‘Plant interactions with others evolved over aeons’

Dr Meghna Krishnadas is senior scientist at the CSIR Centre for Cellular and Molecular Biology (CCMB), India. Speaking to Times Evoke, she discusses intricate links plants evolved with diverse species over time:

Research suggests many of the world’s earliest plants, while they were making their way from sea to land, needed to associate with certain types of fungi — these became mycorrhizal fungi which functioned as primitive roots, helping plants which hadn’t yet developed their full root systems to colonise land. This association has continued to this day. The first such interactions were with arbuscular fungi with tentacles and then, ectomycorrhiza which form a sheath around a plant. These were the first associations plants developed to gain an evolutionary advantage.

As different plants evolved, they developed many other interactions with diverse species. The emergence of flowering plants or angiosperms generated a relationship between pollinating insects and plants which grew flowers to attract them. Angiosperms also show co-evolution between plants and seed dispersers or animals which eat their fruits, gaining nutrition, while carrying the seeds far away, giving the plant an opportunity to colonise new space or escape pathogens. These relationships are called mutualisms which benefit all involved.


Interestingly, some organisms evolved negative associations with plants — and counterintuitively, this helped plants diversify more. Consider herbivorous insects which eat leaves — long back in evolutionary time, when such an insect began eating its leaves, a plant would be under pressure to evolve a defence mechanism against it. So, plants developed secondary metabolites or chemicals they sometimes produce which work in a similar way to a human’s immune system, warding off pathogens. These plant responses can be physical in terms of the thickness of leaves, their structure, developing stickiness, etc. — the evolution of such chemical compounds could have helped many plants diversify incredibly through time. An example is a genus called the Inga located in the neotropics, the tropical forests of the Americas, which has many different species, all within the same patch of forest. Scientists wondered why this happened, when all of them have similar resource requirements — what explains these plants diversifying so much? It turns out there is a signature which shows, how, because of attacks by herbivorous insects, these species of Ingas evolved differ- ent secondary metabolites or compounds and are now differentiated by them. They were also in a kind of arms race with insects, which kept diversifying as well. We may think of this as a negative inter- action but, in fact, it had a very positive result on the evolution of diversity.


We are only just beginning to unpack the consequences of human actions on these ancient interactions. What we do know is the evolutionary force which has been acting for so long has suddenly been diluted in some spaces. Scientists are studying the precise consequences of this. Research in Amazonia has found that in forests where the large seed dispersers have been hunted out, the same plant species have begun developing smaller seeds — this suggests a push to enable smaller animals who are the only seed dispersers left.




IT TAKES A FOREST: Mutualisms over millennia between plants, birds and animals strengthened ecosystems. Photos: iStock


While evolutionary changes are being studied closely, we can clearly see many eco- logical changes due to human actions. The cutting-up of forests into fragments or the loss of forests can result in losing specialist insects which evolved to rely on just one or two plants. This could mean a possible loss of plant diversity over time. We can’t even see a change in the microbe community in soil, air, etc. — this happens almost in the background of human perception but it can lead to a loss of plant diversity because these interactions allow different species to coexist with one another, controlling numbers and stabilising diversity.

All these interactions should teach us to value nature’s diversity and complexity — multiple processes on Earth evolved over deep time. Many of these may not be apparent to us but they are critical in making the world what it is. We should keep this in mind when we embark on changing ecosystems.