Toxicity Of Engineered Lanthanum Oxide Nanoparticles In Higher Plants And Molecular Mechanism Of Water Uptake Inhibition

Due to the increasing demands of engineered nanoparticles(ENPs)in medical,industrial and agricultural products,concerns about the environmental risks of ENPs exposure have increased.As the dominant primary producers in terrestrial ecosystems,higher plants represent a sensitive receptor of concern.However,the mechanisms of ENPs phytotoxicity remain unknown.Phytotoxicity could be caused by ENPs exposure at physiological,biochemical,and molecular levels.In this research,we studied the toxicity effect of lanthanum oxide nanoparticles(La2O3 NPs)on maize(Zea mays L.)and radish(Raphanus sativus L.)seedlings.Furthermore,the molecular mechanisms underlying phytotoxicity,aquaporin genes regulation,ABA-involved signal transduction in maize seedlings and the quality influence in radish tuber exposed to La2O3 NPs were studied.The main results were summarized as follows:1.The biomass of maize seedlings was significantly reduced upon exposure to La2O3 NPs at 50,250 and 500 mg L-1.Maize root biomass was decreased by 22.73%at 6 d of exposure to 500 mg L-1 La2O3 NPs as compared to the control.The roots got thicker and shorter;tissue shrinkage and collapse were also observed.La2O3 NPs exhibited significant nanotoxicity.2.La2O3 NPs significantly weakened the water uptake of maize seedlings.High suppository level and early differentiation of Casparian strip occurred in maize roots upon the exposure of 50 mg L'1 La2O3 NPs treatment for 3 d.Meanwhile,the protein level of ZmPIPl was increased in roots,and the gene levels of most AQPs were decreased in both shoots and roots.The downregulation of AQP genes might reduce the water uptake in maize seedlings,enventualy leading to the growth inhibition.3.Abscisic acid(ABA)was induced in maize seedlings exposed to La2O3 NPs.It could trigger the ABA signaling pathway to activate the antioxidant defense system and induce transcription factors that regulate the expression of several ABA-responsive genes,including AQPs.4.The nutrients were reduced in both shoots and roots under exposure of La2O3 NPs.The causing physiological damage might have potentially harmful effects in food safety and human health.The reduction of Ca,K and Mg could delay the chlorophyll synthesis,which might also be the reason for the growth inhibition.In addition,maize seedlings contained more La at the exposure to La2O3 NPs than to La2O3 BPs,the La content in plants increased over the exposure time.5.The biomass and water uptake of radish were reduced upon the exposure to La2O3 NPs,the tuber collapsed and the roots got shorter.Active endogenous substances such as vitamin C,solute protein and solutes were significantly decreased,leading to the deteriorated quality of radish tuber.Therefore,it is concluded that La2O3 NPs showed phytotoxicity on maize and radish seedlings.The change of AQPs gene expression and ABA content explained the reduced water uptake in maize seedlings on molecular level.Our results provide scientific basis for both the ecological risk assessment and the rational management of NPs.