Study on molecular phylogeny and symbiotic fitness of Bradyrhizobium spp. isolated from different peanut (Arachis hypogaea L.) cultivars

Rhizobia are bacterial symbionts of legumes that can convert atmospheric nitrogen into ammonia via biological nitrogen-fixation within root-nodules. Successful application of rhizobial inoculant can reduce the usage of chemical fertilizer without a decrease in crop yield, and it has been regarded as an environmental friendly approach. Peanut (Arachis hypogaea L.) is an annual herbaceous plant in the Fabaceae. It is usually nodulated by Bradyrhizobium spp., and is regarded as a promiscuous (or less-selective) legume due to its infection is via a less stringent pathway, called crack entry. However, if the indigenous rhizobia with high affinity presented in soil, peanut host will form nodules with them rather than the rhizobial inoculant. This feature causes a severe defect of rhizobial inoculant application in peanut cultivation. The symbiotic fitness between legumes and rhizobia is affected by the genotype of either host or symbiont. In this study, four peanut cultivated varieties (including the Valencia type: Taiwanese cv. BK, and some Spanish type cultivars: Taiwanese cv. TNS 9, Chinese cv. Yuanza9102, and Chinese Shanghua 5) were applied to trap peanut rhizobia inhabited the soil of NTU experimental field. According to the PCR results, there were Bradyrhizobium spp. with Clade III.3 and VIII nodA genotypes in the soil sample. Total 24 strains were isolated from the peanut root-nodules. All of them belonged to Bradyrhizobium genus carrying Clade VIII nodA genotype by multilocus sequence analysis (MLSA) and nodA sequence analysis. Furthermore, comparing the N-fixation ability between Clade III.3 and VIII strains by single strain inoculation test showed that the symbiont with Clade VIII nodA (i.e strain NTU-T-2) performed better symbiotic fitness on cv. TNS 9, and also showed good nodulation ability on the other two cultivars (i.e Valencia type cultivar: Taiwanese cv. TN 16, and Virginia type cultivar: Taiwanese cv. TC 2). Furthermore, in the co-inoculation experiment on cv. TNS 9, the Clade VIII nodA strain (i.e NTU-T-2) also showed better nodulation efficiency than that of type III.3 strains. These results indicated that although peanut is considered as a less-selective legume host, it still showed advantaged nodulation fitness with the Clade VIII nodA strains. This finding not only gives new insights into symbiotic fitness, but can be used as a screening platform for high nodulation competitiveness inoculants to increase peanut rhizobial inoculation efficiency.