Effects Of Rare Earth Element Lanthanum (Ⅲ) On Cell Membrane Of Horseradish

The applications of rare earth elements （REEs） have made the REEs enter into the plantecosystem, and influenced the growth and physiological function of plants, which has beenconcerned by the scientific community. However, the influence mechanism of REEs on thephysiological function of plants is still unclear, and it also is the difficulty and focus problemin related interdisciplinary. The investigation on the distribution, action location, action siteand cytochemical behavior of the REEs in plants are the important pathways for exploring theinfluence mechanism of the REEs affect on the physiological function of plants. Obviously,the plasma membrane is the first target affected by the REEs, because the REEs entering thecells need to pass through the plasma membrane. The selective interaction between receptorsand signaling molecules in cell surface resulted in many physiological and biochemicalreactions in cells, and reflected on the biological effects process in the overall cells. In thispaper, we chose horseradish as the experiment materials, light rare earth La³⁺as a probe,respectively, and used the integrated multidisciplinary methods of flow cytometry,immunofluorescence labeling, scanning electron microscopy, transmission electronmicroscopy, laser confocal microscopy, computer simulation, non-electrical activity of iondetection device, and autoradiography to explore the distribution, action location, action siteand cytochemical behaviors and styles of rare earth in horseradish cells, and revealed themicro-chemical processes of rare earth acting on membrane structure, property and functionin horseradish cells. The results will provide cell and molecular level scientific basis forelucidating the botany mechanism and food safety of REEs.The main results are summarized as follows:1. La³⁺would not enter into the cells as it would mainly distribute on the plasmamembrane when horseradish treated with La³⁺at the low concentration. La³⁺, as a Lewis acid,can coordinate with Lewis base atoms on extracellular matrix and extracellular materialmolecules in horseradish cells, such as O atoms. And then La³⁺can be anchored on plasmamembranes which appeared as rare earth biomolecules complexes in nanoscale, which is rareearth nano-biocomplexes. The ligand-binding anchoring affect the original configuration ofthe molecules on/in the plasma membrane, and then affect the constitution, structure andfunction of horseradish cells by the signal transduction and gene control of plasmamembranes.2. The results showed that the location where La³⁺acting on cells moved to cell interiorwhen horseradish treated with La³⁺at the high concentration. With the increase of theconcentration of La³⁺, the action sites of La³⁺acting on plasma membranes were increased;That is, La³⁺can coordinate with O atoms in protein on/in membrane, membranephospholipids and the part of channel protein outside the membrane to form numerous rareearth nano-biocomplexes in different nano scales. It was clearly observed that the rare earth nano-biocomplexes were transported into cells by endocytosis. Firstly, a part of plasmamembrane invaginated into a pit, which encapsulated the La³⁺nano-biocomplex particles.Secondly, the pit gradually developed into a bud, and then pinched off as a vesicle intocytoplasm. Finally, the bud was fused with lysosome or lytic vacuole in cytoplasm andentered into the cellular compartments. Some nano-biocomplexes with too much La³⁺bustedwhen they were being dissolved by lysosome, and then numerous La³⁺nano complexes indifferent scales were released from the buds into cytoplasm, which was an unusualendocytosis. Then numerous La³⁺nano complexes were self-assembled to form the nano-scaleLa³⁺bioballs with different diameters （80nm-400nm） on the basis of similar consistencyprinciple.3. The cytology mechanisms of La³⁺at the low concentration influence horseradish areas follows.（1） La³⁺and biological molecules on/in plasma membrane formed rare earthnano-biocomplexes by multisite interactions, which activated the endocytosis of horseradishcells. The signal transduction and the genetic expression were started by forms rare earthnano-biocomplexes, resulting in a series of fast responses of cells. For example, Ca2+, animportant signal molecule, activated firstly. The content of Ca2+increased by20.70%compared with that of the control, which indicated that signal transduction was accelerated. Incomparison with those of the control, the content of macroelements, such as N and K, wereincreased by0.50%and131.70%, respectively; Meanwhile, the content of microelements,such as Mg, Fe and Zn, increased47.90%,23.00%and1.10%, respectively. The resultsindicated that the constitution and function of membrane were improved, and then the growthof horseradish cells was promoted.