Defining new roles for plant and rhizobial molecules in sole and mixed plant cultures involving symbiotic legumes

The view that symbiotic legumes benefit companion and subsequent plant species in intercrop and rotation systems is well accepted. However, the major contributions made separately by legumes and their microsymbionts that do not relate to root-nodule N-2 fixation have been largely ignored. Rhizobia (species of Rhizobium , Bradyrhizobium , Azorhizobium , Allorhizobium , Sinorhizobium and Mesorhizobium ) produce chemical molecules that can influence plant development, including phytohormones, lipo-chito-oligosaccharide Nod factors, lumichrome, riboflavin and H-2 evolved by nitrogenase. When present in soil, Nod factors can stimulate seed germination, promote plant growth and increase grain yields of legume and nonlegume crops, as well as stimulate increased photosynthetic rates following plant leaf spraying. Very low concentrations of lumichrome and H-2 released by bacteroids also promote plant growth and increase biomass in a number of plant species grown under field and glasshouse conditions. Rhizobia are known to suppress the population of soil pathogens in agricultural and natural ecosystems and, in addition to forming nodule symbioses with rhizobia, the legume itself releases phenolics that can suppress pathogens and herbivores, solubilize nutrients, and promote growth of mutualistic microbes. Phytosiderophores and organic acid anions exuded by the host plant can further enhance mineral nutrition in the system. This review explores new insights into sole and mixed plant cultures with the aim of identifying novel roles for molecules of legume and microbial origin in natural and agricultural ecosystems.