Plants warn neighbours of overcrowding

Plants actively participate in social interactions with their neighbours, and respond to signals generated by roots that inform them about crowded conditions above-ground.

That’s the conclusion reached by a group of researchers studying maize plants – and one that carries implications not just for agriculturalists, but also for other researchers. The mere act of brushing the leaves of one plant, it turns out, can significantly change the shape and growth direction of another.

A team led by Ali Elhakeem of the Swedish University of Agricultural Sciences set out to discover whether plants, in this case maize (Zea mays), signalled each other in relation to growing conditions – in particular, about their proximity to each other. 

In doing so they were building on earlier studies that established that plants possess signalling systems that transmit information from one to another. A 2012 study, for instance, found that some plants will allocate more biomass to root development and less to fruits if the plants next to them are siblings. Relationships are identified by means of chemicals secreted by the roots.

A 2006 study showed that trees called lodgepole pines stopped expanding their crowns when the outermost leaves brush against those of a neighbouring tree.

Elhakeem and colleagues wondered whether two neighbouring maize plants coming into contact with each other would then signal their close proximity to other, nearby plants, and if so what would happen.

To discover this, they germinated a number of plants in hydroponic containers, some as pairs and some on their own. One plant from each pair was then lightly touched, simulating the pressure of an adjacent leaf brushing up against it.

In the paired containers, the roots of the untouched plants grew significantly more away from the roots of the touched one. New plants put into hydroponic containers that had previously contained a touched plant grew smaller root systems and larger above-ground structures.

Elhakeem’s team found that touched plants exuded a particular mix of chemicals from their roots, which influenced the growth, above and below ground, of their neighbours.

The scientists conclude that the results reveal “a new level of complexity in below-ground plant-plant interactions showing that the direction and extent of plant root responses to neighbours can be affected by the above-ground physical stress to which neighbours are exposed”.

They also suggest the results embody a cautionary tale for other botanists. Researchers studying plants grown in the lab, they say, should henceforth interpret plant shape and root mass in light of how often the plant itself, or its neighbours, have been touched.

The research is published in the journal PLOS ONE.