Polyphasic characterization of rhizobia isolated from Leucaena leucocephala from Panxi, China

Rhizobia population was favoured during in situ phytoremediation of vanadium-titanium magnetite mine tailings dam using Pongamia pinnata

Mine tailings contain toxic metals and can lead to serious pollution of soil environment. Phytoremediation using legumes has been regarded as an eco-friendly way for the rehabilitation of tailings-laden lands but little is known about the changes of microbial structure during the process. In the present study, we monitored the dynamic change of microbiota in the rhizosphere of Pongamia pinnata during a 2-year on-site remediation of vanadium-titanium magnetite tailings. After remediation, overall soil health conditions were significantly improved as increased available N and P contents and enzyme activities were discovered. There was also an increase of microbial carbon and nitrogen contents. The Illumina sequencing technique revealed that the abundance of taxa under Proteobacteria was increased and rhizobia-related OTUs were preferentially enriched. A significant difference was discovered for sample groups before and after remediation. Rhizobium and Nordella were identified as the keystone taxa at genus rank. The functional prediction indicated that nitrogen fixation was enhanced, corresponding well with qPCR results which showed a significant increase of nifH gene copy numbers by the 2nd year. Our findings for the first time elucidated that legume phytoremediation can effectively cause microbial communities to shift in favour of rhizobia in heavy metal contaminated soil.

Inoculation of Sinorhizobium saheli YH1 Leads to Reduced Metal Uptake for Leucaena leucocephala Grown in Mine Tailings and Metal-Polluted Soils

Metalliferous mine tailings have a negative impact on the soil environment near mining areas and render cultivable lands infertile. Phytoremediation involving the synergism of legume and rhizobia provides a useful technique in tackling this issue with cost-effective, environmentally friendly, and easy-to-use features under adverse soil conditions. Leucaena leucocephala has been found to build symbiotic relationships with native rhizobia in the iron-vanadium-titanium oxide (V-Ti magnetite) mine tailing soil. Rhizobia YH1, isolated from the root nodules of L. leucocephala, was classified as Sinorhizobium saheli according to similarity and phylogenetic analyses of 16S rRNA, housekeeping and nitrogen fixation genes. Besides nitrogen fixation, S. saheli YH1 also showed capabilities to produce indole-acetic acid (IAA) (166.77 ± 2.03 mg l⁻¹) and solubilize phosphate (104.41 ± 7.48 mg l⁻¹). Pot culture experiments showed that strain YH1 increased the biomass, plant height and root length of L. leucocephala by 67.2, 39.5 and 27.2% respectively. There was also an average increase in plant N (10.0%), P (112.2%) and K (25.0%) contents compared to inoculation-free control. The inoculation of YH1 not only reduced the uptake of all metals by L. leucocephala in the mine tailings, but also resulted in decreased uptake of Cd by up to 79.9% and Mn by up to 67.6% for plants grown in soils contaminated with Cd/Mn. It was concluded that S. saheli YH1 possessed multiple beneficial effects on L. leucocephala grown in metalliferous soils. Our findings highlight the role of S. saheli YH1 in improving plant health of L. leucocephala by reducing metal uptake by plants grown in heavy metal-polluted soils. We also suggest the idea of using L. leucocephala-S. saheli association for phytoremediation and revegetation of V-Ti mine tailings and soils polluted with Cd or Mn.

Genetic Diversity and Symbiotic Efficiency of Nodulating Rhizobia Isolated from Root Nodules of Faba Bean in One Field

Thirty-one nodulating rhizobium strains were collected from root nodules of spring and winter type faba bean cultivars grown in micro ecoarea, i.e. the same field in Chengdu plain, China. The symbiotic efficiency and phylogeny of these strains were studied. Effectively nitrogen fixing strains were isolated from both winter type and spring type cultivars. Based on phylogenetic analysis of 16S rRNA gene and concatenated sequence of atpD, glnII and recA genes, the isolates were assigned as Rhizobium anhuiense and a potential new Rhizobium species. The isolates were diverse on symbiosis related gene level, carrying five, four and three variants of nifH, nodC and nodD, respectively. Strains carrying similar gene combinations were trapped by both winter and spring cultivars, disagreeing with the specificity of symbiotic genotypes to reported earlier faba bean ecotypes.

Mesorhizobium acaciae sp. nov., isolated from root nodules of Acacia melanoxylon R. Br

Three novel strains, RITF741T, RITF1220 and RITF909, isolated from root nodules of Acacia melanoxylon in Guangdong Province of China, have been previously identified as members of the genus Mesorhizobium, displaying the same 16S rRNA gene RFLP pattern. Phylogenetic analysis of 16S rRNA gene sequences indicated that the three strains belong to the genus Mesorhizobium and had highest similarity (100.0 %) to Mesorhizobium plurifarium LMG 11892T. Phylogenetic analyses of housekeeping genes recA, atpD and glnII revealed that these strains represented a distinct evolutionary lineage within the genus Mesorhizobium. Strain RITF741T showed >73 % DNA–DNA relatedness with strains RITF1220 and RITF909, but < 60 % DNA–DNA relatedness with the closest type strains of recognized species of the genus Mesorhizobium. They differed from each other and from their closest phylogenetic neighbours by presence/absence of several fatty acids, or by large differences in the relative amounts of particular fatty acids. While showing distinctive features, they were generally able to utilize a wide range of substrates as sole carbon sources based on API 50CH and API 20NE tests. The three strains were able to form nodules with the original host Acacia melanoxylon and other woody legumes such as Acacia aneura, Albizia falcataria and Leucaena leucocephala. In conclusion, these strains represent a novel species belonging to the genus Mesorhizobium based on the data obtained in the present and previous studies, for which the name Mesorhizobium acaciae sp. nov. is proposed. The type strain is RITF741T ( = CCBAU 101090T = JCM 30534T), the DNA G+C content of which is 64.1 mol% (T m).

Diversity and phylogeny of rhizobia associated with Desmodium spp. in Panxi, Sichuan, China

Thirty-four rhizobial isolates were obtained from root nodules of four wild Desmodium species growing in Panxi, Sichuan, China. According to the combined ARDRA and IGS-RFLP (CACAI) cluster analysis, Rhizobium, Pararhizobium and Mesorhizobium isolates outnumbered Bradyrhizobium isolates. In general, the isolates representing the same species from the same site clustered together. Furthermore, the four Desmodium species were all nodulated by more than one rhizobial species. AFLP and phenotypic analyses showed that the 34 isolates represented at least 32 distinct strains. None of the strains were found from more than one site or host, indicating a high degree of rhizobial diversity in Panxi. In the multilocus sequence analysis, the isolates were assigned to Pararhizobium giardinii, Bradyrhizobium japonicum, Mesorhizobium septentrionale, and to undescribed species of the genera Rhizobium, Bradyrhizobium and Agrobacterium.