Enhanced phytoremdiation of Robinia pseudoacacia in heavy metal-contaminated soils with rhizobia and the associated bacterial community structure and function.

Heavy metals can cause serious contamination of soils, especially in mining regions. A detailed understanding of the effects of heavy metals on plants and root-associated microbial communities could help to improve phytoremediation systems. In this study, black locust (Robinia pseudoacacia) seedlings with or without rhizobial inoculation were planted in soils contaminated with different levels of heavy metals. Bacterial communities in rhizosphere and bulk soil samples were analyzed using 16S rRNA gene sequencing on the Illumina MiSeq platform and shotgun metagenome sequencing on the Illumina HiSeq platform. Soil bacterial communities varied significantly depending on the level of soil contamination, and planting also had some influence. Although inoculation of Mesorhizobium loti HZ76 (a natural microsymbiont of R. pseudoacacia) was a relatively minor factor, it did influence the soil bacterial community. Under the selective pressure, plant growth promotion-related biomarkers in the rhizosphere increased after inoculation compared with non-inoculated controls, especially those associated with Mesorhizobium, Variovorax, Streptomyces, and Rhodococcus genera. Genes encoding ATP-binding cassette transporters were up-regulated in the rhizosphere after inoculation compared with genes related to sulfur/nitrogen metabolism. These results provide insight into soil bacterial communities and their functions in the R. pseudoacacia rhizosphere in response to rhizobial inoculation and heavy metal contamination. This knowledge may prove useful for improving phytoremediation of metal-contaminated soils.

Fan M ，Xiao X ，Guo Y ，Zhang J ，Wang E ，Chen W ，Lin Y ，Wei G ... -  《-》

Isolation, characterization, and selection of heavy metal-resistant and plant growth-promoting endophytic bacteria from root nodules of Robinia pseudoacacia in a Pb/Zn mining area.

Multiple heavy metals (HMs) commonly coexist in mining areas, which highlights the necessity to select multiple HM-resistant plant growth-promoting bacteria for improving phytoremediation efficiency. In this study, we isolated and characterized 82 endophytic bacteria from the root nodules of black locust (Robinia pseudoacacia) grown in a Pb-Zn mining area. There were 80 isolates showing resistance to four HMs, 0.01-18.0 mM/L for Cd, 0.2-40.0 mM/L for Zn, 0.3-2.2 mM/L for Pb, and 0.2-1.4 mM/L for Cu. Indole-3-acetic acid production, siderophore production, and 1-aminocyclopropane-1-carboxylate deaminase activity were detected in 43, 50, and 17 isolates, respectively. Two symbiotic isolates selected with the highest potential for HM resistance and PGP traits, designated Mesorhizobium loti HZ76 and Agrobacterium radiobacter HZ6, were evaluated for promotion of plant growth and metal uptake by R. pseudoacacia seedlings grown in pots containing different levels of Cd, Zn, Pb, or Cu. HZ76 significantly increased plant shoot biomass, while HZ6 did not, compared with non-inoculated controls. The results indicate that inoculation with HZ76 or HZ6 relieved HM stress in the plants, depending on the type and concentration of HM in the treatment. Mesorhizobium loti HZ76 may be a better candidate for application in phytoremediation than A. radiobacter HZ6. The microsymbiosis between HM-resistant rhizobia and R. pseudoacacia is an interesting mutualistic system for phytoremediation in mining areas contaminated with multiple HMs.

Fan M ，Liu Z ，Nan L ，Wang E ，Chen W ，Lin Y ，Wei G ... -  《-》

Elevated CO(2) benefits the soil microenvironment in the rhizosphere of Robinia pseudoacacia L. seedlings in Cd- and Pb-contaminated soils.

