Decoding plant communication

natural alliances

Human beings talk using one of the most complex systems of communication ever developed — languages. Animals howl, bark or hoot to be heard. A few are special: peacocks fan their feathers, dolphins slap their tails and bees dance. There are some who use their sense of smell to pick up another’s message. All animals, humans included, have mastered a lingo best understood by members of their own species. It is, of course, an essential prerequisite for survival.

Plants, unlike animals, do not make any overt gestures of communication that our eyes can explicitly see and make one believe that they are incapable of communication. However, contrary to popular beliefs, plants are as equally adept in the art of signalling and communicating as their animal counterparts. And this is one of the primary research interests of Professor Renee M Borges from the Centre for Ecological Sciences, Indian Institute of Science, Bengaluru, who has spent many years delving into the numerous aspects of plants’ communication and their language.

Though plants are stationary, a few important phases in their life cycle, such as pollination and seed dispersal, require mobility. During these times, plants seek the assistance of members of the animal world by ‘calling out’ to them. One way they do this is by releasing into air, a molecular cocktail (volatile chemicals) that animals can pick up using their olfactory senses. Some examples of such cocktails are the characteristic smells that plants emit during the blooming of a flower or the ripening of a fruit. And each of these is nothing but ‘plant talk’.

Renee studies the symbiotic relationship that exists between plants and the animals they talk to. In the book Signalling and Communication in Plants: Deciphering Chemical Language of Plant Communication, she explains one of the best examples of ‘plant talk’ illustrating a plant-animal mutual dependence that has been in existence for more than 80 million years — the relationship between the fig tree and the fig wasp. This relationship is a classic representative of mutualistic collaboration, where both, the plant and the animal, benefit.

Fig-wasp relationship
In this case, the fruit of the fig plant, which is actually a hollow flower garden, extends its services to wasps as a nursery for the young ones. Inside the fig are tiny male flowers that contain pollen, or female flowers that require pollen, for pollination. Mother wasps that hear the ‘smelly’ talk of the fig tree from the figs, fly towards such figs and enter them in order to lay their eggs. Once inside, the mother wasp dies as a result of her arduous journey into the fig, after laying her eggs inside the fruit. Upon hatching, the hatchlings feed on nourishing sacks of nutrients available inside the fig. When the female hatchlings exit the nursery, they carry pollen, which can fertilise female flowers inside the next fig they enter.

This mutual dependence is very specific — the wasps cannot breed anywhere else and the plant is exclusively dependent on the wasp for pollination. Both need the other for their survival. Most species of fig trees have their own specific species of pollinator fig wasps, and the smells emitted from a particular kind of fig can only ‘call out’ to a specific kind of wasp. There are, in fact, around 800 various figs-wasp relationships in existence today, each employing a distinct cocktail of volatile molecules.

In the forest, there exist thousands of other such alliances. There are different plants calling out to various insects, birds and four-legged animals. Each plant emits a concoction of specific proportions of assorted molecules during its callout regime. Imagine the spring when every other plant is busy spewing out into the air, their own signature cocktails, using similar (and often even the same) ingredients as its neighbouring plant. How can the olfactory system of any creature distinguish a smell signal that is meant for it, from the molecular soup of smells that exists in forests? How did such specificity evolve? How can animals discern a specific signal from the smelly commotion that surrounds it? Do they respond to a single ingredient or a proportion of ingredients? Can climate changes affect their communication? And more interestingly, how do researchers even attempt to figure all this out?
Renee and her team employ diverse experimentation strategies to study such relationships that exist between plants and the animals they communicate with.

While some studies involve classical ecological methods such as field sampling and direct observations, others include molecular-level studies to analyse the scent molecules emitted by plants, behavioural studies such as observing the response of organisms to an artificially set-up stimulus like smell, etc.

Since their studies include field observations as well as experiments in the lab, there is quite a bit of travel involved. While they have their own grove of ‘study-plants’ within the IISc campus, the environs of Bengaluru and in Kodagu, they also work in the forests of Agumbe, located in the Shivamogga district and the Bhimashankar Wildlife Sanctuary in Pune, both of which are located in the Western Ghats. “We frequently go to the Bhimashankar Wildlife Sanctuary to study pollination systems in seasonal cloud forests. The sanctuary has a host of endemic plants, amphibians, reptiles, eels, fabulous diversity of butterflies and moths, and unique endangered species such as the painted bat,” says Renee. “We also found the world’s only known nocturnal bee that can distinguish colours under starlight illumination here,” she reminisces.

To observe, experiment and decipher results in a field such as this, researchers need an understanding of diverse subjects such as ecology, chemistry, atmospheric science, fluid dynamics, behaviour and neurobiology. As the professor states in her book, the need of the hour is collaboration between various disciplines that will provide better answers to Jean-Henri Fabre (an early French entomologist), who wrote almost a century ago, “It is a recognised fact that smell, ordinary smell and the smell that affects our nostrils, consists of molecules emanating from the scented body. But what is materially emitted by the female Bombyx or great peacock moth? Nothing, according to our sense of smell. Should science one day, instructed by the insect, endow us with a radiograph of smells, this artificial nose will open out to us a world of marvels.”

(The author is with Gubbi Labs, a research collective in Bengaluru)