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Discovery reveals how fungi bypasses plant defenses, kills plants

Closeup of a sunflower plant rotting from white mold.
The fungi that causes white mold rots a sunflower plant.

An insight into how the fungi that causes white mold rot avoids plant defenses could lead to a new tool to combat a pathogen that causes billions of dollars of crop loss worldwide.

A team led by U.S. Department of Agriculture’s Agricultural Research Service and Washington State University researchers found that the Sclerotinia sclerotiorum fungus uses a protein, called SsPINE1, to bypass natural plant defenses and cause extensive rot in hundreds of broad leaf plant varieties.

The findings were just published in Nature Communications.

“Sclerotinia causes stem rot on more than 600 plant species, including peas, lentils, canola, potatoes, soybeans, and many other broad leaf crops,” said Weidong Chen, corresponding author on the paper, adjunct professor at WSU and plant pathologist with the USDA-ARS.

Plants rot when a pathogen degrades cell walls, breaking down the plant by secreting chemicals called polygalacturonases (PG).  Plants protect themselves using an inhibitor protein, called PGIP, which was discovered in 1971. Since that discovery, scientists have wondered how certain pathogens, such as the Sclerotinia fungus, gets around the inhibitor protein to cause extensive rot.

Chen and his colleagues discovered a protein secreted by sclerotinia cells, called SsPINE1, that inactivates a plant’s defenses. The key to finding the protein was looking outside the fungi cells.

“We didn’t know what we were looking for,” Chen said. “When we found this protein, SsPINE1, that interacted with PGIP, it made sense. But we weren’t really looking for it: we found it by looking outside the cells at the materials excreted by the fungus.”

Weidong Chen

Sclerotinia does so much damage around the world that USDA and the U.S. Congress have been funding the National Sclerotinia Initiative for more than 20 years.

“I got goosebumps when we found this protein,” said Kiwamu Tanaka, an associate professor in WSU’s Department of Plant Pathology and a co-author on the paper in Nature Communications. “It answered all these questions scientists had for the last 50 years: Why these fungi always overcome plant defenses? Why do they have such a broad host range, and why are they so successful?”

To prove that the protein SsPINE1 allowed Sclerotinia to bypass plant defenses, the team deleted the protein in fungi in the lab, which dramatically reduced its impact. This discovery could allow breeding of plants with natural resistance to this protein.

“The long-term goal is to reduce the use of chemicals with more useful control management through breeding,” Tanaka said. “The current best way to fight sclerotinia is spraying fungicides, which can unintentionally harm other organisms. Breeding resistance is an environmentally friendly way to manage this pathogen.”

“It’s exciting that we found this new protein,” said Wei Wei, a WSU post-doctoral researcher and first author on the paper. “We hope that this helps increase resistance to fungal infections in plants around the world.”

The research team included scientists from USDA-ARS, WSU, Northwestern A&F University in Shaanxi, China, Wuhan Polytechnic University in Wuhan, China and Huazhong Agricultural University in Wuhan.

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