The Physiological And Molecular Mechanisms Of Silicon-enhancend Resistance Of Rice To High Zinc Stress

Rice is the main food crop in China, with the planting area reached30million ha. The annual rice yield accounts for about44%of the total grain output. In recent years, soil contamination with heavy metal Zinc (Zn) has become more and more serious because of the impacts of industrial wastes, sewage sludge, compost, agrochemicals, and mine tailings, leading to toxicity of excess Zn to rice growth. 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 heavy metal, however, the mechanisms underpinning Si-enhanced metal tolerance remain poorly understood and thus need further investigation.Therefore, hydroponics experiments were carried out in this study. Two rice cultivars:i.e. TY-167, a Zn-resistance cultivar and FYY-326, a Zn-sensitive cultivar, were selected among19rice genotypes based on the root tolerance index to investigate the effects of Si on plant growth, elements uptake, transport, distribution, photosynthetic parameters and antioxidative defenses of both cultivars under Zn stress; Using high-throughput sequencing, we performed a comprehensive analysis of the influence of Zn on gene expression of the sensitive rice (FYY-326) with or without Si. The effect of Si on regulating photosynthesis-related genes of the sensitive rice under high Zn stress with or without Si was conducted using real-time quantitative PCR approach. The main results are presented as follows:1. Effect of Si on antioxidant defense capacity and oxidative of rice in roots under Zn stressSuperoxide dismutase (SOD), catalase (CAT) and asorbate peroxidases (APX) activities were decreased by high Zn stress; the contents of hydrogen peroxide (H2O2) and malondialdehyde (MDA) were increased by high Zn treatment, which were all counteracted by addition of Si. Furthermore, such effects were confirmed by using histochemical staining methods. High Zn stress induced the loss of plasma membrane integrity and the damage of membranes due to lipid peroxidation, which was alleviated by Si. The alleviative effects of Si on Zn toxicity were more significant in the Zn-resistance cultivar than in the Zn-sensitive cultivar.2. Effect of Si on root morphology and ultrastructure of rice in roots under Zn stressZn concentration in both shoots and roots of the two cultivars increased in response to an altered Zn supply in the nutrient solution. Total root surface area (TRSA), total root length (TRL) and total root tip amount (TRTA) were decreased under Zn stress, and transmission electron microscope (TEM) analysis showed that serious untrastructural damage was observed in both cultivar roots treated with Zn. Plasmolysis, concentrated cytoplasm, ambiguity of organelle were all symptoms of root cell damage under Zn stress. Shoot and root growth of both cultivars was negatively affected by a root environment with2mM Zn ion concentrations. However, addition of Si significantly decreased root-to-shoot Zn transport and increased the parameters of root morphology. And exogenous Si alleviated the plasmolysis of root cell, counteracted the inhibitory effect of Zn on plant growth and increased the biomass of rice.3. Effect of Si on relative elements concentration and distribution in rice under Zn stressHigh Zn stress dramatically increased the relative Zn concentration with Zn concentration being significantly higher in roots than in shoots irrespective of cultivar used and silicon added. The balance of K, Ca, Mn, Fe and Cu in shoots and roots of both cultivars tested was disturbed by high Zn stress. However, Si significantly decreased Zn uptake and root-to-shoot transport, improved the unbalanced distribution of some elements in rice, and alleviated Zn toxicity to rice.4. Effect of Si on antioxidant defence capacity in rice plants under Zn stressThe symptoms of Zn toxicity were typically manifested as a yellow colour on the lower leaves starting from the tips and spreading toward the bases of the leaves, which became severer as the experiment continued. Severer leafy symptoms of Zn toxicity were observed in the Zn-sensitive cultivar (FYY-326) than in the Zn-resistant cultivar (TY-167). Under Zn stress, the activities of SOD, CAT, APX decreased, but the contents of H2O2, MDA, ascorbic acid (AsA), glutathione (GSH) and non-protein thiols (NPT) except AsA, GSH and NPT were increased by Zn treatments, which were all counteracted by addition of Si. The toxicity symptoms of Zn were alleviated by addition of Si. The alleviative effects of Si on Zn toxicity were more significant in the Zn-tolerant cultivar than in the Zn-sensitive cultivar.5. Effect of Si on photosynthetic and fluorescence parameters and chloroplast ultrastructure in rice under Zn stressNet photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), water use efficiency (WUE), chlorophyll cocntent, the value of Fv/Fm of the two rice cultivars tested were all decreased under high Zn stress, while intercellular CO2concentration (Ci) was increased. Chloroplast structure was swollen observably with chloroplast granae being destroyed heavily under Zn-stress, which was all counteracted by addition of Si. The results showed that addition of Si improved the photosynthetic efficiency and alleviated the chloroplast ultrastructure in Zn-stressed rice. However, the alleviation of Si on above-mentioned parameters was smaller in Zn-sensitive plant cultivar than in Zn-resistant plant.6. Effect of Si on expression of genes in shoots of rice under Zn stressWe identified2077genes out of56798rice genes which were differentially expressed in the Si-amended plants in the treatments without high Zn added (Si/CK), with101genes up-regulated and1976genes down-regulated. Among3888differentially-expressed genes in rice plants treated with high Zn (Zn/CK),1920genes were up-regulated and1968genes were down-regulated. In the high Zn-treated Si+plants,873genes were differentially expressed, compared with the high Zn-treated Si-plants (Si+Zn/Zn), with504genes up-regulated and369genes down-regulated. In high Zn-treated plants amended with Si,4976genes were differentially expressed, compared with the normal Zn-treated plants amended with Si (Si+Zn/Si), with4115genes up-regulated and861genes down-regulated. These differentially-expressed genes included metabolic regulation, ion transport, transcription and adversity response, etc. On the basis of analyzing expression profile of rice gens, it can be concluded that Si can exhibit obvious impacts on growth and development of plants, especially in rice plants under non-stressed condition, and can regulate natural resistance mechanisms to produce more efficient and timely key genes to alleviate damage caused by high Zn stress.7. Effect of Si on genes responsible for photosynthesis of rice under Zn stressThe gene expression levels of HemA, Os03g36540, PsaH, PetC and PsbY all increased in plants treated with high Zn with or without Si. However, Si could increase expression of these genes earlier and faster in plants under high Zn stress in the early stages, and the increasing extent of expression was higher in Si+plants than in Si-plants. The gene expression of Os02g49870was not remarkable under high Zn stress, but was significantly increased by addition of Si. The results show that Si activated and regulated some photosynthesis-related genes in response to high Zn stress in rice, and consequently increased the photosynthesis.