Pathogenic and mutualistic plant-bacteria interactions: ever increasing similarities

Plant-interacting bacteria can establish either mutualistic or pathogenic interactions that cause beneficial or detrimental effects respectively, to their hosts. In spite of the completely different outcomes, accumulating evidence indicates that similar molecular bases underlie the establishment of these two contrasting plant-bacteria associations. Recent findings observed in the mutualistic nitrogen-fixing Rhizobium-legume symbiosis add new elements to the increasing list of similarities. Amongst these, in this review we describe the role of plant resistance proteins in determining host specificity in the Rhizobium-legume symbiosis that resemble the gene-for-gene resistance of plant-pathogen interactions, and the production of antimicrobial peptides by certain legumes to control rhizobial proliferation within nodules. Amongst common bacterial strategies, cyclic diguanylate (c-di-GMP) appears to be a second messenger used by both pathogenic and mutualistic bacteria to regulate key features for interaction with their plant hosts.