Legume Genomics and Genetics 2024, Vol.15, No.3, 140-151 http://cropscipublisher.com/index.php/lgg 144 4RhizobiumGenetic Diversity and Adaptation 4.1 Genetic variability among Rhizobiumstrains Rhizobium strains exhibit significant genetic variability, which is reflected in their strain-specific traits. For instance, Rhizobium tropici CIAT 899 and Rhizobiumsp. PRF 81 possess a highly conserved symbiotic plasmid (pSym) that includes three distinct nodA genes, contributing to their broad host range and adaptability to various legume hosts. Additionally, these strains have numerous genes encoding drug-efflux systems, which confer high resistance to antimicrobials and enable them to thrive in diverse environmental conditions (Ormeño-Orrillo et al., 2012). The genetic diversity among Rhizobium leguminosarumbiovar viciae strains also highlights the specificity of symbiotic partnerships, with certain genotypes favoring specific legume hosts such as broad bean (Vicia faba) (Mutch and Young, 2004). The genetic variability among Rhizobiumstrains directly impacts their symbiotic efficiency. Studies have shown that the presence of specific genetic elements, such as the nodulation genes in Rhizobium leguminosarum, can influence the effectiveness of nitrogen fixation and the overall fitness of both the host plant and the Rhizobium (Mutch and Young, 2004). Furthermore, the alignment of fitness between host and symbiont is crucial, as ineffective rhizobia are often ‘defective’ rather than ‘defectors’, indicating that genetic mutations can enhance the symbiotic relationship (Friesen, 2012). The interaction between light availability and Rhizobiumstrain variation also plays a role in determining the growth and nutrient composition of legume hosts, further emphasizing the importance of genetic diversity in symbiotic efficiency (Heath et al., 2020). 4.2 Adaptation to environmental stress Rhizobium strains have developed various adaptations to cope with environmental stresses such as drought, salinity, and pH fluctuations. For example, Rhizobium tropici CIAT 899 and Rhizobium sp. PRF 81 are well-equipped to handle low pH, high temperatures, and oxidative and osmotic stresses, which are common in tropical environments. The genetic determinants for these stress responses include genes involved in the biosynthesis and modulation of plant-hormone levels, as well as those encoding surface polysaccharides, uptake transporters, and catabolic enzymes for nutrients (Ormeño-Orrillo et al., 2012). Additionally, the GSK3-like kinase GmSK2-8 in soybean has been identified as a key regulator that inhibits symbiotic signaling and nodule formation under salt stress, highlighting the genetic mechanisms underlying stress tolerance in legume-rhizobium symbiosis (Figure 3) (He et al., 2020). The genetic mechanisms that confer stress tolerance in rhizobia are diverse and complex. Genome sequencing of Rhizobium strains has revealed a wide array of genes involved in stress response, including those encoding drug-efflux systems, iron-acquisition systems, and cell wall-degrading enzymes (Ormeño-Orrillo et al., 2012). These genetic traits enable rhizobia to persist and compete in challenging environments, thereby enhancing their symbiotic efficiency. Moreover, the identification of quantitative trait loci (QTL) and candidate genes associated with symbiotic nitrogen fixation (SNF) in grain legumes provides valuable insights into the genetic basis of stress tolerance and its impact on symbiotic interactions (Dwivedi et al., 2015). 4.3 Co-evolution with legume hosts The co-evolution of rhizobia and legume hosts involves intricate co-adaptation processes that enhance symbiotic efficiency. Rhizobial genetic elements, such as symbiotic plasmids, can be transferred among species and genera, leading to the emergence of symbiotic variants (symbiovars) that are adapted to specific legume hosts (Rogel et al., 2011). This lateral gene transfer facilitates the co-evolution of rhizobia and legumes, allowing for the development of highly specialized and efficient symbiotic relationships. The interplay between host genotype and environmental factors also influences the distribution and diversity of rhizobia, as seen in the Fynbos legumes of South Africa, where soil acidity and site elevation correlate with genetic variation in Mesorhizobium and Burkholderia (Lemaire et al., 2015). The co-evolution of rhizobia and legume hosts has significant implications for symbiotic efficiency. The alignment of fitness between host and symbiont is essential for maintaining stable and effective mutualistic relationships (Friesen, 2012). Studies have shown that rhizobial inoculants based on native strains with high
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