Plant Volatiles in Defence.

Plant odours are the most ubiquitous volatiles in nature. This chapter deals with the biochemistry and molecular biology of plant volatiles that are emitted from vegetative tissues during pathogen- or herbivore-induced stress and the contribution of these volatiles to plant defences. While mechanical tissue wounding causes the non-specific release of volatiles, herbivore-specific elicitors from their saliva distinctly alter the volatile signature. These volatiles acquired diverse roles in ecological interactions. Firstly, they can be toxic to, or repel conspecific or other herbivores from already infested plants. This function is referred to as ‘direct defence’. Secondly, volatiles function as prey-associated signals for foraging carnivorous arthropods. This is referred to as ‘indirect defence’. Finally, volatiles elicit metabolic changes in unattacked neighbouring plants resulting in priming and induction of defences already before herbivores have arrived. This is referred to as ‘plant-plant communication’. Although volatile production by native plants and crops is highly variable under different growth conditions and during development, its function in direct and indirect defences is remarkably robust. With the current state of knowledge, it is now possible to manipulate these direct and indirect defences through breeding or transgenic approaches. Taken together, plant volatiles play profound roles in plant-herbivore and plant-pathogen interactions and are promising targets for improved crop protection.