Eucalyptus trees rely on root fungi to source nutrients and water – but the fungi actually control the genetic development of the tree roots by releasing tiny chunks of RNA
A root fungus that helps eucalyptus trees get nutrients and water has a surprising way of maintaining this symbiotic relation. It releases tiny bits of RNA that manipulate gene activity in the tree roots.
“It is a bit like a key and lock mechanism where the RNA is a key needed to unlock access to the plant,” says Jonathan Plett at Western Sydney University in Australia.
Many trees form symbiotic relations with ectomycorrhizal fungi, which wrap around small roots. “Think like a hot dog in a bun, where the root is the hot dog and the fungus is the bun surrounding it,” says Plett.
This process stops the root growing, but fungal finger-like projections – hyphae – extend out into the soil far beyond the normal root system of the tree. They gather nutrients that the fungus trades for plant sugars.
It is known that ectomycorrhizal fungi “talk” to plants by releasing a variety of proteins. Now Plett and his colleagues have shown that one fungus (Pisolithus microcarpus) also releases a microRNA when it colonises the roots of the flooded gum tree (Eucalyptus grandis).
MicroRNAs are small bits of RNA that reduce the production of certain sets of proteins. Cells normally use microRNAs to control their own gene activity, but some pathogens also release microRNAs to turn off genes involved in cellular defence in a potential target organism.
P. microcarpus does something similar. After discovering that P. microcarpus releases a microRNA called Pmic_miR-8, Plett’s team blocked Pmic_miR-8 in the roots of seedlings growing in the lab. They found that previously colonised roots resumed growth, showing that Pmic_miR-8 is essential to maintaining the symbiotic relationship.
It is likely that many other ectomycorrhizal fungi release similar microRNAs. “By identifying the ‘keys’ used by beneficial microbes to colonise plants, we can hope in future to guide plant breeders to develop plants that will be better able to associate with beneficial microbes,” says Plett.
This could make plants less reliant on fertilisers, making forestry and farming more sustainable, he says.
Journal reference: PNAS, DOI: 10.1073/pnas.2103527119
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