Soil contamination by heavy metals in combination with elevated atmospheric CO2 has important effects on the rhizosphere microenvironment by influencing plant growth. Here, we investigated the response of the R. pseudoacacia rhizosphere microenvironment to elevated CO2 in combination with cadmium (Cd)- and lead (Pb)-contamination. Organic compounds (total soluble sugars, soluble phenolic acids, free amino acids, and organic acids), microbial abundance and activity, and enzyme activity (urease, dehydrogenase, invertase, and β-glucosidase) in rhizosphere soils increased significantly (p < 0.05) under elevated CO2 relative to ambient CO2; however, l-asparaginase activity decreased. Addionally, elevated CO2 alone affected soil microbial community in the rhizosphere. Heavy metals alone resulted in an increase in total soluble sugars, free amino acids, and organic acids, a decrease in phenolic acids, microbial populations and biomass, and enzyme activity, and a change in microbial community in rhizosphere soils. Elevated CO2 led to an increase in organic compounds, microbial populations, biomass, and activity, and enzyme activity (except for l-asparaginase), and changes in microbial community under Cd, Pb, or Cd + Pb treatments relative to ambient CO2. In addition, elevated CO2 significantly (p < 0.05) enhanced the removal ratio of Cd and Pb in rhizosphere soils. Overall, elevated CO2 benefited the rhizosphere microenvironment of R. pseudoacacia seedlings under heavy metal stress, which suggests that increased atmospheric CO2 concentrations could have positive effects on soil fertility and rhizosphere microenvironment under heavy metals.

Huang S ，Jia X ，Zhao Y ，Bai B ，Chang Y ... -  《-》

Physiological tolerance of black locust (Robinia pseudoacacia L.) and changes of rhizospheric bacterial communities in response to Cd and Pb in the contaminated soil.

Luo XF ，Liu MY ，Tian ZX ，Xiao Y ，Zeng P ，Han ZY ，Zhou H ，Gu JF ，Liao BH ... -  《-》

Ancient Heavy Metal Contamination in Soils as a Driver of Tolerant Anthyllis vulneraria Rhizobial Communities.

Anthyllis vulneraria is a legume associated with nitrogen-fixing rhizobia that together offer an adapted biological material for mine-soil phytostabilization by limiting metal pollution. To find rhizobia associated with Anthyllis at a given site, we evaluated the genetic and phenotypic properties of a collection of 137 rhizobia recovered from soils presenting contrasting metal levels. Zn-Pb mine soils largely contained metal-tolerant rhizobia belonging to Mesorhizobium metallidurans or to another sister metal-tolerant species. All of the metal-tolerant isolates harbored the cadA marker gene (encoding a metal-efflux PIB-type ATPase transporter). In contrast, metal-sensitive strains were taxonomically distinct from metal-tolerant populations and consisted of new Mesorhizobium genospecies. Based on the symbiotic nodA marker, the populations comprise two symbiovar assemblages (potentially related to Anthyllis or Lotus host preferences) according to soil geographic locations but independently of metal content. Multivariate analysis showed that soil Pb and Cd concentrations differentially impacted the rhizobial communities and that a rhizobial community found in one geographically distant site was highly divergent from the others. In conclusion, heavy metal levels in soils drive the taxonomic composition of Anthyllis-associated rhizobial populations according to their metal-tolerance phenotype but not their symbiotic nodA diversity. In addition to heavy metals, local soil physicochemical and topoclimatic conditions also impact the rhizobial beta diversity. Mesorhizobium communities were locally adapted and site specific, and their use is recommended for the success of phytostabilization strategies based on Mesorhizobium-legume vegetation. Phytostabilization of toxic mine spoils limits heavy metal dispersion and environmental pollution by establishing a sustainable plant cover. This eco-friendly method is facilitated by the use of selected and adapted cover crop legumes living in symbiosis with rhizobia that can stimulate plant growth naturally through biological nitrogen fixation. We studied microsymbiont partners of a metal-tolerant legume, Anthyllis vulneraria, which is tolerant to very highly metal-polluted soils in mining and nonmining sites. Site-specific rhizobial communities were linked to taxonomic composition and metal tolerance capacity. The rhizobial species Mesorhizobium metallidurans was dominant in all Zn-Pb mines but one. It was not detected in unpolluted sites where other distinct Mesorhizobium species occur. Given the different soil conditions at the respective mining sites, including their heavy-metal contamination, revegetation strategies based on rhizobia adapting to local conditions are more likely to succeed over the long term compared to strategies based on introducing less-well-adapted strains.

Mohamad R ，Maynaud G ，Le Quéré A ，Vidal C ，Klonowska A ，Yashiro E ，Cleyet-Marel JC ，Brunel B ... -  《-》