Diversity and stress-tolerance of symbiotic nitrogen-fixing bacteria from legumes in the dry-hot valleys of southwest China

Abstract

The Dry-Hot valleys in Southwest China are hostile environments for living organisms. The dominant plants in the valleys are drought-resistant legumes. Here, we examined the diversity and physiology of the nitrogen-fixing bacteria associated with these legumes. Thirty-three strains were isolated from the nodules of six legume species located in five valleys. Analyses of their 16S rRNA gene sequences indicated that each strain had a unique 16S rRNA gene sequence and none of the strains had a sequence identical to those reported in the GenBank. The 16S rRNA gene sequence showed little clustering based on host tree species or geographic locations. PCR fingerprinting confirmed the genetic uniqueness of these strains. Physiological tests showed that these strains were all capable of growing at 35°C or above and at hypertonic environments. All strains formed root nodules on Acacia richii, the most common legume in these valleys. The bacterial strains obtained here could help future reforestation efforts in these valleys and other environments with similar conditions. Key words: Rhizobium spp., Bradyrhizobium spp., Mesorhizobium spp., 16S rRNA, high temperature resistance, drought resistanc