Molecular Pathogens, 2025, Vol.16, No.5, 226-235 http://microbescipublisher.com/index.php/mp 231 Rhizobium bacteria vary in nitrogen fixation efficiency and environmental adaptability. Under the same conditions, the amount of nitrogen fixed symbiotically by different strains may vary several times; some strains are highly efficient in fixing nitrogen, while others have nodulation but weak nitrogen fixation (i.e., ineffective root nodules). In addition, different strains have different tolerances to soil pH, moisture, temperature, etc. Therefore, efficient strains should be selected based on soil conditions and varieties to achieve optimal nitrogen fixation. 5.2 Genome comparison reveals functional genes related to nitrogen fixation Comparison of the genomes of different rhizobia strains revealed many genetic differences related to symbiotic nitrogen-fixing functions. Efficient nitrogen-fixing strains usually possess complete and high-expression levels of nitrogen-fixing gene clusters and carry some beneficial auxiliary genes. For example, they often have hydrogen uptake enzyme (hup) genes to recycle hydrogen by-product of nitrogen fixation and improve energy utilization efficiency. Genome comparison also found that different strains have differences in nitrogen fixation regulatory factors and stress resistance genes, allowing some strains to maintain strong nitrogen fixation activity under stress (Black et al., 2012). This information provides important targets and new ideas for screening and transforming efficient strains. 5.3 Screening and engineering improvement of efficient nitrogen-fixing root nodule strains Efficient nitrogen-fixing rhizobia strains can be obtained through traditional screening and modern engineering techniques. In practice, strains isolated from soil in different regions have been selected through nodulation and nitrogen fixation capabilities to select the most efficient ones as inoculants. Classic strains such as BradyrhizobiumUSDA110 have been successfully used in agriculture (Bopape and Hassen, 2013; Liu, 2013). At the same time, researchers use mutation breeding and genetic engineering methods to modify the rhizobia genome to enhance nitrogen-fixing performance, such as introducing hydrogenase genes or knocking out regulatory genes that inhibit nitrogen fixation to create engineered strains with more efficient nitrogen fixation. With the development of screening and improvement technology, it is expected to obtain more efficient, stress-resistant and competitive rhizobia resources, providing a more effective biological nitrogen fixation solution for sustainable soybean production. 6 Influence of Environmental Factors on Nitrogen Fixation Interactions 6.1 Regulation of soil physical and chemical properties Soil pH and moisture conditions directly affect symbiotic nitrogen fixation in soybeans. Most rhizobia are most active in neutral and slightly acidic soil. Strong acid will inhibit the activity of rhizobia and damage the root system, reducing nodulation and nitrogen fixation. Excessive drought or waterlogging in the soil is also not conducive to nitrogen fixation, and appropriate humidity and aeration conditions should be maintained (Kachiguma et al., 2019). Phosphorus (P) and molybdenum (Mo) among soil nutrients are particularly important for nitrogen fixation. Phosphorus deficiency leads to insufficient energy supply and a significant decrease in nitrogen fixation efficiency; appropriate phosphorus supplementation can improve nitrogen fixation levels (Zhong et al., 2022). Molybdenum is an essential element for nitrogenase. Molybdenum deficiency will limit nitrogenase activity and should be supplemented through micro-fertilizer. On the contrary, excessive soil inorganic nitrogen will inhibit nodulation and nitrogen fixation, so soil nutrients need to be balanced to realize the potential of biological nitrogen fixation. 6.2 Effect of stress on nitrogen fixation efficiency Drought stress significantly affects soybean-rhizobia symbiosis. When there is insufficient water, plant photosynthesis is limited, the supply of carbon sources to root nodules is reduced, and nitrogen fixation activity decreases. Drought also reduces the activity of rhizobia in the soil and the nodulation rate is significantly reduced. Plants produce more stress hormones such as abscisic acid and ethylene under drought, which will inhibit the formation of new root nodules and trigger premature senescence of existing root nodules (Racca et al., 2017). Continuous drought significantly reduces or even stops the activity of root nodule nitrogenase, severely weakening biological nitrogen fixation and causing plant nitrogen deficiency symptoms.
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