Opportunities for improved legume inoculants: enhanced stress tolerance of rhizobia and benefits to agroecosystems

Environmental stress conditions influence the growth and survival of rhizobia by affecting the signalling and infection process, nodule development and function. Stress factors such as osmotic stress, extremes of temperature and pH and accumulation of heavy metals result in reduced nodulation, leading to low levels of nitrogen fixation and crop yield. Some species of rhizobia are known to be tolerant to biotic and abiotic stresses and utilization of these stress-tolerant rhizobia strains as inoculants, can greatly improve biological nitrogen fixation. This review highlights the main environmental stresses known to cause cause rhizobial cell damage and death, including temperature, desiccation, drought, salinity, pH and heavy metal stresses. An understanding of the key physiological and molecular factors of how these stress responses affect the survival of rhizobia is crucial in the development of strains with high potential in symbiotic nitrogen fixation. Key responses range from expression of stress-linked genes and proteins which aid in cell repair and protection, accumulation of compatible solutes such as sugars and polymers, carbon enrichment in drought stress, to extrusion of heavy metals. Biological nitrogen fixation can be improved by the selection of nitrogen-fixing endosymbionts that are well-adapted and tolerant to a broad range of environmental stresses is important in alleviating the effects of adverse conditions, potentially increasing the chances of success of legume inoculation with rhizobia thus improving the contribution of atmospheric nitrogen fixation in agroecosystems.