Silicon and Biotic Stress: Suppressing Plant Diseases.

Silicon has been shown to suppress a number of foliar and root diseases in both dicots and monocots. This suppression has been effective against not only fungal diseases but those caused by the fungal-like Oomycota, bacteria, nematodes and viruses.  Silicon appears to affect a number of components of host plant resistance that includes delaying the incubation and latent period, reducing lesion expansion rates, lesion size, and lesion number. Subsequently, disease progress and/or final disease severity is dramatically reduced, and the resistance of susceptible cultivars is augmented to almost the same level as those with complete or partial resistance. Silicon may even suppress plant disease as effectively as a fungicide. As the silicon concentration (insoluble or soluble) increases in plant tissue, plant disease suppression greatly improves. Furthermore, the silicon supply to the plant must be continuous or disease suppressive effects are reduced or non-existent. Two hypotheses have been proposed to explain how silicon enhances plant resistance against pathogen infection, i) the insoluble silicon layer is deposited in epidermal cells preventing penetration by the pathogen (i. e.  ‘mechanical barrier hypothesis’) and ii) that the soluble silicon affects the response of the plant at the biochemical and molecular level.  For the latter hypothesis, a number of studies have shown increases in plant defensive compounds such as phenolics, phytolexins and plant resistance proteins.  Genome wide studies for Arabidopsis, tomato, rice and wheat amended with silicon and compared to non-amended control plants also has shown a differential expression of a large number of genes, and these genes are known to be involved in host plant defense mechanisms or metabolism. Clearly, this quasi-essential element can play a major role in suppressing diseases of plants grown under greenhouse and field conditions, especially for soils or soilless mixes deemed to be low or limiting in plant available silicon.