Nanoparticles mitigate arsenic stress in plants by modulating defence mechanisms

Arsenic stress greatly affects plant production, threatening food security, and also human health through food chain. Arsenic alters various physiological processes that subsequently affect plant’s normal metabolism. Plant has different mechanisms to protect from stress, where nanoparticles improve plant metabolisms and defence system, thereby alleviating arsenic stress in plant. Therefore, this review discusses the effects of arsenic in plant at different levels, and the roles of NPs in modulating plant defence system against As stress. This review will be encouraging in future research on plant protective mechanisms against stress and the significance of NPs in plant science and agriculture. nitrite reductase, and glutamate dehydrogease, thereby, reducing NO 3̄ and NO 2 ̄ contents and glutamic acid and glutamine ratio 24,25 . AsV uncouples oxidative phosphorylation, which resulted into the inhibition of ATP synthesis 26 . ATP is formed by phosphorylation of ADP in mitochondria, but due to the interference of AsV with the mitochondrial enzymes, F 1 F o ATP synthase, the enzyme reacts with AsV and formed ADP-AsV and thereby inhibiting the normal metabolism. AsV interferes the activity of polynucleotide phosphorylase enzyme (PNPase), the enzyme that catalyze phosphorolysis and also the exchange of the terminal phosphate group of ADP and Pi. In presence of AsV, PNPase catalyze the arsenolysis of RNA and ADP giving AMP-arsenate 27 . AsV also alters the activity of glycolytic ezymes by substituting the Pi group. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is the glycolytic enzyme that catalyzes the oxidative phosphorylation of D -glyceraldehyde 3-phosphate (G3P) to 1,3-biphospho- D -glycerate (1,3BPG), but presence of AsV inhibited the formation of 1,3BPG, and formed 1-arseno-3-phosphoglycerate (1As3PG) 28 . The study of Tariang et al. 29 also revealed that As altered activity of hexokinase, phosphofructokinase and pyruvate kinase enzymes, which might have caused to inhibition of carbohydrate metabolism. cell walls by decreasing degree of pectin methylesterification. The SiO 2 NPs treated cell show higher expression of OsNIP1; 1 and OsNIP3; 3 and lower expression of OsLis1 and OsLis2 genes. These findings provide the possibility of using SiO 2 NPs in As-contaminated paddy soil. It has been reported that many other abiotic stresses such as salinity stress, drought and Cd stresses were mitigated by different NPs by increasing antioxidant enzyme activities while lowering ROS 48, 49 . The recent study of Hussain et al. 50 on the use of different NPs (ZnO, FeO and Si) under Cd stress in wheat plant also revealed that NPs ameliorate Cd stress by increasing nutrient uptake and antioxidant enzyme activities, while reducing Cd intake by the plant. The prospective of NPs to mitigate abiotic stress in crop plant was reviewed by Das and Das 51 where the significant roles of NPs in mediating different stresses have been explained. Abiotic stress such as drought, flood or salinity stress was mediated by different NPs including Ag, Al 2 O 3 , Fe 3 O 4 , TiO 2 , SiO 2 , ZnO NPs in different plants by increasing essential nutrient content and enzymic and non-enzymic antioxidant, altogether increased total antioxidant capacity of plant. Gohari et al. 48 , Duo et al. 52 and Mahammadi et al. 53 also demonstrated stimulation of antioxidant enzymes with the supplementation of NPs, enhancing plant defence system and tolerance against salt, drought and cold stress respectively. NPs possess great potential towards amelioration of different stresses by counteracting stress induced oxidative damage with the increase of antioxidant activities. Studies of Khan et al. 54 and Praveen et al. 55 , also revealed the prominent role of antioxidants in the mitigation of As in plants.