Physio dynamism related to phytodesalination and phytoremediation of copper in Volkameria inermis L

Abstract

The present study was carried out to investigate the NaCl and CuSO4-induced physiological dynamism in Volkameria inermis L., a mangrove-associated plant. The healthy stem cuttings of the plants were collected fromthe natural habitats and grown in the polyhouse. After 80d of growth, therooted cuttings were transferred to 300 mL half-strength modified Hoaglandmedium. The healthy plantlets of V. inermis were then subjected to varyingconcentrations of NaCl (0, 100, 200, 300, 400, and 500 mM) and CuSO4 (0, 100,150, 200 and 250 µM) for 20d under hydroponic culture conditions. From theresults of the preliminary phase of the study, it was clear that V. inermiscould effectively tolerate up to 400 mM of NaCl and 200 µM CuSO4, anddeleterious stress effects were shown in plantlets grown in concentrationabove it. Further detailed analysis was carried out to investigate thetolerance potential of the V. inermis plantlets subjected to variousconcentrations of NaCl and CuSO4 in the tolerance limit. The plant growthunder NaCl and CuSO4 was assessed using various plant growth parameterssuch as fresh and dry weight, leaf area, tissue water content, and toleranceindex. The supra optimal concentration of NaCl and CuSO4 affects plantgrowth negatively and at the same time, the lower modest concentration ofNaCl (100 mM) and CuSO4 (100 µM) exhibits growth promotion ascompared to control plantlets. The stress imparted by differentconcentrations of NaCl and CuSO4 was evaluated by assessing the stressintensity factors such as accumulation of hydrogen peroxide (H2O2),superoxide (O2•-), membrane stability index (MSI), electrolyte leakage, andextent of lipid peroxidation. The exposure of V. inermis to higher NaCl andCuSO4 treatments causes oxidative stress by the upregulation of the reactiveoxygen species such as H2O2 and O2•-, which causes damages to thecomponents of photosynthesis and affects the plant growth negatively.Hence, the photosynthetic efficacy was modulated under differentconcentrations of NaCl and CuSO4. The total chlorophyll content in theleaves of the plantlets decreased with increasing concentrations of NaCl andCuSO4 and lower concentrations of NaCl and CuSO4 enhanced the totalchlorophyll content over control. The osmotic stress induced by NaCl andCuSO4 was countered by the upregulation of osmolytes in V. inermis. Theenhanced accumulation of osmoprotectants safeguards the organelles, bio-membrane, and cytosolic enzymes. The augmented antioxidative machineryincluding non-enzymatic antioxidants (total phenolics, ascorbate,glutathione, and flavonoids) and enzymatic antioxidants (superoxidedismutase, ascorbate peroxidase and guaiacol peroxidase) in V. inermisactively scavenge reactive oxygen species (ROS) and maintains the cellularhomeostasis under stress. The higher bioaccumulation of sodium andcopper in the root tissues and lower translocation of the same to the shootregions proved its phytostabilization ability. The subcellular distributionstudies in the different fractions of plantlets exhibit higher sequestration ofNa in the soluble fraction and Cu in the cell wall fraction. The Cu ions werefound to be complexed with the polysaccharides of cell walls and therebylimiting the cellular toxicity to a greater extend. The sequestration of toxicions in the cell wall and wall debris fraction of roots reduce the metaltoxicity reaching the leaves and thus protects the photosynthetic machinery.Higher level of cell wall lignification helps to sequester toxic metal in the cellwall with the support of carboxyl and hydroxyl groups of the lignin whichtightly binds the Cu ions. Moreover, the higher lignin content specificallyprovides rigidity and hydrophobicity, which supports in encountering themetal toxicity. Higher lignified cells in the root, especially in the xylemwalls, validate the Cu phytostabilization potential of V. inermis. Themodulations in the secondary metabolites were analysed by HR-LCMS anda higher number of compounds were detected in treated plantlets, which areknown to play roles in metal chelation and stress tolerance. The increasedgene expression of phenylalanine ammonia-lyase and phytochelatinsynthase genes shows the activation of the phenylpropanoid pathway forproducing different phenolic secondary metabolites, having a role in metalchelation in V. inermis. This study clearly shows that V. inermis is a goodcandidate for phytodesalination and copper phytoremediation in NaCl andcopper-affected areas since it is equipped with strong tolerance strategies.