Plants call out with chemical signals in times of stress, summoning microbes that can unlock bound nutrients and find water in soil pores too small for the finest roots. In return, microbes get a safe place to live or a sugary drink.
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It’s a classic you-scratch-my-back-I’ll-scratch-yours scenario. Except when it’s not.
New research from the University of Illinois Urbana-Champaign challenges conventional wisdom to show free-living soil microbes are just looking out for themselves.
In a multi-generation experiment, researchers from the College of Agricultural, Consumer and Environmental Sciences found microbes helped plants cope with drought, but not in response to plants’ cries for help. Instead, the environment itself selected for drought-tolerant microbes. And while those hardy microbes were doing their thing, they just happened to make plants more drought-tolerant, too.
“It was a surprise because I expected to see evidence of coevolution and mutualism between the microbes and plants. I think people, myself included, forget that just because microbes do something adaptive or beneficial to the plant, it doesn’t necessarily mean they’re doing it for the plant,” said Kevin Ricks, who completed the project as part of his doctoral degree. Ricks is now a postdoctoral researcher at the University of Toronto.
To learn how microbes help plants deal with drought, Ricks established live soil communities in pots with or without plants. He watered half of the pots well and imposed drought conditions in the other half, then repeated these treatments for three generations. The idea was to allow time for selection to occur — potentially for plants to signal their need for help and select for microbes that came to their aid.
In phase two of the experiment, Ricks mixed everything up. He again grew plants in soil from phase one and kept the same watering treatments, but some plants were now experiencing drought in soils that had been well-watered for generations, and vice versa.
He expected soil microbes from historically dry pots would have adapted to those conditions, helping plants withstand drought more than microbes from historically wet pots. And that is what he found. Plants experiencing drought were bigger when grown with drought-adapted microbes.
But — and this is key — that was true for soils grown with or without plants in phase one. In other words, microbes adapted to drought over time even without plants selecting for them through chemical signals. Yet they still provided benefits when grown with plants generations later.
It was proof these microbes were doing their own thing, only helping plants incidentally.
Tony Yannarell, associate professor in the Department of Natural Resources and Environmental Sciences, at Illinois, is the co-author of the research, which has been peer-reviewed in the Proceedings of the Royal Society B: Biological Sciences.
