Species diversity and plant growth-promoting effects of bacteria isolated from the root nodules of Sophora davidii

Abstract:

[Objective] There are numerous non-rhizobia in addition to rhizobia in the root nodules of leguminous plants. Despite the extensive studies about the endophytic bacteria in other plant tissues, little attention has been paid to the species diversity of non-rhizobia in root nodules. Therefore, further research is needed to explore the significance and ecological roles of non-rhizobia. [Methods] The root nodules of wild Sophora davidii (Franch.) Skeels, a leguminous nitrogen-fixing shrub growing in the hilly-gully loess region of northern Shaanxi, were collected. The species diversity of rhizobia and non-rhizobia in the root nodules of S. davidii was systematically studied by the conventional culture method. The plant growth-promoting effects of the strains were evaluated and their growth-promoting effects on wheat seedlings were verified. [Results] A total of 320 strains of endophytic bacteria were isolated from the root nodules of S. davidii in six counties/districts in northern Shaanxi. The phylogenetic analysis based on the 16S rRNA gene sequences identified the strains into 55 genera, 35 families, 17 orders, 17 classes of 4 phyla. Pseudomonas (18.44%), Bacillus (17.81%), and Mesorhizobium (11.56%) were the dominant genera. The results of re-inoculation experiments showed that Mesorhizobium sp. and Ochrobactrum sp. HL-2 formed root nodules with the host plant. Furthermore, the plant growth-promoting characteristics of 192 strains isolated Baota District were studied. The results showed that 115, 20, 78, and 18 strains possessed the abilities of fixing nitrogen, solubilizing phosphorus, producing indole-3-acetic acid (IAA), and secreting siderophores, respectively. Four elite strains were selected and inoculated alone or in combination into the wheat seedlings in pots, and their growth-promoting effects on the agronomic traits of wheat seedlings were evaluated. The results indicated that the treatment HIJ increased the plant height and fresh weight of wheat seedlings by 49.65% and 140.00% respectively. The treatments HK and IK increased the root length and chlorophyll content by 45.84% and 25.48%, respectively. [Conclusion] There are diverse non-rhizobia in the root nodules of S. davidii. The results of this study have great scientific significance for exploring the role of such microbial resources in natural ecosystems and enriching the resource pool of endophytic bacteria. Additionally, the results provide a theoretical basis for the application of these strains in ecological restoration in the arid region of northern Shaanxi.