Plant Defences against Herbivore Attack

Abstract

Abstract Plants deploy a number of defences against attack by insects and other herbivores. Direct defence is conferred by plant products and structures that deter or kill the herbivores. Chemical toxins and deterrents vary widely among plant species, and some typical toxins include alkaloids, terpenoids, steroids, phenylpropanoids and precursor compounds that, on damage of the plant tissue, can be degraded to cyanide or other toxic molecules. Physical defences include thorns, barbs and unpalatable tissues. Other defences are indirect, such as defensive symbioses with microbes, harbouring protective ants or recruitment of parasitoids and predators that attack the herbivores. Recent experiments, and deployment of transgenic plants with Bt toxins, demonstrate the potential for novel plant defences in agriculture. Key Concepts: Plants employ various direct and indirect mechanisms to defend against herbivores. Direct defences include chemical defences and physical defences such as tissue toughness, thorns and barbs. Chemical defences against herbivores are numerous and diverse. Direct chemical defences can be toxic or deterrent to herbivores, or reduce nutritive value of the plant tissues. Some chemical defences act indirectly, for example, by attracting wasps that are parasitic to the herbivores. According to optimal defence theory, young tissues of high value to plant fitness tend to be chemically defended, whereas older tissues have relatively strong physical defences. Chemical defences, especially induced chemical defences, are tightly regulated mainly through the jasmonate signalling pathway.