The Physiological And Molecular Mechanisms Of Silicon-enhanced Resistance To Bacterial Blight In Rice

Previous studies on silicon-enhanced disease resistance in rice were mainly focused on rice blast and sheath blight that were caused by fungi. To date, the effect of silicon on bacterial blight (Xoo) has never been reported. A series of hydroponics experiments were performed in the present study in a controlled rice-Xoo pathosystem to examine the silicon-enhanced disease resistance, silicon absorption and rice growth. The effects of silicon on activities of antioxidant defense enzymes in rice leaves in relation to induced resistance were also investigated. The defense-related genes regulated by silicon in rice-Xoo interaction pathosystem were quantified by using real-time fluorescence quantitative PCR technique. The comprehensive and systematic study of mechanisms of silicon-enhanced resistance to bacterial blight in rice was conducted. The main results are presented as follows:(1) Silicon addition could significantly enhance rice resistance to Xoo, thus alleviating damage resulting from infection by Xoo strain. The severity index of the infected plants was decreased by 11.83-52.12% by addition of silicon compared with the control with the relative immunization efficiency of 16.55-75.82%. The plants that were switched from Si+ (with Si added) to Si- (without Si added) nutrient solution lost partial resistance to Xoo, however, rice plants that were switched from Si- to Si+ nutrient solution exhibited the same high resistance as the plants treated continuously with silicon. Silicon played a role as a physical barrier in controlling bacterial blight in rice, although this was not a major mechanism. Application of silicon was beneficial for rice growth, resulting in an increased silicon content and total dry weight. The shoot and root dry weights were significantly decreased by inoculation with Xoo strain. Compared to the control plants, necrosis spots caused by the programmed cell death occurred rapidly in the leaf tissues of Si-fed plants infected by Xoo, thus preventing from further disease development. However, the leaves in rice plants grown in Si- nutrient solution appeared obvious symptoms of desiccation, immature death, curliness and wilt.(2) The contents of malondialdehyde (MDA) and peroxide hydrogen (H2O2) in Si-fed rice plants were significantly increased during the 48-h period after inoculation with Xoo strain. The activities of superoxide dismutase (SOD) and lipoxygenase activity (LOX) were significantly higher, while the activities of catalase (CAT), peroxidase (POD) and ascorbate peroxidase (APX) were lower in the Si-fed plants than in the Si-deprived Xoo-inefectd plants, resulting in accumulation of peroxide hydrogen (H2O2) accumulation in Si-fed plants and intensification of membrane lipid peroxidation intensification. It can be concluded that Si triggered hypersensitive reaction (HR) through mediating the activities of antioxidant defense enzymes, therefore inducing rice resistance to bacterial blight.(3) Silicon application significantly increased activities of phenylalanine ammonia-lyase (PAL) and polyphenoloxidase (PPO) , as well as contents of soluble phenolics and lignin in rice leaves infected by Xoo strain. The results demonstrated that silicon actively participated in metabolism of phenolics to accelerate accumulation of antimicrobial compounds, and therefore enhancing the resistance to bacterial blight in rice. (4) The activities ofβ-1,3-glucanase, exochitinase and endochitinase were all quickly increased after inoculation with Xoo strain. Theβ-1,3-glucanase activity was significantly higher in Si-supplied plants than that in Si-deprived plants within the eight days, and reached the maximum on the eighth day after inoculation with Xoo strain. The exochitinase and endochitinase activities in leaves of Si-fed plants significantly increased throughout the experimental period. The results showed that silicon application improved the enzyme activities ofβ-1,3-glucanase, endochitinase and exochitinase, thus enhancing the resistance to bacterial blight in rice.(5) Application of silicon could activate the expression level of the gene Os03g0109600, a transcript factor, after inoculation with Xoo strain. The expression quantity in Si-fed plants was significantly higher than in Si-deprived plants, which was benefical to increasing the gene's activity, In the later stages of infection, the expression of Os03g0126000 gene was depressed, which was beneficial to maintaining normal physiological metabolisms and disease resistance. Silicon could regulate the expression of PAL gene. In the early stages of infection, its expression quantity was higher in Si-fed plants than in Si-deprived plants. Silicon induced expression of Pr1a and Rcht2 genes earlier and faster, and the expression quantity was also significantly higher in Si-fed plants than in Si-deprived plants. Application of silicon could significantly enhance the expression quantity of Lox2osPil gene and regulate the expression level of CatA gene. In the early stages of infection, silicon significantly depressed the expression of CatA gene. The results showed that silicon actively induced and regulated some defense-related genes in response to bacterial blight attack in rice-Xoo interaction and this was the main mechanism of Si-enhanced resistance to bacterial blight in rice.In conclusion, silicon is beneficial for rice growth. Silicon addition may significantly enhance the resistance to Xoo caused by bacilli. Silicon can actively participate in physiological metabolisms, induce and regulate defense-related genes of plant and induce a series of defense mechanisms to impede the invasion of pathogens. Using Si to control plant bacilli diseases is an economical, highly-effective and environmently-friendly technology. It is theoretically and practically important to develop alternative novel technologies for controlling plant diseases.