## 5 Silicon Accumulation Improves Plant Disease Resistance
### Formation of Biochemical Barriers as Influenced by Si
One is to enhance the activity of protective enzymes related to plant disease resistance, such as peroxidase (POD), phenylalanine ammonia-lyase (PAL) and polyphenol oxidase (PPO), as well as enhance membrane integrity and stability, thereby maintaining plant cell membrane permeability [89]. Protective enzymes are not only involved in the metabolism of phenolic substances, but also in the synthesis and accumulation of secondary metabolic antimicrobial compounds such as lignin which are regarded as regulators of the entire metabolic pathway. In particular, the two main components of fungal cell wall chitin and \(\beta\)-1,3-glucan are degraded by chitinase (CHT) and \(\beta\)-1,3-glucanase, respectively. Some reports have shown that the activities of CHT, POD and PPO in leaves of cucumbers inoculated with _Podosphaera xanthii_ are enhanced after Si applications [90]. In addition, Si applications in peas can also reduce the incidence of leaf blight by enhancing the activities of CHT and \(\beta\)-1,3-glucanase [76]. Increased resistance to brown spot in rice treated with Si fertilizers is also associated with enhanced CHT and POD activities [91]. These findings indicate that the accumulation of Si can enhance the activities of plant leaf protective enzymes and that changes in secondary metabolic antimicrobial compounds play substantial roles in plant disease resistance. The second aspect is to increase resistance by inducing secondary metabolites. Briefly, Si can improve the host's resistance to disease by inducing the production of secondary antimicrobial compounds such as phytochemicals, lignin, phenolic substances and pathogen-related proteins. Among secondary metabolites, Si-mediated regulation of phenolics has been extensively studied. Studies have shown that phenolic compounds such as hydroxycinnamate and flavonoids act as ROS scavengers, preventing the accumulation of ROS by activating antioxidant enzymes, such as guiaacol peroxididases [92]. Several studies have also shown that the beneficial effects of Si on plant disease resistance can be attributed to the induction of phenolic compounds. For example, [PERSON] et al. [93] showed that exogenous application of Si stimulates the accumulation of phenolic compounds in rose to enhance its resistance to powdery mildew. Similarly, Si enhances rice resistance to bacterial leaf blight by stimulating the accumulation of soluble phenols and lignin in rice leaves and the activities of PPO and PAL [94]. Si enhances chestnut resistance to chestnut blight by increasing the synthesis of phenolic compounds in leaf tissues [95]. [PERSON] et al. [96] first extracted a phytoalexin (3, 5, 3c, 4c-tetrahydro-7-O-methoxy flavone) from powdery mildew-infected cucumber leaves treated with Si. It is believed that Si participates in the antibacterial activities of the invaded host, causing the plant to produce some small molecular metabolites (such as flavonoids). In contrast to salicylic acid or jasmonic acid-induced plant resistance, soluble Si acts as a regulator of plant resistance induction, allowing plants to defend against pathogen attack faster or more effectively. The Si accumulation plays a preventive role but does not directly affect plant metabolism. The rice treated with Si after inoculation produce large amounts of phenolic substances and phytochemicals, thereby enhancing the resistance of rice to rice blast [97]. Moreover, plant terpenoid metabolites play important roles in adaptation to adverse environments, such as those with biotic and abiotic stresses [98]. A study has shown the accumulation of diterpene phytoalexins in rice as a potential mechanism against rice blast [99]. Finally, nitrogen-containing metabolites are an important class of secondary compounds in plant defense, especially in mitigating oxidative damage. When pathogens infect plants, plant tissues accumulate large amounts of polyamines, which limit the growth and reproduction of plant pathogens [100].