Diversity and distribution of soybean-nodulating bradyrhizobia isolated from major soybean-growing regions in Myanmar

Abstract

Rhizobia play an important role in plant nutrition by fixing nitrogen that is subsequently available for uptake by plants. In Myanmar, it has been difficult to characterize the nitrogen-fixing indigenous bradyrhizobia strains. In this study, 120 strains were isolated from five major soybean-growing regions of Myanmar. The strains were characterized based on sequence analysis of the 16S-23S rRNA internal transcribed spacer (ITS) region. Determination of nodulation types of native isolates was based upon their compatibility between strain and soybean cultivars by inoculation test. Our goal was to describe the polygenetic diversity among indigenous bradyrhizobial strains isolated from Myanmar and to identify their nodulation types. Sequence analysis indicated that all isolates belonged to the Bradyrhizobium genus and were conspecific with B. liaoningense,B. elkanii,Bradyrhizobium spp., B. japonicum and B. yuanmingense. A phylogenetic tree showed that 40, 33.3, 19.2, 5 and 2.5% of the isolates were related to B. liaoningense,B. elkanii,Bradyrhizobium spp., B. japonicum and B. yuanmingense, respectively. These results suggested that B. liaoningense and B. ellkanii were the predominant Bradyrhizobium species present in Myanmar soils.B. elkanii, B. liaoningense and Bradyrhizobium spp. strains were dominant in acid soils, slightly acid to slightly alkaline soils and alkaline soils, respectively. Among tested isolates, Type A strains accounted for 74% of the isolates, while Type B and C strains accounted for 22 and 4%, respectively.It can be concluded that the diversity and distribution of indigenous bradyrhizobia differed markedly among sampling sites and among sites within the same region, and were affected by soil pH and climate. Determination of nodulation types of indigenous brayrhizobia provides useful information for selection of strains compatible with different soybean cultivars. This is the first report describing B. liaoningense strains isolated from soybean in Myanmar.   Key words: Indigenous bradyrhizobia, internal transcribed spacer (ITS), Myanmar, phylogenetic diversity, soybean, nodulation type.