Although the function of silicon (Si) in plant physiology has long been debated, its beneficial effects on plant resistance against abiotic and biotic stresses, including insect herbivory, have been well documented. In addition, the jasmonate (JA) signaling pathway plays a crucial role in mediating antiherbivore defense responses in plants. However, potential interactions between JA and Si in response to insect attack have not been examined directly. To explore the role JA may play in Si-enhanced resistance, we silenced the expression of allene oxide synthase (OsAOS; active in JA biosynthesis) and CORONATINE INSENSITIVE1 (OsCOI1; active in JA perception) genes in transgenic rice plants via RNAi and examined resulting changes in Si accumulation and defense responses against caterpillar Cnaphalocrocis medinalis (rice leaffolder, LF) infestation. Si pretreatment increased rice resistance against LF larvae in wild-type plants but not in OsAOS and OsCOI1 RNAi lines. Upon LF attack, wild-type plants subjected to Si pretreatment exhibited enhanced defense responses relative to untreated controls, including higher levels of JA accumulation; increased levels of transcripts encoding defense marker genes; and elevated activities of peroxidase, polyphenol oxidase, and trypsin protease inhibitor. Additionally, reduced Si deposition and Si cell expansion were observed in leaves of OsAOS and OsCOI1 RNAi plants in comparison with wild-type plants, and reduced steady-state transcript levels of the Si transporters OsLsi1, OsLsi2, and OsLsi6 were observed in Si-pretreated plants after LF attack. These results suggest a strong interaction between Si and JA in defense against insect herbivores involving priming of JA-mediated defense responses by Si and the promotion of Si accumulation by JA.Oryza sativa | induced defense | jasmonic acid | mitogen-activated protein kinase

Drought is a major constraint for rice production in the rainfed lowlands in China. Silicon (Si) has been verified to play an important role in enhancing plant resistance to environmental stress. Two near-isogenic lines of rice (Oryza sativa L.), w-14 (drought susceptible) and w-20 (drought resistant), were selected to study the effects of exogenous Si application on the physiological traits and nutritional status of rice under drought stress. In wet conditions, Si supply had no effects on growth and physiological parameters of rice plants. Drought stress was found to reduce dry weight, root traits, water potential, photosynthetic parameters, basal quantum yield (F(v)/F(0)), and maximum quantum efficiency of PSII photochemistry (F(v)/F(m)) in rice plants, while Si application significantly increased photosynthetic rate (Pr), transpiration rate (Tr), F(v)/F(0), and F(v)/F(m) of rice plants under drought stress. In addition, water stress increased K, Na, Ca, Mg, Fe content of rice plants, but Si treatment significantly reduced these nutrient level. These results suggested that silicon application was useful to increase drought resistance of rice through the enhancement of photochemical efficiency and adjustment of the mineral nutrient absorption in rice plants.

Rice (Oryza sativa L.) blast disease caused by Magnaporthe grisea is one of the most destructive diseases in the rice-growing areas of the world. Silicon is an important nutritional element especially for rice. Two near-isogenic lines of rice with different resistance to blast disease, i.e. CO39 (susceptible) and C101LAC (Pi-1) (resistant), were selected to determine the effects of Si amendment on the severity and incidence of rice blast disease. The physiological and cytological mechanisms involved in the induced disease resistance by silicon were investigated. Exogenous Si application at a concentration of 2 mM reduced the disease index by 45% for CO39 and 56% for C101LAC (Pi-1). Si application alone did not change lignin content and the activities of defense-related enzymes including peroxidase (POD), polyphenol oxidase (PPO) and phenylalanine ammonia-lyase (PAL) in rice leaves of both isogenic lines. However, after inoculation with M. grisea, Si-treated rice plants significantly increased the activities of POD, PPO and PAL in leaves of both isogenic lines. Si and lignin content were also significantly increased in Si-treated inoculated seedlings. Environmental scanning electron microscope observations revealed that Si amendment resulted in higher Si deposit on dumbbell bodies in the rice leaves and silicon papilla accumulation on the guard cell of stoma. These results suggest that silicon-induced defense response and cell silicification of rice leaves altogether contribute to the silicon-induced rice resistance to blast disease.

