Short-term exposure to silicon rapidly enhances plant resistance to herbivory

Silicon (Si) can adversely affect insect herbivores, particularly in plants that evolved the ability to accumulate large quantities of Si. Very rapid herbivore-induced accumulation of Si has recently been demonstrated, but the level of protection against herbivory this affords plants remains unknown. Brachypodium distachyon, a model Si hyperaccumulating grass, was exposed to the chewing herbivore, Helicoverpa armigera, and grown under three conditions: supplied Si over 34 days (+Si), not supplied Si (-Si), or supplied Si once herbivory began (-Si→+Si). We evaluated the effectiveness of each Si treatment at reducing herbivore performance and measured Si-based defenses and phenolics (another form of defense often reduced by Si). Although Si concentrations remained lower, within 72 hr of exposure to Si, -Si→+Si plants were as resistant to herbivory as +Si plants. Both +Si and -Si→+Si treatments reduced herbivore damage and growth, and increased mandible wear compared to -Si. After 6 hr, herbivory increased filled Si cell density in -Si→+Si plants, and within 24 hr, -Si→+Si plants reached similar filled Si cell densities to +Si plants, although decreased phenolics only occurred in +Si plants. We demonstrate that plants with short-term Si exposure can rapidly accumulate Si-based anti-herbivore defenses as effectively as plants with long-term exposure.