How plants distinguish between good and bad microbes

In a new study from the American Phytopathological Society, researchers are shedding new light on how plants distinguish between microbes that are beneficial to them and those that cause disease. 

Plants are constantly exposed to different types of microbes, including harmful pathogens and mutualists that promote plant growth and development. For example, most land plants form mutually beneficial relationships with arbuscular mycorrhizal fungi to improve nutrient uptake. 

“Maintaining a balance in plant disease resistance where plants can fight off pests and pathogens but still engage with microbes that can help with nutrient uptake is essential for the health of our crops,” explained study co-author Dr. Cara.

The experts set out to investigate the process that allows plants to fight off pathogens without also killing beneficial microbes, and without wasting energy on commensals that cause no harm or benefit.

According to Dr. Haney, this question is so broad that the review actually raises more questions than it answers. “We attempted to highlight much of what was known, but also what wasn’t known and to provide paradigms and models that could be frameworks going forward.”

The researchers explored three principles – chemical selection with metabolites, dual receptor recognition, and tuning of an immune thermostat.

Plants first use metabolite compounds like chemical signals to recruit beneficial bacteria and restrict pathogens, but not all pathogens are excluded.

Study co-author Dr. David Thoms theorizes that once the plant and microbe are in contact, a “dual input model” is used by the plant to distinguish the type of microbe present and whether it is a mutualist, commensal, or a pathogen. 

Microbes are detected by receptor proteins on the surface of plant cells. When receptor proteins detect chitin, the plant knows that a fungal microbe is present.

At the same time, chitin and other microbe-associated molecular patterns (MAMPS) are often shared among microbes. This means they are insufficient signals to distinguish commensals from pathogens, according to Dr. Thoms.

“Unlike most animals, plants do not have an adaptive immune system. However, plant genomes contain many more innate immune receptors than animals,” explained study co-author Dr. Yan Liang. “Plants also use similar receptors to sense signaling molecules from beneficial microbes, environments, as well as their own cells.” 

The experts propose that additional receptors use a second layer of information to identify pathogens versus mutualists. Symbiosis receptors can identify signaling molecules specifically produced by beneficial microbes, while immune receptors can identify pathogen proteins intended to shut down a plant’s defense mechanisms. 

“I think it’s cool that plants can perceive so many types of MAMPs across different kingdoms of life and I think one use of that is recognizing where the microbe is coming from to give the appropriate physiological response,” said Dr. Thoms.

“Many living things, ranging from plants to humans, are faced with the challenge of engaging with beneficial microbes while restricting pathogens,” noted Dr. Haney. 

“As a result, paradigms in plant-microbe interactions may shed light on eukaryotic interactions with microbes in diverse organisms.”

The study is published in the journal Molecular Plant-Microbe Interactions.

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By Chrissy Sexton, Earth.com Staff Writer