Figure 1.

Mechanism of Si-induced apoplastic obstruction and its beneficial effect on the applied plants. (a) Conditions before (−Si) and after (+Si) Si application; Si blocks the apoplastic entry route of toxicants, which is the common pathway in addition to the symplastic route to reach the xylem. Casparian bands (CBs) are present, stopping the entry of toxicants, although breaks may occur allowing for bypassing routes, particularly under low or −Si conditions. High or +Si improves CB development [76], as well as apoplastic Si deposition (as silica, SiO₂ [77], effectively blocks bypass routes resulting to root-to-shoot translocation of toxicants). (b) In +Si plants, when Si translocates to the plant shoot cells, it deposits in the form of SiO₂, forming a thick layer in the apoplastic region or a mechanical barrier blocking the pathogenic entry, and protects from other abiotic stresses. Furthermore, it modulates reactive oxygen species (ROS) level leading to detoxification and reduction of stress effect and increased plant growth. However, in the case of −Si plants, toxicant levels in shoots accumulate to a greater extent causing adverse effect on plants. (c) In +Si plants, the formation of an SiO₂ barrier layer in the apoplastic region disrupts the process of establishing specificity between a plant and insect by altering the flow of molecules (e.g., effectors). This also helps the plant to prevent piercing by insects, resulting in the limited translocation and release of effectors in +Si plants compared with that in −Si plants [78]. (d) The overall effect of Si application influences the other general mechanisms of plants, altering plant attributes, including abiotic and biotic stress resistance, which leads to yield enhancement. For further details about the general mechanisms, see the references in [79,80,81,82,83,84,85,86,87,88,89,90,91,92,93].