Abstract
Heavy metals in soil, as selective agents, can change the structure of plant-associated bacterial communities and their metabolic properties, leading to the selection of the most-adapted strains, which might be useful in phytoremediation. Trifolium repens, a heavy metal excluder, naturally occurs on metal mine waste heaps in southern Poland characterized by high total metal concentrations. The purpose of the present study was to assess the effects of toxic metals on the diversity and metabolic properties of the microbial communities in rhizospheric soil and vegetative tissues of T. repens growing on three 70–100-years old Zn–Pb mine waste heaps in comparison to Trifolium-associated bacteria from a non-polluted reference site. In total, 113 cultivable strains were isolated and used for 16S rRNA gene Sanger sequencing in order to determine their genetic affiliation and for in vitro testing of their plant growth promotion traits. Taxa richness and phenotypic diversity in communities of metalliferous origin were significantly lower (p < 0.0001) compared to those from the reference site. Two strains, Bacillus megaterium BolR EW3_A03 and Stenotrophomonas maltophilia BolN EW3_B03, isolated from a Zn–Pb mine waste heap which tested positive for all examined plant growth promoting traits and which showed co-tolerance to Zn, Cu, Cd, and Pb can be considered as potential facilitators of phytostabilization.
Highlights
As a result of extensive anthropogenic industrial and agricultural activities, numerous soils are polluted with metals at concentrations that are toxic to ecosystems and to human health [1,2]
Since there exists a huge shortage of knowledge about the microbiome of the potential phytostabilizer T. repens, and its probable evolutionary adaptation to long-term metal exposure, the purpose of the present study was to determine the effect of metals on the diversity and phenotypic properties of the cultivable bacterial communities inhabiting soil, nodules, roots, and leaves of T. repens growing on three 70–100-year old Zn–Pb waste heaps (Bolesław, Bukowno, and Saturn) in comparison to T. repens originating from a reference non-polluted grassland in Bolestraszyce
In order to search for novel heavy metal tolerant strains equipped with plant growth promotion properties and determining their taxonomic position, 113 endophytes from leaves, roots, and nodules of T. repens as well as bacteria from the T. repens rhizosphere, originating from three 70–100-year old
Summary
As a result of extensive anthropogenic industrial and agricultural activities, numerous soils are polluted with metals at concentrations that are toxic to ecosystems and to human health [1,2]. Phytoremediation relies on various mechanisms which plants can use to deal with metal toxicity, avoiding the transfer of metals into the cells or activation of intracellular mechanisms, which provide the basis of the whole plant tolerance system against deleterious metals [9] Plants express their tolerance to heavy metals using a strategy of (i) accumulation, i.e., extracting toxic elements from soil and accumulating them in the harvestable, above-ground plants parts; or (ii) exclusion, which relies on the ability to exclude metals from the shoots, and concentrate them on/in roots, reducing their further spreading due to wind or water erosion, percolation to the groundwater and the risk of additional environmental harm [5,10,11,12]. Such evolutionary acquired adaptations of plants to withstand metal toxicity can be exploited for phytoextraction or phytostabilization, respectively
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