Last updated February 22, 2018 at 2:54 pm
Big leaves and pretty flowers had nothing to do with it. The spread of plants around the globe was enabled by developments out of sight and underground.
As the land-based plant kingdom commenced its gradual spread north and south from its tropical beginnings it did so by evolving ever thinner roots, according to a new study.
A team led by Lars Hedin of the Department of Ecology and Evolutionary Biology at Princeton University analysed root traits in 369 plant species found in natural plant communities across seven biomes, and find a clear pattern: the further from the tropics a plant species resides, the narrower its exploratory roots.
Geographic location also determines a second, equally important survival trait. Plants that live in tropical environments – genetically, the most diverse and with the oldest ancestries on the planet – very often exist in symbiotic relationships with fungi.
The fungi – known as mycorrhiza – colonise plant roots and assist with the gathering of essential nutrients, such as nitrogen and phosphorus.
How plants took over the world
Plants that colonised less resource-rich environments, characterised by poorer soil and, in some circumstances, periodic drought, harsh winters or snow, ditched the fungus and developed narrower roots with stronger tips.
These adaptations, the researchers write, allowed them to dramatically increase the amount of soil they could explore in search of nutrients in relation to each unit of carbon invested. Also, no longer reliant on mycorrhiza, they were free to enter territories that their former partners might have found inhospitable.
“These are the secret strategies that plants have used over time to take over the world,” Hedin says.
“Our goal was to unlock the understanding of those strategies, and our findings offer a new global theory for plant evolution. Hidden underground there has been a tremendous game of survival-of-the-fittest and we are fortunate to have the first-ever view of the science of that game.”
Taking a decade to gather and analyse their samples, the scientists discovered that plants resident in tropical biomes tend to have fine root tips between a 0.25 and one millimetre wide. Living in soil that is consistently wet and full of helpful fungi, these plants use what Hedin describes as a “conservative” strategy.
Coping with harsher environments
In contrast, plants in harsher environments tend to have fine root tips smaller than 0.25 of a millimetre, made for efficient and rapid exploration to allow for the discovery and exploration of scarce food resources.
The study represents the first large-scale comparison of root structures.
“Thus far everybody has quite naturally tried to understand how plants are organised by looking at above-ground traits,” says Hedin. “But our findings do not follow the above-ground theories – that was a surprise.”
Co-author Mingzhen Lu adds that the theory that root traits condition at least in part the ability of plant species to survive in particular environments means the patterns unearthed by the study carry lessons for ecological management.
“Our findings simplify how we can practically characterise a plant’s strategy for obtaining nutrients,” she says. “Knowing their underlying nutrient strategy will help us know how to preserve them, or know the conditions under which they could or could not survive.”
The paper was published in the journal Nature.