A breakthrough study published in Molecular Plant-Microbe Interactions® (MPMI) reveals how the destructive fungal pathogen Fusarium graminearum uses a specialized protein to weaken plant immune defenses and cause Fusarium head blight (FHB), a devastating disease that severely damages wheat and barley crops worldwide. These new insights into how F. graminearum attacks crops could lead to the development of genetically engineered disease-resistant grains.

5606297-PPT

Source: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

Fusarium head blight (Fusarium graminearum Schwabe)

This collaborative research team, led by Matthew Helm of the U.S. Department of Agriculture–Agricultural Research Service (USDA-ARS), West Lafayette, Indiana, Roger Innes at Indiana University Bloomington, and Kim Hammond-Kosack at Rothamsted Research in the United Kingdom identified and functionally characterized a fungal protein called TPP1.

READ MORE: Soil’s secret language: Researchers decode plant-to-fungi communication

READ MORE: Soil microbiome yields revolutionary new biofungicide

This effector protease is secreted by F. graminearum during infection and plays a central role in helping the fungus overcome plant defenses by targeting the chloroplast—an essential part of the plant cell responsible not only for energy production but also for immune signaling.

“What excites us most is that this effector protease not only promotes disease but also targets a specialized plant cell structure known as the chloroplast, which is an unexpected and strategic location for disarming the plant’s immune system,” Helm said. “This study could be transformational for developing disease-resistant crops.”

Global food security threat

FHB continues to threaten global food security, causing significant yield losses and contaminating grain with mycotoxins that are harmful to humans and livestock. In this study, researchers showed that when the gene for TPP1 was knocked out, the fungus became significantly less virulent, confirming that this protein is essential for infection. The finding sheds light on a largely unexplored mechanism in fungal pathogenesis.

Low-Res_Helm_TPP1

Source: Courtesy of Matthew Helm.

The PH-1-ΔFgtpp1-1 mutant did not show an observable virulence defect when wheat spikes were inoculated using the top-inoculation method. Yellow arrows indicate inoculation points. Wheat spikes infected with either the ΔFgtpp1-1 mutant (PH-1-ΔFgtpp1-1) or wild-type F. graminearum PH-1 strain showed similar disease symptoms (left). The number of diseased spikelets throughout the time courses for the ΔFgtpp1-1 mutant and wild-type F. graminearum PH-1 were similar (middle), and the area under the disease progress curve (AUDPC) values calculated for the mutant and wild type did not reveal significant differences (right). Violin plots show distribution of the AUDPC values (black dots), average AUDPC values, and confidence intervals (rectangles) for each fungal strain. The statistical analysis (t test) included pooled data from three independent replicates. Each replicate consisted of 12 plants, where 6 plants (one spike per plant) were inoculated with either ΔFgtpp1-1 mutant or wild-type F. graminearum PH-1 strains (the total number of plants inoculated was 17 and 18 for the ΔFgtpp1-1 mutant and wild-type F. graminearum PH-1 strain, respectively).

This is the first report to identify an effector protease from F. graminearum that targets the chloroplast and directly contributes to disease development by suppressing plant immune responses.

“The discovery of TPP1’s role marks a significant advancement in our understanding of fungal pathogenesis. It also opens up exciting possibilities for using ‘decoy’ engineering strategies to develop wheat and barley varieties with built-in resistance,” the research team noted.

Highly conserved across pathogens

“In addition, TPP1 appears to be highly conserved across a broad group of fungal pathogens, making it potentially a prime target to deliver plant disease resistance against other problematic fungal species” said Hammond-Kosack.

With implications for both the plant-microbe and broader host-microbe research fields, this foundational work brings researchers one step closer to long-term goals of reducing crop loss and ensuring food security. It lays the groundwork for bioengineering more resilient wheat and barley varieties, which is an urgent need in the face of a changing climate and rising global food demand.

“Our ultimate goal is to protect global food supplies by reducing crop losses from Fusarium head blight. This study is an important step toward that goal,” the authors said.