Fungal Facebook and Airborne Signals: How Plants Communicate

Love the smell of freshly cut grass? You’re a monster. That smell is cut grass signalling distress. It’s also the smell of the grass releasing airborne volatile organic compounds (VOCs) that repel herbivorous pests that could be feeding on the grass, and a way of attracting parasitic wasps. The wasps come and attack or lay eggs on the pests, killing the grass’s predators. So rather than a fresh aroma on the breeze, reminding you of innocent summers past, that smell is the grass summoning its allies to plant chest-bursting eggs inside its enemies, calling for a medic, and is just one of the many ways that plants communicate with one another.

We now know that many plants release VOCs. In the case of cut grass, these VOCs summon allies and repel predators. In other cases, VOCs signal to neighbouring plants that they are under attack. For example, lima bean plants under attack from beetles release VOCs into the air that warn other lima bean plants of the risk, causing them to grow faster and become more resistant to attack. Plants don’t discriminate either: signals from a cucumber plant can produce a defensive response in a chilli bush, and damage to a sagebrush can warn nearby tobacco plants to release their own defensive enzymes.

This kind of communication between plants has obvious benefits for the plants receiving the distress message. But it is not yet clear what the advantage is for the selfless plants emitting a signal/working in a soup kitchen. One explanation is that this kind of communication enhances the ability of the plant species to survive, a kind of ‘TAKE THE KIDS AND GET IN THE CAR’ type message. This strategy, known as kin selection, has gained some support with research showing that VOC communication is more effective between plants that are closely genetically related.

But then why are the messages received and responded to by plants of different species? One theory is that other plants are merely ‘eavesdropping’, and then using the information to strengthen their own defences. Quite sensible really, like reading the Neighbourhood Watch newsletter, or tapping the neighbour’s phone lines. Or perhaps plants are truly altruistic, and we all have something to learn from their selflessness?

As well as sending messages through the air, plants can communicate via a subterranean web of fungi and plant roots, much like the NBN, only functional. Part of the fungus, known as the mycelium, connects to the plant roots to form Common Mycelial Networks (CMNs). These CMNs carry messages from one plant to the next.

Researchers in Israel planted a series of pea plants so that each pot contained the roots of two different plants, essentially creating a CMN. They then exposed one of the plants to stress in the form of drought-like conditions. Within only 15 minutes, the stressed plant closed the pores on its leaves to conserve water. Shortly after, the plants that were connected to the stressed plant via the CMN showed the same response even though they weren’t exposed to the drought-like conditions. Plants not sharing roots with the stressed plant did not respond. In a similar study, tomato plants that were connected by a fungal network, but isolated in terms of airflow, activated genes to combat leaf blight when only one plant was actually infected.

CMNs have been shown to connect entire forest’s worth of trees, communicating information much further than airborne VOC emissions. In 2009, researchers documented a fungal network that connected dozens of trees over distances of up to 20 metres. Scientists are now also examining the possibility that plants may be communicating with one another via soundwaves undetectable by human hearing, after the publication of a study showing inter-species plant communication despite all known communication pathways being blocked.

While this all sounds a bit crazy, or like the plot line of another underwhelming M. Night Shyamalan spook-fest, keep in mind that plants have been around much longer than us: they must be doing something right. They might not have invented cars, understood negative gearing, or even worked out what a joke is, but they sure did invent Facebook long before we even crawled out of the water.

Dr Jessica L Paterson, Senior Research Fellow, CQUniversity, Appleton Institute