How plants defend themselves against fungal infections

How plants defend themselves against fungal infections

Strawberries affected by mold. © Steven Koike/UCCE

Plants are not only susceptible to animal pests – they are also often affected by mold. But as researchers have now discovered, plants defend themselves against mold with a previously unknown weapon: In small lipid bubbles, they introduce different types of RNA into the mold’s cells. The RNA disrupts important cellular processes of the fungus, making it harder for the mold to spread. The findings could help develop new, environmentally friendly means of combating fungi.

If you leave fruit lying around for too long, you can see a greyish, furry coating forming on the surface: gray mold. The mold responsible for this, Botrytis cinerea, is widespread worldwide and can infect more than 1,400 host plants, including almost all fruits and vegetables and many flowers. Every year it causes billions of dollars in crop losses. But the plants are not defenseless against the infestation. They use various molecular strategies to defend themselves against the fungus.

Molecular battle between plant and fungus

A team led by Shumei Wang from the University of California at Riverside has now uncovered a previously unknown defense mechanism: “We discovered that the host plant thale cress (Arabidopsis thaliana) introduces small vesicles filled with mRNA into the fungus,” reports the team . “The mRNA is read in the fungal cells and translated into proteins, which in turn damage the fungus and reduce its spread.”

It was already known that when plants and molds are infected, they exchange numerous molecules with each other, which they use to defend themselves and harm each other. “In the past, we mainly looked at the exchange of proteins,” says Wang’s colleague Hailing Jin. “Thanks to modern technology, we have now discovered another important group of players in this fight.” Around ten years ago, Jin and her team had discovered that plants and fungi use small RNA molecules as weapons in addition to proteins. However, it was previously unknown that messenger DNA (mRNA) is also transferred, which is translated into proteins at its target.

Bubbles with harmful cargo

This strategy is very efficient for plants because even a few mRNA molecules can have a major effect. “The advantage of using mRNA as opposed to other forms of molecular weapons is that a single mRNA can be translated into many copies of proteins. This increases the effect,” explains Jin. “For example, the mRNAs can provide the blueprint for certain proteins that end up in the mitochondria of the mold cells, i.e. in the cells’ power plants. There they disrupt the structure and function of the fungal mitochondria, which inhibits the growth and virulence of the fungus.”

Why exactly the fungus ingests the bubbles containing the harmful cargo is not yet fully understood. “The fungus probably ingests bubbles because it wants nutrients. He doesn’t know that these RNAs are hidden in the vesicles,” says Jin. The fungus also introduces lipid vesicles containing harmful RNAs into the plants in order to weaken their defense system. The lipid shell not only serves as a kind of camouflage, but also protects the sensitive mRNA from being broken down prematurely. According to a similar principle, the mRNA of common corona vaccines is protected by a lipid shell.

New approaches to fighting mold?

From the research team’s perspective, the discovery of plant mRNA fungal defense could help develop novel, environmentally friendly means of combating fungi in the future. “RNA-based fungicides would not leave toxic residues in the environment and would not affect humans or animals,” says Jin. “RNA is naturally present in most foods and can be easily digested.” Fungal agents that take advantage of plants’ natural defense strategies could therefore help plants win the fight against mold without releasing toxins into the environment enter the food chain.

Source: Shumei Wang, (University of California, Riverside) et al., Cell Host & Microbe, doi: 10.1016/j.chom.2023.11.020

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