Molecular Approaches in Microbial Ecology to Assess Genomic Diversity and Stress-Induced Gene Expression in Plant-Associated Diazotrophs

During the last two decades a tremendous amount of progress has been made in elucidating the molecular basis of endosymbiotic and associative interactions of diazotrophic bacteria with plants. In fact, in the area of the Rhizobium-legume symbiosis, a true explosion of information has been generated with regard to plant-derived signal molecules, the rhizobial nodulation (nod) genes they induce, as well as the structure, host-specificity and putative mode of action of the nod-genes encoded nodulation factors (Nod factors). The regulatory circuits underlying the control of nitrogen fixation gene expression in strictly endosymbiotic, associative or free-living diazotrophs have been examined in detail and shown to be highly complex and diverse, while revealing common themes. Moreover, the mechanisms underlying the induction of a variety of plant genes specifically expressed or enhanced as a result of infection by, or association with diazotrophic bacteria are now beginning to be understood.