Mechanisms Of Effects Of Silicon Nutrition On Different Genotypes Rice Resistance Toward Insect Herbivore

Rice (Oryza sativa L.) is one of the world’s most important crops. It plays an extremely important role in grain production in China. Rice leaf folder (Cnaphalocrocis medinalis, LF), a chewing specialist caterpillar, can cause great damage to the rice production. Rice is a typical of silicon (Si) accumulation plants. Rice plant absorb silicon nutrient by Lsil transporter. The responsive gene Lsil is exclusively expressed in the cortex and outer cortex only in the roots.Silicon not only increases physical resistance of the host plant, but also induce chemical resistance in host plant. Jasmonic acid and methyl jasmonate and other analogs (collectively jasmonates, JAs) are a class of plant hormones, commonly found in plants, it is an important group of defense signaling substances in plants, JAs play an important role in anti-herbivore defense responses in plants. In this paper, transgenic line OsLsil-RNAi silencing Lsil via RNA interference was used as a research material, the corresponding wild-type (WT) rice plant severed as a control. Physiological and biochemical defense responses, insect resistance of the WT and RNAi lines were examined under hydroponic conditions with Si application (Si+) or without Si application (Si-) to reveal the mechanism of effects of silicon nutrition on rice resistance toward insect herbivore LF. The study will provide theoretical guidance for Si fertilizer in future agricultural production to increase insect resistance of rice and to reduce pesticide pollution and to ensure food security. The main results are as follows:1. Si promoted rice growth. The root and shoot lengths were increased after Si amendment. Si application also significantly increased Si contents in the leaves and roots in WT rice plants. Wax contents that are relarted to physical defense were significantly higher in Si-treated WT plants than those in RNAi+ Si, WT and RNAi treatments. Bioassay showed that weight gain of LF caterpillars fed on Si-treated WT plants (WT+Si) was reduced by 55.3%,46.3% and 63.4%, respectively, compared to those fed on plants of RNAi+Si, WT and RNAi treatments, indicating that silicon deposited in the rice plant increased its pest resistance.2. Activities of defensive enzymes and protective enzymes in leaves and roots were measured in WT and Lsil-RNAi plants. It was found that activities of peroxidase (POD),polyphenol oxidase (PPO), superoxide superoxide dismutase (SOD), catalase (CAT) activity peaked at 12 h,24 h and 48 h. The activity of POD was significantly higher in Si-treated WT plants (WT+Si) which were attacked by LF, especially in 24 h. In contrast, the difference in POD activity in RNAi+ Si, WT and RNAi treatments was not significant compared to one another during the most experiment time. The activity of POD in roots was showed a similar tendancy to POD activity in leaves. The PPO activity in leaves and roots in Si-treated WT plants (WT+Si) started to increase 3 h after LF infestation, and peaked at 12 h. The activity of PPO was significantly higher in Si-treated WT plants (WT+Si) than those in RNAi+ Si, WT and RNAi treatments after attacked by LF. Although the induction of SOD activity was significantly higher in wild type (WT) after LF infestation, however the SOD activity in leaves and roots in Si-treated WT plants (WT+Si) was the highest, suggesting that Si pre-treatment increased defense response against LF. The CAT activity in leaves and roots was showed a similar tendancy. And the PPO activity was significantly higher in WT+Si than those in RNAi+ Si, WT and RNAi treatments after attacked by LF. At the same time, the content of malondialdehyde (MDA) in WT+Si was the lowest, but the content of MDA in RNAi treatments was most active. The content of free proline was significantly higher in WT+Si than those in RNAi+ Si, WT and RNAi treatments after attacked by LF at 24 h. silicon nutrition plays a very important role in insect resistance in plants. Our findings demonstrate that Si play an extremely important role in insect resistance in host plant.3. Real-time quantitative PCR was used to detect the relative expression levels of defense-related genes and key genes in JA signal transduction pathway. The results showed that LF feeding induced expression of OsLOX, OsAOS and COR-INSENSITIVE1 (COI1a and COI1b). Si amandment further increased expression of these genes and Protease Inhibitor gene OsBBPI. The highest gene expression levels were found in Si-treated WT plants (WT+Si) after insect infestation, and the lowest expression levels were found in RNAi Si-untreated plants. Silicon amandment alone did not induce expression of OsBBPI, OsLOX, OsAOS, OsCOIla and OsCOI1b. OsLsil expression level was positively correlated with silicon level,no correlation was found between OsLsil expression level and LF feeding. In this paper, the important role of silicon in rice resistance toward chewing insects at the molecular level was revealed.