TheFusarium graminearumeffector protease FgTPP1 suppresses immune responses and facilitates Fusarium Head Blight Disease

Abstract

Most plant pathogens secrete effector proteins to circumvent host immune responses, thereby promoting pathogen virulence. One such pathogen is the fungusFusarium graminearum, which causes Fusarium Head Blight (FHB) disease on wheat and barley. Transcriptomic analyses revealed thatF. graminearumexpresses many candidate effector proteins during early phases of the infection process, some of which are annotated as proteases. However, the contributions of these proteases to virulence remains poorly defined. Here, we characterize aF. graminearumendopeptidase, FgTPP1 (FGSG_11164), that is highly upregulated during wheat spikelet infection and is secreted from fungal cells. To elucidate the potential role of FgTPP1 inF. graminearumvirulence, we generatedFgTPP1deletion mutants (ΔFgtpp1) and performed FHB infection assays. While the number of completely bleached spikes infected byF.graminearumwild-type reached 50% of total infected spikes, the number of fully bleached spikes infected byΔFgtpp1mutants was 25%, suggesting FgTPP1 contributes to fungal virulence. Transient expression of green fluorescent protein (GFP)-tagged FgTPP1 revealed that FgTPP1 localizes, in part, to chloroplasts and attenuates chitin-mediated activation of mitogen-activated protein kinase (MAPK) signaling, reactive oxygen species production, and cell death induced by an autoactive disease resistance protein when expressedin planta. Notably, the FgTPP1 protein is conserved across theAscomycotaphylum, making it a core effector among ascomycete plant pathogens. These properties make FgTPP1 an ideal candidate for decoy substrate engineering, with the goal of engineering resistance to FHB, and likely other crop diseases caused by ascomycete fungi.