Evidence of Diazotrophic Symbionts in the Leguminous Cover Crop Mucuna bracteata

New studies point to an increasing number of identified bacteria that can nodulate and fix N2 in legumes which do not belong to the original genus of Rhizobium and the rhizobial phylogenetic lineages. This study was conducted to isolate and identify diazotrophic microsymbionts from the root nodules of Mucuna bracteata (an important cover crop for oil palm) based on nitrogenase gene (nifH) isolation and partial 16S rDNA sequence analysis. The findings of this study indicated that the isolated microsymbionts could nodulate and promote N2-fixation activity in M. bracteata. These also contributed to enhanced plant growth in terms of leaf protein and chlorophyll content, as well as in the biomass of whole plants and nodules. Additionally, nifH gene fragments were successfully amplified at ~380 bp from eight of the isolates (USM accessions A11, B4, B9, B12, B19, C1, C4 and C8) using nifH3 primers, while the remaining isolates (namely, USM accessions B14, B15, B20, C2 and C9) were successfully amplified at various sizes (~550, 650, 350, 450, and 900 bp, respectively) using nifH4 primers. The partial 16S rDNA sequencing revealed that the diazotrophic microsymbionts were not only from the traditional Alphaproteobacteria class (Brevundimonas sp.), but also from the Betaproteobacteria class (Achromobacter sp. and Burkholderia sp.) and the Gammaproteobacteria class (Stenotrophomonas sp.). Five non-rhizobial isolates were obtained and identified as Bacillus sp. from the root nodules of M. bracteata. The findings indicate the diversity of potentially-beneficial diazotrophic microsymbionts active in this emerging legume species