Silicon is the second most abundant mineral element in soil, it has important role in alleviating various environmental stresses and enhancing plant resistance against pathogen, but the exact mechanism by which Si mediates pathogen resistance remains unclear. One of the resistance mechanisms is related to silicon deposition in leaf that acts as a physical barrier to hinder pathogen penetration. But more evidence show that silicon can induce defense responses that are functionally similar to systemic acquired resistance, Si-treated plants can significantly increase antioxidant enzyme activities and the production of antifungal compounds such as phenolic metabolism product, phytoalexins and pathogenesis-related proteins etc. Molecular and biochemical detections show that Si can activate the expression of defense-related genes and may play important role in the transduction of plant stress signal such as salicylic acid, jasmonic acid and ethylene.

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Excessive use of agro-chemicals (such as mineral fertilizers) poses potential risks to soil quality. Application of organic amendments and reduction of inorganic fertilizer are economically feasible and environmentally sound approaches to develop sustainable agriculture. This study investigated and evaluated the effects of mineral fertilizer reduction and partial substitution of organic amendment on soil fertility and heavy metal content in a 10-season continually planted vegetable field during 2009-2012. The experiment included four treatments: 100% chemical fertilizer (CF100), 80% chemical fertilizer (CF80), 60% chemical fertilizer and 20% organic fertilizer (CF60+OM20), and 40% chemical fertilizer and 40% organic fertilizer (CF40+OM40). Soil nutrients, enzyme activity and heavy metal content were determined. The results showed that single chemical fertilizer reduction (CF80) had no significant effect on soil organic matter content, soil catalase activity and soil heavy metal content, but slightly reduced soil available N, P, K, and soil urease activity, and significantly reduced soil acid phosphatase activity. Compared with CF100, 40% or 60% reduction of chemical fertilizer supplemented with organic fertilizer (CF60+OM20, CF40+OM40) significantly increased soil organic matter, soil catalase activity and urease activity especially in last several seasons, but reduced soil available P, K, and soil acid phosphatase activity. In addition, continuous application of organic fertilizer resulted in higher accumulation of Zn, Cd, and Cr in soil in the late stage of experiment, which may induce adverse effects on soil health and food safety.

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Bacterial wilt is a devastating disease of tomato caused by soilborne pathogenic bacterium Ralstonia solanacearum. Previous studies found that silicon (Si) can increase tomato resistance against R. solanacearum, but the exact molecular mechanism remains unclear. RNA sequencing (RNA-Seq) technology was used to investigate the dynamic changes of root transcriptome profiles between Si-treated (+Si) and untreated (−Si) tomato plants at 1, 3, and 7 days post-inoculation with R. solanacearum. The contents of salicylic acid (SA), ethylene (ET), and jasmonic acid (JA) and the activity of defense-related enzymes in roots of tomato in different treatments were also determined. The burst of ET production in roots was delayed, and SA and JA contents were altered in Si treatment. The transcriptional response to R. solanacearum infection of the +Si plants was quicker than that of the untreated plants. The expression levels of differentially-expressed genes involved in pathogen-associated molecular pattern-triggered immunity (PTI), oxidation resistance, and water-deficit stress tolerance were upregulated in the Si-treated plants. Multiple hormone-related genes were differentially expressed in the Si-treated plants. Si-mediated resistance involves mechanisms other than SA- and JA/ET-mediated stress responses. We propose that Si-mediated tomato resistance to R. solanacearum is associated with activated PTI-related responses and enhanced disease resistance and tolerance via several signaling pathways. Such pathways are mediated by multiple hormones (e.g., SA, JA, ET, and auxin), leading to diminished adverse effects (e.g., senescence, water-deficit, salinity and oxidative stress) normally caused by R. solanacearum infection. This finding will provide an important basis to further characterize the role of Si in enhancing plant resistance against biotic stress.