The Fusarium graminearum candidate effector protease FgTPP1 targets chloroplasts and suppresses immunity to facilitate Fusarium Head Blight disease

Authors: DARINO, MARTIN - Rothamsted Research; Jaiswal, Namrata; KROLL, ERICA - Rothamsted Research; URBAN, MARTIN - Rothamsted Research; DARMA, REYNALDI - Rothamsted Research; XIANG, YOUHUANG - Indiana University; SRIVASTAVA, MOUMITA - Rothamsted Research; Scofield, Steven; HAMMOND-KOSACK, KIM - Rothamsted Research; Helm, Matthew; Kim, Hye-Seon; MYERS, ARIANA - US Department Of Agriculture (USDA); INNES, ROGER - Indiana University

Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/22/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: Fungal plant pathogens need to circumvent the plant immune system in order to promote their growth and sustain colonization. To do this, fungi often use molecules (i.e. proteins) called 'effectors' to “shut-off” the plant immune system, thereby allowing the pathogen to colonize the host. One such fungal pathogen, Fusarium graminearum, uses effector proteins to cause a disease called Fusarium Head Blight (FHB) on wheat. Importantly, FHB is one of the most important diseases of wheat and is responsible for significant wheat yield losses. Therefore, there is an urgent need to investigate how this fungal pathogen uses its effector proteins to infect wheat. In this study, we identified a protein from Fusarium graminearum called TPP1 and showed that when this gene is removed from the fungal genome, FHB disease symptoms are significantly weakened in wheat. These results indicate that TPP1 contributes to FHB disease development in wheat. To further examine how TPP1 affects FHB disease progression, we show that this fungal protein targets host chloroplasts and suppresses immune responses. Collectively, our results presented in this study indicates this fungal pathogen targets chloroplasts to suppress plant defense responses, thereby enabling FHB disease development. Our results are expected to significantly contribute to the development and deployment of enhanced disease management strategies for U.S. wheat farmers.

Technical Abstract: Most fungal pathogens secrete effector proteins inside host cells to circumvent host immune responses, thereby promoting pathogen virulence. One such fungal pathogen is Fusarium graminearum, which causes Fusarium Head Blight (FHB) disease on wheat and barley. Recent transcriptomic analyses revealed F. graminearum likely secretes nearly six hundred candidate effector proteins during the early phases of the infection process, some of which are annotated as proteases. However, the function of F. graminearum candidate effector proteases in plant pathogenesis remains unknown. Here, we identified a F. graminearum endopeptidase, FgTPP1 (FGSG_11164), that is highly upregulated during the early stages of wheat spikelet infection and is secreted from fungal cells. To elucidate the potential role of FgTPP1 in F. graminearum virulence, we generated FgTPP1 deletion mutants ('Fgtpp1) and performed FHB virulence assays. While the 'Fgtpp1 mutant was able to colonize the inoculated spikelet, the disease symptoms were consistently reduced when compared to wild-type F. graminearum strain PH-1, suggesting FgTPP1 contributes fungal virulence. Laser scanning confocal microscopy of Nicotiana benthamiana epidermal cells transiently expressing green fluorescent protein (GFP)-tagged FgTPP1 revealed this candidate effector protease localizes to the chloroplast stroma as well as the nucleus and cytosol. Furthermore, FgTPP1:GFP consistently attenuated cell surface- and intracellular-mediated immune responses in N. benthamiana including chitin-triggered reactive oxygen species production, activation of mitogen-activated protein kinase (MAPK) signaling, and NLR-mediated cell death. Our results provide new insights into the functions of a F. graminearum candidate effector protease in plant pathogenesis.