Silicon Uptake Mechanism and its Multidimensional Influences on Stress Mitigation in Rice (Oryza sativa L.)

Silicon (Si) is the second most abundant element in the earth crust constituting 27.7 per cent. The beneficial effects of Si includes mitigation of various forms of abiotic and biotic stresses. Rice (Oryza sativa), a typical Si accumulator, takes up Si actively in the form of silicic acid. There are three transporters involved in the uptake of Si viz, LSi1, LSi2 and LSi3. Influx transporter (LSi1) takes up silicic acid from soil solution and mediates its transport upto the exodermal layer of root system, followed by the efflux transporter (LSi2), which transports it across the aerenchyma. Further movement of Si to the aerial parts of the plant is mediated by another influx transporter, LSi6 and finally gets deposited as silica in the plant parts. Silicon present in soil solution as well as its deposition in plants helps in mitigating various stresses in rice. Si application during drought stress prevents compression of xylem vessels and thereby resulting in reduction of transpiration rate. Sufficient supply of Si stabilises the culms and serves to decrease the risk of lodging. Rice is sensitive to metal toxicities like Iron (Fe), Manganese (Mn) and Aluminium (Al). Si complexation with these metal ions decreases their translocation rate and reduces the toxic effects. Heavy metal toxicity due to accumulation of cadmium (Cd) and arsenic (As) can be alleviated through supplementation of Si. Si also ameliorates salt stress by decreasing Na uptake and its root-to-shoot translocation. Silicon deposition in plant parts provides a mechanical barrier against pathogens and pests. The plants supplied with Si produce phenolics and phytoalexins in response to fungal infection and cuticle-Si double layer act as a defence mechanism preventing pests. The key mechanisms of Si-mediated alleviation of stresses in rice include stimulation of antioxidants, complexation of toxic metal ions with Si, immobilization of toxic metal ions and compartmentation of metal ions within plants.