Molecular Pathogens, 2025, Vol.16, No.5, 226-235 http://microbescipublisher.com/index.php/mp 233 7.3 Molecular strategies for collaborative improvement of hosts and rhizobia Synergistically improving the host soybean and symbiotic rhizobia is an effective strategy to improve nitrogen fixation efficiency. Simply optimizing the performance of plants or strains often has limitations, but "host-strain co-breeding" can play a complementary role in the advantages of both parties. On the one hand, we need to cultivate excellent soybean varieties with stronger symbiotic affinity and stress tolerance for efficient rhizobia; on the other hand, we screen or engineer rhizobia strains that are more adaptable to the rhizosphere environment of the variety and can efficiently fix nitrogen, so that the two can achieve the best match. Studies have attempted to pair engineering strains with high nitrogen fixation efficiency with improved soybean genotypes, and the results showed that the symbiotic nitrogen fixation efficiency was significantly improved (Barros-Carvalho et al., 2019). As our understanding of the host-strain interaction mechanism deepens, we are expected to design the ideal combination of plants and strains to maximize the benefits of biological nitrogen fixation. 8 Rhizosphere Microbial Communities and Multiple Interaction Mechanisms 8.1 The supporting role of the rhizosphere microbiome in nitrogen fixation interactions The soybean rhizosphere microbial community as a whole affects symbiotic nitrogen fixation efficiency. In addition to rhizobia, beneficial microorganisms such as arbuscular mycorrhizal fungi and plant growth-promoting bacteria (PGPR) also exist in the rhizosphere, which can indirectly improve nitrogen-fixing capacity by improving host nutrition and health. For example, mycorrhizal fungi help plants absorb phosphorus and other minerals, providing sufficient nutrients for the energy-intensive nitrogen fixation process (Zhang et al., 2020); for another example, some rhizosphere bacteria produce plant hormones or ACC deaminase to alleviate plant stress and also promote root nodule development and function (Sarrette et al., 2024). Therefore, different members of the rhizosphere microbiome work synergistically to assist and enhance symbiotic nitrogen fixation in soybeans in manyways. 8.2 Signal competition and resource coordination in symbiotic systems In complex rhizosphere environments, symbiotic systems face competition for signals and resource allocation. Different microorganisms release similar signaling molecules, and plants need to accurately identify them in order to respond correctly. Multiple strains of rhizobia may compete for the nodulation site of the same plant, and plants often optimize resource utilization by "rewarding" efficient nitrogen-fixing bacteria and suppressing inefficient bacteria (Kachiguma et al., 2019). In addition, the carbon source allocated by plants to rhizobial symbiosis also needs to be weighed between rhizobia and other symbiotic or endophytic microorganisms: when other microorganisms consume large amounts of host resources, nodule nitrogen fixation may be limited. Through the dynamic regulation of signal games and resource allocation, the benefits of symbiotic nitrogen fixation in the context of the community are ultimately determined. 8.3 Development and application prospects of microbial inoculants In view of the diversity of rhizosphere microorganisms and their interactive effects, the development of new microbial inoculants is moving towards multi-bacterial compounds. Traditional soybean inoculants usually only contain rhizobia, but current research attempts to combine rhizobia with other beneficial microorganisms, such as phosphate-solubilizing bacteria, growth-promoting bacteria, or anti-stress bacteria, to form a functionally complementary flora agent. This type of compound inoculant has shown the potential to increase nitrogen fixation, promote growth and enhance stress resistance of soybeans in field trials. Acknowledgments Thank you to Marry T. for her technical support in data analysis and visualization, and also thank the members of the research team for their discussions and suggestions during the paper writing. Conflict of Interest Disclosure The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.
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