The Physiological And Molecular Mechanisms Of Silicon-enhanced Resistance To Rice Blast

Silicon (Si) has been proven to be beneficial for healthy growth and development of many plant species and plays an important role in enhancing plant resistance against fungal pathogens, however, the mechanisms involved are still unclear. Traditionally, it is thought that silicon acts as a mechanical or physical barrier in response to fungal attack. However, there have been increasing bodies of evidence showing that silicon is associated with host defense responses and plays an active and physiological role in enhancing resistance of the host plant. A series of hydroponics experiments were performed in a controlled rice-Magnaporthe grisea pathosystem to study the effects of silicon on rice growth, disease development and regulation of induced resistance. The effect of silicon on regulating defense-related genes in the rice-M. grisea interaction was conducted using real-time quantitative PCR approach. Using a cDNA chip containing 60727 rice cDNAs representing 36926 unique genes, we performed a comprehensive analysis of the influence of Si on gene expression of rice inoculated with or without M. grisea. The comprehensive and systematic study of mechanisms of silicon–enhanced resistance to rice blast was conducted. The results are presented as follows:(1) Rice plants that were switched from Si- (without Si added) to Si+ (with Si added) nutrient solution and simultaneously inoculated with M. grisea exhibited the same high resistance as the plants treated continuously with silicon, with control efficiencies of 67.0 % and 69.3%, respectively. However, rice plants switched from Si+ to Si- nutrient solution lost partial resistance to blast, with control efficiency of 47.7% only. It seems to suggest that there are still some roles that silicon plays as a physical barrier in controlling rice blast though these roles are very limited and not a major mechanism for Si-enhanced resistance to rice blast. Application of silicon was beneficial for rice growth and alleviated damage resulting from infection by M. grisea. Regardless of inoculation with M. grisea, Si concentration of shoots or roots in rice plants amended with 1.7 mM Si was significantly higher than that of the non-Si-amended plants. The Si concentrations in all treatments were significantly higher in rice shoots than in rice roots.(2) Silicon induced a rapid and transient burst of H2O2 at 24 h after inoculation. Catalase (CAT) activity in leaves of Si+ plants was lower at 24 h after inoculation compared with Si- plants, then rapidly increased and reached a peak at 72 h after inoculation, significantly higher than in Si- plants. Compared with Si- plants, malondialdehyde (MDA) concentration and membrane permeability in Si+ plants were significantly higher at 48 h after inoculation but lower from 96 h and onward. Two peaks of lipoxygenase (LOX) activity in Si+ plants were observed at 12 and 48 h after inoculation, respectively, and were earlier than in Si- plants which occurred at 24 and 72 h, respectively. The maximum LOX activity was significantly higher in Si+ plants than in Si- plants. In the early stages of infection by M. grisea, silicon induced a rapid and transient burst of H2O2, and consequently resulted in membrane lipid peroxidation and membrane damage, which may be closely correlated with HR. Silicon alleviated the oxidative damage during later periods of the rice-M. grisea interaction.(3) Silicon application significantly increased activities of polyphenoloxidase (PPO) and phenylalanine ammonia-lyase (PAL), as well as contents of total soluble phenolics and lignin. However, leaf peroxidase (POD) activity was significantly lower in Si+ plants than in Si- plants after inoculation and was not related directly to blast resistance. The results demonstrate that silicon actively participates in metabolism of phenolics to accelerate accumulation of antimicrobial compounds.(4) Exochitinase and endochitinase activities were significantly higher in Si+ plants than in Si- plants and maintained longer. However, silicon application decreasedβ-1, 3-glucanase activity in rice leaves infected by M. grisea. The results show that silicon improved chitinase activities to enhance resistance to rice blast. However, the influence of silicon onβ-1, 3-glucanase activity was not observed.(5) Regardless of silicon applied, the gene expression levels of PAL, CatA, Rcht2 and Pr1a all increased after inoculation with M. grisea. In the early stages of infection, increased expression of these genes in Si+ plants occurred earlier and faster with the extent of increment being also significantly higher than in Si- plants. Compared with Si- plants, expression of OsBTF3 in Si+ plants occurred earlier and faster, and its expression level was also significantly higher than in Si- plants. The results show that Si activated and regulated some defense-related genes in response to blast attack in rice.(6) We identified 1210 genes which were differentially expressed in the Si-amended plants without inoculation (Si/CK), with 126 genes being up-regulated and 1084 genes being down-regulated . Among 670 differentially-expressed genes in rice plants inoculated with M.grisea (M/ CK), 346 genes were up-regulated and 324 genes were down-regulated . After inoculation with M.grisea, 483 genes in Si+ plants were differentially expressed, compared with the Si- plants (Si+M/ M), with up-regulation of 27 genes and down-regulation of 456 genes. These genes included stress-related transcription factors, and genes involved in signal transduction, the biosynthesis of stress hormones (SA, JA, ethylene), the metabolism of reactive oxygen species, the biosynthesis of antimicrobial compound, primary and/or secondary metabolism, defense response, photosynthesis, and energy pathways, etc. On the basis of analyzing expression profile of rice genes, it can be concluded that silicon can exhibit obvious impacts on growth and development of plants, especially in rice plants, which were not infected by pathogens, but can regulate natural resistance mechanisms of plant to produce more efficient and timely defense response, and consequently alleviate damage caused by pathogens in plants subjected to pathogens stress.In conclusion, silicon is very important and beneficial for plant growth, and may be essential for gramineous plant species such as rice. Silicon can activate and regulate defense-related genes of plant, participate in physiological metabolism and induce a series of defense mechanisms to impede the invasion and cloning of pathogens. Silicon, acting as a regulator of induced resistance, is active in response to pathogens. The Si-enhanced plant resistance to pathogens is not solely limited to a mechanical barrier as previously proposed.