Abstract
AimsWetlands play vital roles as sinks for metal contaminants. Some wetland plants accumulate manganese (Mn) oxides in the black biofilm around roots and rhizomes, although the underlying mechanism is still unclear. Our aim is to determine the role of endophytic bacteria in the formation of Mn deposits in the wetland plant Suaeda salsa Pall. as well as the underlying chemical and molecular mechanisms.MethodsManganese-oxidizing endophytic bacteria were isolated with leucoberbeline blue (LBB) and further identified via the phylogenetic analysis. The Mn content and black deposit characteristics of laboratory-cultivated plants before/after co-cultivation of bacteria were investigated by inductively-coupled plasma optical emission spectrometry (ICP-OES), a scanning electron microscope equipped with an energy energy-dispersive X-ray spectroscopye (SEM-EDX), and X-ray fluorescence (XRF). The chemical structures of the biogenic Mn minerals were characterized via spectra of X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), and selected area electron diffraction (SAED). Proteomic analyses, coupled with the enzymic assays were performed to identify the enzymes involved in the Mn oxidation.ResultsWe observed black deposits containing Mn oxides in the belowground and aboveground tissues of S. salsa. Three Mn-tolerant bacterial strains were isolated from the plants, and two of them possessed Mn(II) oxidation capacities, which were identified as Pantoea eucrina SS01 and Pseudomonas composti SS02. Co-cultivation of the two isolates with S. salsa showed promoted plant growth and facilitated the formation of black precipitations on roots. Further results showed the different chemical compositions and cellular localizations of biogenic Mn oxides from the two strains. Hydrogen peroxide-detoxifying enzymes were involved in Mn oxidation, most likely mitigating oxidative stresses.ConclusionsWe suggest a role of endophytic bacteria in Mn uptake and accumulation in the wetland plant S. salsa; our study thereby contributes to a better understanding of the plant-endophyte symbiosis in biogeochemical Mn cycling and wetland soil phytoremediation.
Highlights
Wetlands are often considered as sinks for metal contaminants
Three Mn-tolerant bacterial strains were isolated from the plants, and two of them possessed Mn(II) oxidation capacities, which were identified as Pantoea eucrina SS01 and Pseudomonas composti SS02
We suggest a role of endophytic bacteria in Mn uptake and accumulation in the wetland plant S. salsa; our study thereby contributes to a better understanding of the plant-endophyte symbiosis in biogeochemical Mn cycling and wetland soil phytoremediation
Summary
In June, we collected plants of S. salsa Pall. from the coastal area of the Yellow River, China. Plants from G2 contained lower amounts of Mn, namely 9, 4, and 4 mg/kg for root, shoot, and leaves, respectively (Fig. S2) These results indicate that the presence of black precipitates was associated with elevated Mn concentrations in the tissues of S. salsa Pall. The XRF showed the presence of Mn in the black precipitates of the plants cultivited with strains SS01 and SS02, whereas no Mn signal could be detected for untreated plant roots (Fig. S4) These results indicate the involvement of endophytic Mn-oxidizing bacteria in the accumulation of Mn oxides in S. salsa Pall.plants. We tried to isolate and purify the proteins responsible for Mn oxidization in strain SS01, and the active components were tested by LBB staining and collected after DEAE-Sepharose chromatography; SDS-PAGE showed that the band of the active parts was weak (Fig. 5d), and was identified as catalase by mass spectrometry (Table 2). The activities of the protein solutions were determined by LBB methods
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