Plant volatiles carry both public and private messages

Abstract

Plants have never worried about compliance with the Kyoto Protocol. † For millennia, they have released large amounts of gaseous compounds, such as carbon dioxide, oxygen, water vapor, and ethylene, from their foliage into the lower atmosphere. Thanks to improvements in headspace sampling techniques and mass spectrometry in the last 20 years, the list of plant volatiles has greatly expanded and now includes methanol, acetone, formaldehyde, and other short-chain carbonyl compounds, plus a host of terpenes, phenylpropanoids, benzenoids, and fatty-acid derivatives (1, 2). Are plants just “passing gas” by emitting compounds that are by-products of essential processes, or do the released substances have any real function in their lives? A major breakthrough in answering this question occurred in the early 1990s when it was discovered that the emission of plant volatiles could be triggered by herbivore feeding. The emitted compounds were shown to act as a botanical cry for help, attracting predatory and parasitic species that attack herbivores (3, 4). Herbivore-induced plant volatiles have also been demonstrated to have other ecological roles, for example, in direct deterrence of herbivores (5, 6) or in plant-to-plant communication. However, surprisingly little attention has been paid to the possible internal roles of these substances in the life of the plant until the work of Heil and Bueno in this issue of PNAS (7).