Mechanism Of Lead Uptake And Tolerance In Typical Accumulator Plants

The bioavailability of heavy metals in soil,the uptake and tolerance of heavy metals by plants are important factors that limit remediation efficiencies.Heavy metal lead?Pb?in tailings and soils of lead-zinc mining area often exists as lead-containing minerals,which is low bioavailable and difficult to be taken up by plants.Under natural conditions or with the action of plant roots,the surface of lead minerals will be weathered and release toxic element Pb,which not only damages plant growth,but also harms the health of human beings through the food chain.Root exudates play important role in the interaction between plants and minerals,which not only promote the weathering of minerals and the release of elements,but also affect the uptake and accumulation of these elements by plants.Exploring the weathering process of lead minerals induced by plant,as well as the uptake,distribution,translocation and speciation transformation of Pb in mineral/soil-plant systems,which are helpful to understand the microscopic mechanism of root Pb uptake from Pb minerals.It also reveals the role of rhizosphere organic acids in the release of mineral elements and the uptake of Pb,as well as the tolerance mechanism of Pb in the cells of accumulator plants.These can lay a theoretical foundation for the efficient phytoremediation for the Pb polluted soil.In this paper,based on comparing the ability of Pb uptake and accumulation of four plants in the field,the action mechanism of organic acids at the interface of lead mineral-root and the tolerance mechanism of Pb in plant cells were investigated.Various analytical techniques were used in the study,including micro-X-ray fluorescence?Micro-XRF?,X-ray absorption near edge structure?XANES?and transmission electron microscopy?TEM?energy dispersive X-ray spectroscopy?EDS?.We systematically studied the release of lead from galena,Pb-goethite and pyromorphite under the action of various organic acids and the uptake of Pb from lead minerals by Bidens pilosa L.It was also investigated that the effects of citric acid?natural low weight molecule organic acid?,humic acid?natural high weight molecule organic acid?and EDTA?synthetic chelating agent?on the uptake of Pb by plants,the transformation of the subcellular cells binding state of Pb and the translocation of Pb into shoots.Detoxification and tolerance mechanisms of Pb in plant cells were also discussed.The main results are as follows:?1?The release of lead induced by root exudates,humus and EDTA is not only related to the types of lead minerals,but also closely related to the formation of lead-organic acid complexes on the surface of minerals and the coordination structures of Pb-organic acids.The results of XANES showed that galena,Pb-goethite and pyromorphite are the three main lead minerals in the soil of lead-zinc mining area.Mineral dissolution experiments showed that EDTA was the organic acid with a strongest ability of Pb release from the three lead minerals,and humic acid was the humus with a strongest ability of Pb release from galena and pyromorphite.Moreover,citric acid,histidine and oxalic acid were the most potent root exudates for driving Pb release from galena,Pb-goethite and pyromorphite,respectively.Strong Pb-EDTA interactions originate from six strong donor atoms in EDTA?two nitrogen atoms of amines and four oxygen atoms of carboxylates?.Humic acid is a kind of natural macromolecular organic acid,which contains a variety of functional groups,such as-COOH?-NH₂?-OH and Ar-OH,can form complex with Pb.Citric acid?CA?and oxalic acid?OA?contain three and two carboxylic groups,respectively.These carboxyl groups in the molecular structure of CA and OA are more easily complexed with Pb.?2?Bidens pilosa L.has a potential to absorb and accumulate Pb from galena,Pb-goethite and pyromorphite.The release,translocation and transformation of Pb in the mineral-root interface are the important prerequisites for the Pb uptake from mineral sources by root.It was found that the ability of Pb upatke by Bidens pilosa L.from Pb minerals decreased in the order galena>Pb-goethite>pyromorphite through the laboratory cultivation experiment.The results of XANES showed that galena and anglesite were weathered into lead stearate((C₁₇H₃₅COO)₂Pb)and cerussite?PbCO₃?,and Pb-goethite was weathered into lead carbonate?2PbCO₃?Pb?OH?₂?and Pb₃?PO₄?₂ under the action of Bidens pilosa L.The speciation of Pb in the rhizosphere solution released from the three Pb minerals was similar,which were Pb-organic ligands complexes.After these Pb in the rhizosphere solution was absorbed by Bidens pilosa L.roots,it was mainly deposited in the roots as Pb-phosphate precipitate.This revealed the process of roots Pb uptake from minerals.?3?Rhizosphere organic acids play an important role in improving the bioavailability of soil Pb and the ability of Pb uptake and translocation by roots,which can promote the uptake and accumulation of Pb mainly through changing the uptake pathway of Pb in roots,increasing the detoxification ability of root cells for Pb and improving the translocation ability of stem cells for Pb.Compared with citric acid?CA?and humic acid?HA?,EDTA can promote the transport of Pb from roots to shoots.Uptake pathway experiments showed that Pb was taken up by Bidens pilosa L.through the symplast pathway,calcium channel and part of potassium channel without the addition of EDTA,while it was absorbed through the symplastic pathway,apoplastic pathway,transporter and potassium channel with the addition of EDTA.Micro-XRF analysis showed that EDTA alleviated the deposition of Pb in the root meristem of root apex and the junction of lateral root zone and increased the distribution of Pb in the stem vascular bundles.The extraction experiments of plant cells showed that EDTA significantly increased the proportion of water soluble Pb complexes situated in cytosol and vacuole and Pb species in cell wall,while decreased the proportion of Pb in residue state.The results of XANES showed that Pb-EDTA was the main Pb speciation in the organs of plants under the condition of EDTA treatment.It was suggested that EDTA promoted the transport of Pb from roots to shoots by affecting the uptake pathway of Pb,the distribution of Pb in roots and stems and the transformation of Pb species.The extraction experiments of plant cells showed that the proportion of residual Pb in root cells and the proportion of cell-bound Pb which was easily transfered in stem cells increased with the addition of organic acids.The higher the proportion of residual Pb in root cells,the more Pb absorbed by roots,and the higher the proportion of cell-bound Pb that was easily transfered in stem cells,and the more Pb accumulated in leaves.?4?Understanding the damage of Pb to cell ultrastructure and the distribution and speciation of Pb in cells at the cellular/subcellular scale is the key to exploring the tolerance and detoxification mechanism of Pb in plants.The extraction experiments of plant cells showed that the proportion of cell-bound Pb decreased in the order residual state>hydrophobic proteins binding state>water-soluble in cytoplasm and vacuoles for the roots cells of Bidens pilosa L.and Bidens bipinnata Linn.However,in stems cells,the proportion of cell-bound Pb decreased in the order residual state>water-soluble in cytoplasm and vacuoles>hydrophobic proteins binding state.In leaves cells,Pb was mainly distributed in the water-soluble state of the cytoplasm and vacuole,followed by cell-bound Pb.In general,the proportion of residual Pb decreased gradually,while the proportion of water-soluble Pb in cytoplasm and vacuoles increased gradually during the process of Pb translocation from roots to shoots.The results indicated that the distribution of cell-bound Pb was related to the location of cells.The result of TEM-EDS showed that Pb exposure could lead to cells and subcellular organelles distortion,plasmolysis,plasma membrane blurring,intercellular space narrowing and cell wall thickening and some folds.With the increase of Pb concentration,the plant cells ultrastructural damage levels gradually increased.Cell walls,vacuoles and intercellular spaces were the major position of Pb deposition in plant cells,which prevented cells from disrupting the normal physiological and metabolic function by immobilization and sequesteration free Pb.