Why do essential oils exist? Scientists discover plants detect insect attacks by ‘sniffing’ each other

Why do essential oils exist? Did you know that the gorgeous smelling essences we so prize in perfumes are actually a way plants communicate with each other (and defend themselves from insect attacks?) This fascinating report reveals all…

‘Plants have nowhere to run from their enemies – flying, crawling and jumping insects want to eat them alive. But plants are not defenceless. They deploy chemical toxins to deter insects. These can make the plant taste bitter, inhibit the herbivore’s digestive enzymes, disrupt their metabolism or poison them.

But they have a more subtle defence too – perfumed chemical compounds, known as volatiles, that they emit into the air to warn neighbours of danger or convey when they’re hurt. An example is the smell of cut grass, a mix of molecules called ‘green leaf volatiles’ which are released when a plant is damaged.

‘Plants are nature’s chemists. They take a few simple inorganic molecules and produce thousands of different organic molecules by just adding (energy from) sunlight,’ said Professor Matthias Erb, a plant scientist at the University of Bern, Switzerland. He investigates the volatiles that plants emit when attacked by insects for a project called PERVOL.

‘Some of these volatiles attract natural enemies of the herbivore, so, friends of the plant,’ said Prof. Erb. For example, if a caterpillar attacks a plant, these volatiles may attract parasitoid wasps or trigger defence responses in neighbouring plants. He says plants don’t help one another by signaling ‘I’m under attack’. Rather, they snoop on one another’s chemical signals to warn themselves about imminent threats.

Insects

Decoding these signals could teach us how to better protect crops against insects, according to Prof. Erb.

Insects are responsible for destroying one-fifth of the world’s total crop production each year. This is predicted to rise further for grain crops with climate change, hitting the temperate zones hardest.

‘These (plant-derived) molecules can be useful for agriculture in that they are natural protective mechanisms of plants. We could use them instead of synthetic chemicals,’ he said.

Prof. Erb works with maize, a strong emitter of volatiles. One chemical it emits is indole, which has a pleasant flowery aroma in small concentrations. Indole is not released by cutting maize. It requires the presence of a molecule in moth caterpillar saliva that activates defence responses in the plant. ‘(Healthy) maize plants do not emit indole. It is only triggered by herbivory,’ he said.

Prof. Erb and his colleagues found that when indole wafts towards the part of the plant that is not under attack, it triggers what he calls a primed state. ‘(Indole) doesn’t induce a defence response, but it prepares the plant, so that when the plant is attacked by a herbivore, it will respond quicker and stronger,’ he said.

Doing this means it can fend off its attacker more effectively, he says.

 

Why do essential oils exist?

One limitation of indole, however, is that it is also released by some flowers, such as jasmine and orange blossom. To prevent confusion, as a single volatile might be misleading, maize plants often tune into chemical mixtures to deduce attacks.

‘We have shown that indole and green leaf volatiles act synergistically to induce defences in an even stronger fashion than an individual volatile,’ said Prof. Erb.

‘Plants are nature’s chemists. They take a few simple inorganic molecules and produce thousands of different organic molecules by just adding sunlight.’

Professor Matthias Erb, University of Bern, Switzerland

 

To paint a fuller picture of plant behaviour, scientists are also exploring the impact of insect saliva on green leaf volatiles.

This is something that Dr Silke Allmann at the University of Amsterdam in the Netherlands has investigated in her work looking at how the green leaf volatiles of hurt plants is perceived by both plants and insects.

She experimented on tobacco plants by mechanically cutting them and applying water or the saliva of a tobacco hornworm caterpillar. The results surprised her: overall, the amount of green leaf volatiles did not change much, but the composition of the volatiles shifted dramatically. An enzyme in the caterpillar’s spit changed the compound, causing it to shift from a grassy to a sweet smell.

 

Why do essential oils exist?

 

She then discovered that a shift to the sweet-smelling compound attracted big-eyed bugs, which are natural enemies of the hornworm caterpillar, to the tobacco plant. This seemed puzzling to Dr Allmann, as the caterpillar’s own enzyme helped alert its presence to its enemies.

However, the sweet smell also warned adult tobacco hawk moths that a tobacco plant had already been colonised by caterpillars and steered them towards those with fewer competitors and fewer predators.

Dr Allmann is now studying this compound further as part of a project called VOLARE, and exploring practical uses.

‘A big challenge with plant volatiles is finding applications in agriculture. That is the holy grail,’ said Dr Allmann.

These chemicals can help farmers in a greener way, say the scientists.

‘You could imagine applying plant volatiles at the right moment to trigger specific reactions in a plant, for instance, resistance to herbivory,’ said Prof. ‘That would be a far more environmentally friendly strategy of boosting plant immunity or resistance to stress than applying a bioactive chemical to kill insects.’

Such natural chemicals could be released into fields under threat from pests to activate plant defences at the right moment. Insights into how plants detect warning smells could also allow breeders to develop crop varieties that are responsive to the signals.

Nose

What remains puzzling for scientists is how plants sniff out volatiles in the first place. They don’t have noses like us but can smell.

‘Our hypothesis is that volatiles enter through the stomata, small pores in leaves. We expect that there are sensors inside the leaf, perhaps proteins on the surface of cells, that the volatiles bind to,’ said Prof. Erb.

Dr Allmann is also hunting for these sensors. ‘If we found these receptors, we could find ligands (a type of molecule) that bind to them and switch them on. We could perhaps breed plants to be more or less sensitive to volatiles,’ she said. Plants could be bred that are easily triggered and could serve as sentinels to warn other plants nearby.’

