Gene controls plant-fungi relationship

The breakthrough could lead to crops that are better able to cope with harsh growing conditions and resist pests

Plants have a complex relationship with mycorrhizal fungi, which have a symbiotic association with a plant’s root system.

When joined, the fungi form a sheath around the roots and then spread beyond the roots to increase the uptake of nutrients from the surrounding soil for the benefit of the plant. In return, plants absorb carbon from the atmosphere and feed it through their roots to the fungi, promoting their continued growth and sequestering carbon dioxide in the soil.

Now, scientists at the U.S. Department of Energy’s Oak Ridge National Laboratory (ORNL) in Tennessee have discovered the specific gene that controls this symbiotic relationship, which could lead to the development of food and bioenergy crops that would be better able to cope with harsh growing conditions, resist pests, need less chemical fertilizer and produce greater yields per acre.

The researchers focused on the bioenergy feedstock crop Populus, or poplar tree, and the fungus Laccaria bicolor. They were able to narrow down the search to a particular receptor protein named PtLecRLK1, which was the most likely candidate gene.

Finding the genetic trigger that starts symbiosis took 10 years and came out of two projects. One was a study of the poplar tree’s native ecosystems and its interactions with microbes and microbiomes and the other was a study to further develop bioenergy and bio-materials using Populus and switchgrass feedstocks, both of which are among the best plants for the current sustainable development of diverse biofuel.

“We recently reported that a gene coding a lectin receptor-like kinase (a receptor that has a regulating activity) is critical for the initiation of the mycorrhizal symbiosis, particularly the ectomycorrhizal symbiosis between Populus and the fungus Laccaria bicolor,” said molecular geneticist Jessy Labbe.

“These molecular mechanisms underlying mycorrhizal symbioses are poorly known. More is known about how they function than how they happen and are regulated.”

He said that by understanding the mechanisms controlling the relationship between plants and fungi, researchers would be able to use the knowledge to establish better coping abilities in plants leading to greater resistance to drought and pathogens while improving nitrogen and nutrition uptake.

“The importance of this work is that we demonstrated these beneficial interactions are under the control of more or less specific gene-coding for a receptor with kinase function,” he said.

“This allows recognition and specific downstream regulation of the plant for the symbiosis.”

Labbe said these kinds of symbioses are very common — 80 to 90 percent of plants host them. It is believed that these mycorrhizal symbioses supported the ancient colonization of land by plants, enabling successful ecosystems such as vast forests and grasslands to survive and thrive.

However, some plant species resist fungi. In order to test whether a plant that does not normally host fungi can actually develop a symbiotic relationship with L. bicolor, the researchers used Arabidopsis, an easy-to-grow plant popular among scientists for genetic study.

According to the news release issued by ORNL, the researchers created an engineered version of the plant that expressed the PtLecRLK1 protein and then inoculated it with the fungus. The fungus completely enveloped the plant’s roots, forming a classic fungal sheath.

“We demonstrated that transferring this gene to another plant conferred the ability to get colonized by beneficial fungus,” said Labbe.

“If further work characterizes the diverse relative receptor coding gene for other fungi with other specific functions, this study will prove that such a gene can be transferred across plants. This is enormous; we may have the possibility to increase plant health (and) grow crops in areas not (currently) suitable. For instance, use of marginal land for bioenergy production (would be) more possible. Microbial inoculants are already used in agriculture. Now we could extend this use and foster microbial communities toward a specific function that would be needed to grow on a specific soil type, a climate region or a harsh environment. While a lot of work would need to be done, this could be used to increase carbon sequestration in soil and maintain it longer in response to the challenges of climate change. Along with my colleagues, we are continuing this work.”

In an independent study done by the International Institute for Applied Systems Analysis in Laxenburg, Austria, ecosystems encompassing mycorrhizal vegetation store about 350 gigatons of carbon annually, compared to just 29 gigatons stored by plants that do not have the symbiotic relationship.

“We showed that we can convert a non-host into a host of this symbiont,” Wellington Muchero, an ORNL quantitative geneticist, said in the news release.

“If we can make Arabidopsis interact with this fungus, then we believe we can make other biofuel crops like switchgrass, or food crops like corn, also interact and confer the exact same benefits. It opens up all sorts of opportunities in diverse plant systems. Surprisingly, one gene is all you need.”

Labbe said there is enthusiasm among farmers for the research being done and the genetic testing that is continuing.

“One implication of this work is that the design of host–microbe symbioses in non-host organisms is feasible, and the gene we discovered provides an immediate target for design. We are now testing this in food and bioenergy crops while we further characterize the complete downstream mechanisms in Populus, particularly with the hope for improving pathogen and other stress tolerance.”

The potential is for bioenergy crops that could thrive on marginal, non-agricultural land, which could mean as much as 20 to 40 million acres producing hardy bioenergy crops and driving rural, bio-based economies.

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