（2） It was observed through the cell dynamic microscopethat the series of responses promoted the endocytosis and exocytosis of plant in return. Thenutrient substance transported out from the cell interior added into the substances which werethe constitution, structure and nutrition of the plasma membrane and cell wall. And then theamount of plasma membrane and cell wall were increased; thereby, plasma membraneelasticity was promoted and the growth of cells was improved.4. The cytology mechanisms of La³⁺at the high concentration influence horseradishare as follows.（1） The number of binding site of La³⁺and biomolecules on plasma membraneincreased when horseradish were treated with La³⁺in high concentration, that is, the numberof rare earth nano-biocomplexes was increased. The increase induced to the change of themicrostructure of biological molecules on/in the plasma membrane, and then inhibited themolecular function of the plasma membrane.（2） It was appeared that endocytosis in cells wasover-activated by La³⁺when horseradish was treated with La³⁺at the high concentration,leading to the time of endocytosis and exocytosis shortened, namely, notable excessiveendocytosis. The excessive endocytosis made the amount of nutrient elements in theintra/extra cellular substance exchange increased. However, compared with those of thecontrol, the content of saturated fatty acid in plasma membrane was increased by20.80%,while the content of unsaturated fatty acid was decreased by9.1%. It indicated that thedamage of plasma membrane would happen following the excessive endocytosis, and then the plasma membrane elasticity was decreased.（3） A great amount of nutrition has beenconsumed during the excessive endocytosis caused by the treatment with La³⁺at the highconcentration. Compared with those of the control, the content of the nutrient elements suchas N, K, Mg, Fe and Zn decreased by2.00%,45.20%,5.80%,0.63%and47.60%,respectively. A great amount of nutrition was needed for the excessive energy dissipation;meanwhile, the amount of the intracellular nutrition elements was decreased, which wasdisadvantageous for the growth of the horseradish cells. Thus horseradish can do nothing forkeeping cells alive but decreasing the biosynthesis, that is, horseradish decreased thebiosynthesis by inhibiting the cells growth or lessening cells to resist the lethal damage fromLa³⁺at the high concentration. The strategy is a protection mechanism of self rescue.（4） AsLa³⁺entered into cells by endocytosis, an unusual endocytosis emerged. La³⁺self-assembledinto La³⁺nano-bioballs （80nm-400nm） in cytoplasm. Most of them can be transported tothe outside of cells by exocytosis reported by experiments, then the La³⁺nano-bioballsmigrated into soil through the roots. Therefore, the accumulation of rare earth in cell wasdecreased, and the damage to cells from the rare earth was also decreased.（5） Meanwhile, itwas found that a little number of La³⁺nano-bioballs deposited in cell interior, which do notchange with time. These La³⁺nano-bioballs, which disturbed the identification signals ofbiomolecules and lytic enzyme, can not be dissolved thus permanently accumulated in cellinterior.（6） The deposition of these La³⁺nano-bioballs induced the decrease of thebiological utilization of biomolecules and then inhibited the growth of the horseradish cells.Meanwhile, it could decrease the damage of the plasma membrane and organelle from La³⁺atthe high concentration by the La³⁺nano-bioballs deposited in cell interior, which was also adetoxification mechanism of decrease the cell damage in plants from La³⁺at the highconcentration.5. Plasma membrane is one of the major sites which La³⁺action on plants.（1） The sitewhich La³⁺acting on the horseradish cells from outward to inward followed the increase ofLa³⁺concentration. The distribution, action location, action site and cytochemical behavior ofLa³⁺in the horseradish leaf cells depended on the concentration of La³⁺. Meanwhile, thepositive effects at the low concentrations and the negative effects at the high concentrations ofLa³⁺depended on the distribution, location and sites of La³⁺acting on the plasma membrane.（2） The location and sites of La³⁺acting on the plasma membrane can directly start the signaltransduction and genetic expression of plasma membrane, lead to induce a series ofintracellular responses and regulate the constitution, structure and function of plasmamembrane. Regardless of the concentration is low or high, the plasma membrane, which is thefirst protective screen that La³⁺must pass through into cells, and being one of the major siteswhich La³⁺acts on horseradish.