Silicon: A bio stimulant for plant defence against herbivores

Abstract

Herbivorous performance mainly depends on the defensive and nutritional traits of the plants they consume. Herbivory can inhibit plant growth and yield, or may affect plant responses at morphological, physiological, biochemical and molecular levels. Plants have developed complex anatomical, physiological and biochemical protectors to recognise the herbivorous attacker and activate the appropriate defensive reactions. Plant defence mechanisms are divided into two categories: direct or indirect strategies. Direct defence involves physical barriers (e.g., trichomes), toxic chemicals (e.g., alkaloids) and antinutritional factors (e.g., protease inhibitors), whereas indirect defence involves the release of volatile compounds, which lure the herbivore's natural predators and thus reduce herbivore populations. Silicon (Si)-rich plants activate various defence responses and phytochemicals which alleviate different biotic and abiotic stresses, mainly against vertebrate and invertebrate herbivores. Silicon may form a discrete structure or deposit in the cell wall, which confers physical resistance to herbivory. Silicon may interact with the defence phytochemical pathways of herbivores and can enhance the production of secondary metabolites for defence. This review presents a comprehensive overview of the complex interactions between plants and herbivores and the relationship between Si-mediated defence responses and phytohormonal regulation of plant immunity and outlines potential future research areas for developing more sustainable pest management strategies.