Rhizobia for Biological Control of Plant Diseases

Rhizobia are a diverse group of nodule-forming bacteria known for inhabiting the soil and establishing functional symbiotic associations with legume plants. Rhizobial inoculants are widely employed in agricultural practices to reduce nitrogen fertilizer inputs on legume crops due to rhizobial ability to fix atmospheric nitrogen. Here we argue that rhizobia should also be considered an alternative method to agricultural pesticides use in plant disease management. Several rhizobial strains have been reported leading to disease resistance, while also promoting plant yield and biomass increases. The biocontrol properties of rhizobia could be associated with lytic enzymes and antimicrobial secondary metabolite production, especially when regarding diseases affecting root systems of plants. Aside from the action of antifungal molecules, suppression of plant diseases could be related to rhizobial plant growth promotion and/or symbiotic efficiency. Moreover, rhizobia have been found to induce systemic resistance to immunize plants, which is a valuable process, considering foliar and viral diseases. This review will focus on rhizobial mechanisms and efficacy to biocontrol diseases caused by different classes of pathogens affecting leguminous and even non-leguminous plants.