The research in this article has been funded by the EU’s European Research Council. This post Plants can detect insect attacks by ‘sniffing’ each other’s aromas was originally published on Horizon: the EU Research & Innovation magazine | European Commission.

Forage – Stop to smell the flowers… then eat them, too?

Forage for your food, lately, or too scared to pick your own? There’s a whole world of edible plants growing around us, but if the closest you’ve ever been to foraging for food is scrumping apples (or more recently, scrabbling at the red-stickered items in your supermarket’s Reduced section), you need this beautiful book…

Yes, it’s another book we’ve eagerly added to our Fragrant Reads shelves, but although the publishers of Forage: Wild Plants to Gather and Eat say ‘Anybody can enjoy the increasingly popular back-to-nature activity of foraging’, the truth is, very few of us feel confident enough to start picking some of the foliage we see on our daily walks. Thanks to author Liz Knight’s clear descriptions, and the stunning botanical illustrations of Rachel Pedder-Smith, the identification is made far easier and reading this, you’ll really feel encouraged to explore and diversify with wild ingredients.

What’s more, it’ll certainly make you look at flowers in a different way. From honeysuckle cordial (which sounds like something the fairies would drink in A Midsummer Night’s Dream), pickled cherry blossoms, linden leaf madeleines, dandelion petal cake to damson and rose petal preserves, the accompanying recipes sound like a feast for all the senses.

 

Forage: Wild Plants to Gather and Eat by Liz Knight, illustrated by Rachel Pedder-Smith [Laurence King Publishing]
Buy it at Waterstones

There’s such an elegant and understated confidence to Forage, and no wonder – Liz has a wealth of experience, having spent years learning the ways of foraging, founded Forage Fine Foods – a business she runs from her kitchen in rural Herefordshire – where she teaches courses on foraging and cooking wild ingredients, and also sells some delicious foodie finds. You may also have seen her appearing on the eight-part series of Channel 5’s Escape to the Farm with presenter Kate Humble. But if you’ve the idea that Liz was born in the bosom of the country and learned such skills at her mother’s knee, it certainly didn’t come naturally.

 

 

‘I grew up in normal street in a normal town just outside London,’ says Liz, and it turns out she gradually grew to love freshly picked food having tasted the tomatoes from a neighbour’s greenhouse, and later, worked in care homes and talked to the older residents. Explains Liz:

‘These people knew food; they taught me how to make butter, what cuts of meat to buy and how to cook it, what leaves to nibble on and what food should really taste like…Thanks to them I got a fire in my belly about the wild, wonderful food of Britain and that fire turned into Forage.’

Nowadays we’re becoming used to seeing ‘foraged food’ celebrated on menus of fine dining restaurants, but really Liz wants everyone to feel confident enough to try their hand at picking ingredients growing wild locally. Because Liz’s life now truly is spent searching the local hedgerows in search of scrumptious finds, and we’re sure reading this book will sew some more seeds of the passion for foraging. Now you won’t only want to stop to smell the roses (and wild cherry blossom, linden trees, honeysuckle, gorse…) but eat them (once safely identified!) too.

By Suzy Nightingale

Chelsea Fringe: Free fragrant talks, from Georgian perfume to the scented room and beyond…

Perhaps seen as a floral equivalent of the Edinburgh Fringe, The Chelsea Fringe Festival is actually entirely independent of The Chelsea Flower Show, though acts with its full support. Intended to extend the enjoyment of gardens and all things verdant to well beyond a show ground setting, there are all manner of events taking place around the UK (and beyond) to celebrate this year.

We were particularly excited to hear about a series of talks focusing on perfume, with subjects ranging from Georgian and 19th Century perfume, the scented room, and the simulation of nature in 20th Century perfume (the last in that list being hosted by our very dear friend, Lizzie Ostrom -aka Odette Toilette!)

Chelsea Fringe say:Stephen Nelson is a plantsman and perfumer who specialises in re-creating historic fragrances. As part of the Chelsea Fringe, Town House will host talks by Stephen Nelson, centred around the English garden and its direct link to perfume over the past four centuries.’

Stephen has been commissioned to create everything from historically accurate pomander beads to perfumed leather from the 16th century, an 18th century spicy pot pourri and a handkerchief scent from the 19th century.

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Working extensively within horticulture and running his own nursery, Stephen breeds a number of fragrant plants including pinks, lilacs and lavender – some of which have been featured in planting schemes at the official Chelsea Flower Show.

What’s more, Stephen personally cultivates many of the ingredients used in his historical perfume re-creations – including damask roses, patchouli, orris, verbena, lavender and many more – putting him in the unusual position of seeing the fragrance through from seed to finished scent.

Have a look at the full range of events happening around the country, as there’s bound to be one near you!

A number of the talks are taking place at the historic Town House in London’s East End, and the series there has been dubbed ‘Garden Extracts’. Here’s the scent-centric talks that we’ll be writing in our diaries – and we can’t wait to get (our noses) stuck in…

TUE 31 MAY, 14:30pm: Free
Perfume in Georgian London – talk by Stephen Nelson.

THU 2 JUN, 14:30pm: Free
The Scented Room – talk by Stephen Nelson.

SUN 5 JUN, 14:30: Free
20th Century Perfume and the Simulation of Nature – talk by Lizzie Ostrom.

Written by Suzy Nightingale