| In the present study, the common floating plant—Spirodela polyrrhiza Schleid was chosen as the experimental material, rare earth elements (REEs)—terbium (Tb) and neodymium (Nd) were chosen as the stress factors. The mechanisms of REEs toxicity in floating plant were investigated by applying technology of plant physiology and biochemistry, combined with comparative proteomics technology. The results are as follows:1. The effects of Tb concentrations of 0-50 μmol·L-1 applied for 14 d were assessed by measuring changes in subcellular distribution, surface morphology, mineral nutrition, photosynthetic pigments, reactive oxygen species, antioxidant system, soluble protein, ultrastructure and proteomics in S. polyrrhiza Schleid. Our results indicated that, (1) Tb stress led to a substantial accumulation of Tb in S. polyrrhiza Schleid, the highest concentration of Tb was found in the cell wall (74-79%), followed by the cell organelles (12-14%) and then the soluble fraction (8-12%). (2) Tb stress resulted in serious imbalance of nutrient elements, such as inhibiting the absorption of Zn, Ca, K, Mg, B and S, promoting the content of Cu, Mn, Fe and P. (3) Tb treatment also severely affected the surface morphology of leaf and reduced photosynthetic pigments (chlorophyll a, chlorophyll b, carotenoids) concentration. (4) The content of soluble protein reached a peak value at 20 μmol·L’-1 and then decreased, as well as the antioxidant system significant changed under Tb stress. A large number of accumulation of superoxide anion (O2-) and hydrogen peroxide (H2O2) caused the content of malondialdehyde (MDA) increased in S. polyrrhiza Schleid. In comparison with the control, Tb treatment induced the activities of superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR) and ascorbate peroxidase (APx) decreased, while the anctivity of peroxidase (POD) increased. Meanwhile, the exogenous Tb enhanced the content of micromolecule antixoidants such as glutathione (GSH), ascorbic acid (AsA), non-protein thiols (NP-SH) and phytochelatins (PCs) in S. polyrrhiza Schleid. (5) Morphological symptoms of senescence, such as chlorosis and damage to chloroplasts, mitochondria and cell nucleus were observed under Tb stress. (6) Proteomics analysis found that, in response to Tb stress, S. polyrrhiza Schleid exhibited 50 differential expressed proteins:45 up-regulated and 5-down-regulated. These significantly changed proteins were further indentified by MALTI-TOF MS and may be approximately classified into seven functional categories, such as photosynthesis, energy, transport, cell rescue, signal transduction, transcription and translation, unkown, etc. Among them, the proteins related to photosynthesis, energy metabolism, transcription and translation take relatively major proportion.2. After exposing S. polyrrhiza Schleid to various concentrations of Nd (0,20,40, 60,80 μmol·L-1) for 14 d, the proportion of leaf area damaged (PLAD), photosynthetic pigments, fatty acids, ROS, soluble protein, photosystem, antioxidant system, thylakoid membrane proteins and ultrastrure were investigated. Our study demonstrated that, (1) the influences of exogenous Nd in S. polyrrhiza Schleid led to PLAD increase, the content of photosynthetic pigments and soluble protein decrease. (2) With the increasing concentration of Nd, O2- generating rate and H2O2 content progressively rose, the behavior induced the decrease in unsaturated fatty acids and the increase in the peroxidation of membrane lipid, as well as enhanced the content of MDA. Thereby, the structure of cell membrance was damaged. Meanwhile, the exogenous Nd enhanced the activities of POD and APx, decreased the activities of SOD and GR, while the activity of CAT initial rose and followed by declined. It also significantly enhanced the levels of AsA and PCs, but the levels of GSH and NP-SH rose in the first stage, and then decreased. (3) Thylakoid membrane proteins and the activity of photosystem Ⅱ (PS Ⅱ) were changed significantly compared with the control.23kDa,25kDa,130kDa thylakoid membrane proteins were changed significantly under Nd stress, the changes in the peptide components content might involve in the PS Ⅱ changes.(5) It was found by transmission electron microscope that exogenous Nd induced the damage of chloroplasts, such as swelling of chloroplast, vacuolation of the thylakoid and disorderly arrangement of cristae under the lower Nd level. The extent of ultrastuctural damage was much more serous with the increasing of Nd concentration:disappearing of the outer membrances of chloroplast, vacuolization and further disintegration of chloroplast.3. The results of exogenous REEs in physiological and biochemical indexes in S. polyrrhiza demonstrated that accumulation of REEs were highest in cell walls, they changed surface morphology of plant, imbalance of nutrient elements, photosystem, antioxidant system, protein components and organelle structure. These results suggested that the exogenous REEs in water bodies might produce harmful effects on 5. polyrrhiza Schleid. |
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