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.

Why are smell memories so strong? New research reveals startling results

We all know how transporting smell memories can be – the whiff of someone’s perfume as they pass by immediately propelling you to another time, place or person you associate it with. It has long been known our sense of smell is the strongest link to unlocking these memories, but new research has only just revealed why

An international team of scientists, led by Christina Zelano from the Northwestern University Feinberg School of Medicine, used neuroimaging and intercranial electrophysiology to discover why certain areas of the brain, such as the hippocampus, are more strongly linked with smell than any other sense. According to a report on the science news website New Atlas:

‘This new research is the first to rigorously compare functional pathways connecting different human sensory systems with the hippocampus. The striking findings reveal our olfactory pathways connect more strongly with the hippocampus than any other sense.’

‘During evolution,’ Zelano explains, ‘humans experienced a profound expansion of the neocortex that re-organised access to memory networks.’ Basically put, all other senses got re-routed as sections of our brains expanded, but smell remained intrinsically (and directly) connected to the hippocampus. Or as Zelano more scientifically puts it: ‘Vision, hearing and touch all re-routed in the brain as the neocortex expanded, connecting with the hippocampus through an intermediary-association cortex-rather than directly. Our data suggests olfaction did not undergo this re-routing, and instead retained direct access to the hippocampus.’

While this is, of course, fascinating; perhaps the more practical outcome of this, and other continuing research, is a reaffirmation of how important our sense of smell is to our wellbeing, and impacts on our every day lives even more than was previously assumed. Indeed, the discoveries of links between our sense of smell and depression (and how scent might be used in the future to treat it), has been significantly highlighted because of Covid-19 cases often suffering with anosmia (a lack of smell) and parosmia (a distorted sense of smell).

 

 

You can read more about anosmia and parosmia on our website by searching for those terms, and also in Louise Woollam’s piece about how devastating it was to lose her sense of smell as a fragrance blogger. It’s a subject Louise wrote about so movingly, again, more recently for our magazine, The Scented Letter: Perfume’s Bright Future edition. VIP Subscribers receive this magazine FREE, but you can also buy print copies, here, or purchase an International Online Subscription at only £20 for a full year of fragrant reading.

By Suzy Nightingale

Eating garlic makes men smell more attractive to women [apparently…]

Although it may sound completely counterintuitive, results of a 2015 study by University of Stirling and Prague’s Charles University revealed that women who sniffed the heady scent of male body odour perceived it to be ‘significantly more attractive and less intense,’ when the men had eaten garlic.
82 women were asked to smell odour samples from 42 men, collected on pads worn in their armpits, and judge them for ‘pleasantness, attractiveness, masculinity and intensity.’
[Permit us to wonder, with eyebrows firmly raised, if the same study has been repeated with the gals eating garlic.]
Researchers concluded the results possibly showed that as ‘…the health benefits of garlic consumption include antioxidant, immunostimulant, cardiovascular, bactericidal and anti-cancer effects, it is plausible that human odour preferences have been shaped by sexual selection.’

So that’s vampires bang out of luck, then.
Previous research has shown that many animals’ noses are honed to select mates in the best physical condition, and therefore human noses might well be similarly evolutionary primed to seek out partners with the most promising whiffs. Something to think about the next time your special someone reaches for that extra slice of garlic bread.
Those of you wishing to learn more about the fascinating science of taste and smell can get the juices flowing by reading our latest issue of The Scented Letter Magazine. Full to bursting with the latest news, exclusive interviews and award-winning fragrance journalism – we think you’ll find this is our most delicious issue yet…
Written by Suzy Nightingale