The significance of microbial-root symbioses for the salt tolerance of N2-fixing plants

• Saline soils are widely distributed in many countries and regions around the globe • The symbiosis between BNF plants and rhizosphere microorganisms plays a crucial role in improving plant resilience and to cope with salt stress • We summarize the effects of salt stress on N2-fixing plants and their root colonizing microorganisms • With a focus on mechanisms of interaction between N2-fixing bacteria and mycorrhizal fungi under salt stress • Research gaps of the analyses of the tripartite symbioses of plants-N2-fixing-bacteria-mycorrhizal fungi are identified to elucidate future research directions Soil salinization and its accompanying soil degradation pose a major threat to plant growth and the sustainability of terrestrial ecosystems worldwide. Therefore, exploring methods to improve the efficiency of phytoremediation of saline soils has been the focus of current research. The symbiosis between biological nitrogen-fixing (BNF) plants and rhizosphere microorganisms plays a crucial role in improving plant resilience and to cope with salt stress. Especially the interaction between nitrogen (N 2 )-fixing bacteria and mycorrhizal fungi can mitigate the negative effects of salt stress on N 2 -fixing plants. However, a comprehensive review on the mechanisms of interaction between N 2 -fixing bacteria and mycorrhizal fungi that confer salt tolerance to N 2 -fixing plants is still lacking. In this review, we summarize the effects of salt stress on N 2 -fixing plants and their root colonizing microorganisms with a focus on mechanisms of interaction between N 2 -fixing bacteria and mycorrhizal fungi under salt stress. These interactions enhance host plant resilience through nutrient complementation, hormonal regulation, and improved antioxidant capacity, but may also be antagonistic through nutrient competition. Finally, we identified research gaps of the analyses of the tripartite symbioses of N 2 -fixing plants, N 2 -fixing bacteria, and mycorrhizal fungi to elucidate future research directions.