Diversity of Nitrogen-Fixing Bacteria Associated with Yams (Dioscorea spp.)
Michel Ouyabe, Hidehiko Kikuno, Naoto Tanaka, Pachakkil Babil, Hironobu Shiwachi
Tokyo University of Agriculture, Japan
Yams are important tuber crops in the tropical zones. Nitrogen is one of the limiting nutrients in yam production. In Africa, where the highest production is held, yam tuber yields are very low due to loss of soil fertility, and to unaffordable and expansive mineral fertilisers which are also not eco-friendly. In this context, identifying yam accessions that can rely on biological nitrogen fixation is necessary for achieving efficient and sustainable yam production. The aim of this study was to isolate and identify endophytic diazotrophic bacteria of yam. Endophytic bacteria were isolated from surface-sterilized roots, stems and leaves of yam accessions grown in nutrient-poor subsoil 160 days after planting. The nitrogen-free MR medium was used for culturing and purifying the endophytic bacteria. Sequencing of the 16S rRNA genes was performed for the fast-growing pure colonies. In total, 41 endophytes were found, belonging to 18 bacterial genera including Bacillus, Rhizobium, Burkholderia, Curtobacterium, Devosia, Mesorhizobium, Paenibacillus, Sphingomonas, Achromobacter, Bradyrhizobium, Ensifer, Enterobacter, Kocuria, Mycobacterium, Neorhizobium, Pseudaminobacter and Pseudoxanthomonas. Strains of Proteobacteria accounted for 56.10 % of total isolates, followed by Firmicutes (34.15%) and Actinobacteria (9.75%). Most endophytes were isolated from stems (21) and roots (14), while only six were from the leaves. This is, to our knowledge, the first report where bacterial strains related to Curtobacterium sp., Ensifer sp., Kocuria sp., Mesorhizobium camelthorni, Mycobacterium cosmeticum, Pseudoxanthomonas indica, Paenibacillus panacisoli and Neorhizobium huautlense were fast grown on nitrogen-free medium, and their nitrogenase activities assessed through acetylene reduction assay. In D. esculenta, the highest nitrogenase activity (57.4 nmol C2H4tube-1day-1.) was recorded in the isolate E1T0R_Y6, belonging to the genus Bradyrhizobium sp., while it was 164 nmol C2H4tube-1day-1 in D. alata, strain A23T1R_Y40, a strain related to the bacterial genus Ensifer sp. The plant growth promotion activities (PGP) of these isolates are to be investigated for selecting most promising nitrogen-fixing bacteria of yams.
Keywords: Diazotrophes, Dioscorea, endophytes, yams
Contact Address: Michel Ouyabe, Tokyo University of Agriculture, Tamaku Mita 2-3297, 214-0034 Kawasaki, Japan, e-mail: miki.1983.ouyabegmail.com