3SS and T4SS), which pathogenic bacteria use for delivery of virulent factors in host cells. It was demonstrated, that T3SS is able to activation of signal transduction pathway, leading to nodule development, independently from Nod-factor. Different surface polysaccharides (exopolysaccharides, lipopolysaccharades and cyclic b-glucans) are used by rhizobia as well as pathogenic bacteria for suppression of plant defense reactions. In nodules of some legume plants bacteria undergo terminated differentiation into bacteroids under action of antimicrobial NCR peptides.