The Effect of Climate Variables, Soil Characteristics, and Peanut Cultivars on the Rhizobial Bacteria Community

Peanuts are widely cultivated across the world; however, peanut's rhizobial community and the determinant factors of their composition are still to be elucidated. This study investigates the biogeography and determinant soil environmental factors for peanut rhizobia. A total of 1001 rhizobial isolates were obtained from the peanut root nodules, mainly belonging to two cultivars (X9 and M6) cultivated in 20 sampling sites across China. According to recA sequence analysis, all the isolates were classified as 84 haplotypes, and a representative strain for each haplotype was randomly selected to perform subsequent analyses. Based on multilocus sequence analysis (MLSA) of housekeeping genes dnaK, glnII, gyrB, recA, and rpoB, all the representative strains were classified as 42 genospecies in the genus Bradyrhizobium, including 12 effectively published and 30 undefined genospecies. Strains belonging to six genospecies were predominant (>5%), including B. ottawaense, B. liaoningense, B. yuanmingense, Bradyrhizobium sp. XXIX, B. guangdongense, and B. nanningense. However, only a single isolate was obtained for 15 genospecies. The diversity indices of peanut rhizobia distributed in South China are obviously higher than those in North China, but no obvious peanut cultivar selection for rhizobial genospecies was found. Correlation analyses indicated that the community composition of peanut rhizobia was mainly affected by MAP, MAT, soil AP, and pH. Nodulation tests indicated that the 79 representative strains belonging to 37 genospecies with both nodC and nifH could perform nitrogen-fixing symbiosis with peanuts. This study revealed the great diversity and varied composition of communities of peanut rhizobia in different geographic regions across China.