Something in the Air

Discovering the complex roles of plant emissions
So striking is the blue haze which rises above mountainsides on hot days that it features in the names of mountain ranges around the world—from […]

Art by Maria Demidova

So striking is the blue haze which rises above mountainsides on hot days that it features in the names of mountain ranges around the world—from the Blue Ridge Mountains of Virginia, to Jamaica’s Blue Mountain Peak and the Blue Mountains of New South Wales. The phenomenon is not new, the journals of Leonardo da Vinci note the haziness of the Tuscan hills, but it was only 50 years ago that plant physiologist Frits Went postulated that emissions from plants were behind this spectacle, scattering sunlight to create the familiar blue haze. People have been searching for explanations for these plant emissions—their answers are numerous, complex and fascinating.

Initially, it was assumed that plant emissions simply represented the release of unwanted metabolic by-products. Yet, as the loss of these volatile organic compounds (VOCs) can represent over 30% of the carbon assimilated by photosynthesis, it makes plant metabolism seem extraordinarily inefficient! More recently, numerous hypotheses rationalising VOC production have been put forward.

In the 1990s, observations of a link between emissions and plant tissue damage, led to the suggestion that VOCs could defend plants from herbivore attack. One key finding was the discovery that chemicals in the oral secretions of herbivores induced the release of VOCs from some plants, and that these had powerful repellent effects on attacking species. Further work revealed even more complex ecological interactions associated with VOC emissions. Whilst plants may use repellent chemicals as ‘direct defence’ mechanisms, more commonly they are found to attract the predators or parasitoids of herbivores (parasitoids, unlike parasites, always kill their host), in a so-called ‘indirect defence’ approach. These interactions are remarkably sophisticated; plants can detect herbivore eggs on their leaves and initiate a response which reduces herbivore numbers by up to 90%.

Even more exciting was the discovery that plants ‘eavesdrop’ on the VOC emissions of their neighbours, gleaning information on the abundance of herbivores nearby. VOCs from damaged plants lead undamaged ones to ‘prime’ themselves, ready to mount a larger defence when the risk of herbivore attack is high. Lima beans, for example, prepare themselves by secreting more nectar, attracting ants which devour herbivorous insects.

Another form of VOC emission with which everyone is familiar underlies floral scent. Plants do not produce their fragrances for our enjoyment but to attract the animal pollinators on which they depend for seed production. Recent work has demonstrated that in some cases the release of these chemicals is even more important than floral display and, through complex mixtures of different compounds, can act both as attractants to effective pollinators and repellents to poor ones, enabling highly specific, and efficient, pollinator interactions.

Finally, after plants have successfully defended themselves from herbivores and attracted pollinators to their flowers, they must spread their seeds. A strong correlation has been demonstrated between the ripeness of fruits and the presence of VOCs. This is likely no coincidence; plants lose out if their fruits are eaten before they are ripe, so unripe fruits are often distasteful. Conversely, VOCs make ripe fruit so attractive that animals go to great lengths to acquire and disperse them. Indeed, we humans fly, ship and drive them all over the world.

In recent years plant VOCs have become the focus of intense interest, as their importance and range of functions raises the prospect of manipulating their metabolism for our own benefit. For example, researchers have already engineered plants with enhanced protection from unfavourable environmental conditions. However, even with thousands of different compounds recognised, we still have a long way to go before our understanding of all their roles is anywhere near complete. At present it can perhaps best be described as hazy!

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About Charles Brabin

Charles Brabin is a DPhil. student studying the molecular genetics of cell proliferation and differentiation in nematode C. elegans (round worms).