Strategies to deal with drought-stress in biological nitrogen fixation in soybean

The soybean [Glycine max (L.) Merrill] production system in Brazil and other South American countries relies on the biological nitrogen fixation (BNF) in symbiosis with Bradyrhizobium elite strains, which provides nearly all the crop N requirements, about 83 kg per ton of grains. However, drought impairs several plant metabolic processes, especially BNF, limiting development and yield. Strategies to deal with drought comprise breeding, including genetic modification of the host plant, selection of rhizobial strains for more tolerance to water stress, co-inoculation with other plant growth-promoting microorganisms, and their metabolites to stimulate the symbiosis. A better understanding of physiological mechanisms involved in the symbiosis related to the host plant and the symbiont are crucial to development inoculants, and also soybean genotypes more adapted to drought, assuring lower impairment of drought on the N supply to the crop. Soil management based on no-till, and chemical and biological improvement to allow roots to reach deeper layers are also allied strategies to deal with drought. The maintenance of mulching and increase of soil organic matter levels, in addition to improvement of soil chemical conditions, allow more access to water in the soil profile, and a better condition for the symbiosis and other beneficial biological processes. Strategies to stimulate or maintain BNF can be one of the main strategies to mitigate drought stress in soybean. This review discusses the progress so far on strategies aiming to lessen the impairment of BNF under drought and suggests using integrated tools to alleviate the effects of drought on the soybean crop for a more stable and sustainable yield. •BNF is pivotal for sustainability of soybean production.•Drought is one of the most limiting environmental factors to BNF.•Integrated strategies are tools for increasing tolerance to drought.•Plant breeding, tolerant strains, microbial metabolites, plant nutrition, and soil management increase tolerance to drought.