SELECTION AND CHARACTERIZATION OF MUNGBEAN ROOT NODULE BACTERIA BASED ON THEIR GROWTH AND SYMBIOTIC ABILITY IN ALKALINE CONDITIONS
Keywords:
Alkaline tolerance, root nodule bacteria, mungbean, RAPD, nodulation assay, BradyrhizobiumAbstract
Ten strains of root nodule bacteria, isolated from mungbean (Vigna radiata) grown in five provinces of Thailand, were examined for their ability to grow in alkaline medium. Seven strains were able to grow at pH 8.0, whereas the growth of three strains was inhibited at pH 8.0. Therefore, the strains were grouped into 2 categories consisting of alkali-sensitive strains and alkali-tolerant strains. The growth of all strains tested was inhibited at pH 8.5, with a survival rate ranging between 0.03% to 2.13%. Of ten strains tested, the highest cell number was obtained from strain DASA02008 at pH 8.0. The cell number of strain DASA02008 reached 1.98 × 108 } 5.90 × 107 CFU/ml., which differed significantly from other strains. At pH 8.5, the highest cell numbers were obtained from strains DASA02008, DASA02009 and DASA02010. The cell numbers obtained from strains DASA02008, DASA02009 and DASA02010 were 2.13 × 103 } 9.97 × 102, 1.83 × 103 } 4.80 × 102, and 1.97 × 103 } 9.50 × 102 CFU/ml, respectively. In a nodulation test at pH 8.0, all strains could nodulate mungbean, and acetylene reduction activity (ARA) could be detected in all nodulated plants. However, there was no significant difference between alkali-tolerant strains and alkali-sensitive strains based on nodule numbers, nodule dry weight, plant shoot dry weight and acetylene reduction activity. Alkali-tolerant strains and alkali-sensitive strains were placed in the same cluster based on random amplified polymorphic DNA (RAPD) analysis. No small plasmids were found in any strains. The strain DASA02008, which had the highest cell number at both pH 8.0 and pH 8.5, was selected to analyze partial sequence (approximately 500 bp.) of the 16S rDNA. The nucleotide sequence indicated high homology (98%) with members of Bradyrhizobium japonicum, Bradyrhizobium liaoningense and Bradyrhizobium sp.
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