Characterization and nodulation capacity of native bacteria isolated from mung bean nodules used as a trap plant in Brazilian tropical soils

Abstract

Abstract Mung bean [Vigna radiata (L.) Wilczek] is a legume of Asian origin with great importance worldwide, particularly in developing countries. It is known for its promiscuous association with soil rhizobia, which provides nitrogen via biological nitrogen fixation (BNF). From the prospective of increasing productivity, efforts are ongoing to identify bacteria that form more efficient symbiotic associations with mung bean. In this regard, we evaluated the nodulation of two mung bean cultivars with native rhizobia from 10 samples of tropical Brazilian soil and performed the isolation of bacterial strains present in the nodules. These isolates were characterized morphologically, and grouped based on sequencing of the 16S rRNA gene. The isolates were also evaluated for nodulation capacity in mung bean and BNF efficiency. We found differences in mung bean plant development depending on the soil for both cultivars. In general, the plants cultivated in soil samples from the Atlantic Forest region showed greater nodulation and shoot dry weight compared to the Brazilian Cerrado (tropical savanna). On the basis of an evaluation of nodulation by native rhizobia, we isolated 101 bacterial strains, which were classified according to 16S rRNA gene into nine genera: Bradyrhizobium (66), Rhizobium (19), Mesorhizobium (4), Ensifer (3), Leifsonia (3), Bacillus (3), Agrobacterium (1), Mycolicibacterium (1), and Kaistia (1). Bradyrhizobium was the most abundant genus, and its isolates formed 12 phylogenetic groups. Inoculation assessments revealed that only Bradyrhizobium isolates induced nodule formation in mung bean, with the BNF efficiency of isolates differing depending on phylogenetic group and soil of origin. Plants inoculated with isolates grouped with Bradyrhizobium yuanmingense B071 were found to show greater increases in shoot dry weight. Considering the soil origin, we observed that plants inoculated with Bradyrhizobium isolated from Mato Grosso state soil samples showed smaller increments in nodulation and shoot dry weight. These results suggest that the Bradyrhizobium is the predominant symbiont in the nodules of mung bean grown in Brazilian tropical soils, and its isolates have the potential for mung bean inoculation, especially when grown in soils that provide low nodulation by native rhizobia.