Leaf volatile isoprenoids: an important defensive armament in forest tree species.

Current knowledge on the ecological impact of the emission of volatile isoprenoids by plants is reviewed. This trait is common to many terrestrial species but is scattered across different taxonomic groups; it appears to be related to other ecological traits rather than to phylogenetic relationships. Plants invest high resources to produce volatile isoprenoids, which are likely to play multiple roles in the defence against biotic and abiotic stressors. We describe how constitutive and induced volatile isoprenoids may directly or indirectly defend plants, and briefly address how indirect defence may involve communication with other trophic levels beyond the simple plant-herbivory interaction. It is discussed that, as metabolically costly defensive mechanisms are only activated after attacks, induced volatile isoprenoids may also prime other biochemical pathways that are involved in stress resistance responses. It is also surmised that attacked plants may also use volatiles as an airborne communication to signal the attack to other leaves or other plant organs, or even other plants, eliciting defence responses.