Abstract
Fusarium head blight (FHB) caused by the ascomycete fungus Fusarium graminearum can result in significant crop losses and render the crops harmful to human health due to contamination with mycotoxin. Although the pathogenesis of F. graminearum is widely investigated by molecular genetics approaches, detailed studies about its cellular and developmental processes at the initial stages of infection are very limited. We applied live-cell imaging approach to characterize the spatial and temporal development of growing hyphae and plant responses during F. graminearum and wheat coleoptile interactions. The present investigation demonstrates that F. graminearum uses two strategies to penetrate the host epidermal cells. The pathogen breaks through the host cell wall with appressoria-like structures derived from surficial hyphae, and also with the narrow pegs from thick and bulbous intracellular hyphae. Live cell imaging in the presence of the endocytic tracker FM4–64 showed that the plasma and intermembranes of the invaded wheat coleoptile cells were intact. Invasive hyphae exhibit branching, budding, pseudohyphae-like growth, cell-to-cell spreading ability, and were sealed within a plant membrane, indicating a biotrophic lifestyle of this fungus inside the invaded cells. Time-lapse imaging suggested that there were callose depositions at the plant cell walls in the form of continuous lines and also on the outer linings of the fungal invasive hyphae at colonization stage. In addition, our studies demonstrate that the activation of the toxisome-related gene (TRI4) requires external stimuli and is spatio-temporally modulated. Generally, this study provides new insights into the colonization strategies and host response features during F. graminearum-plant interactions. Further tracing of cellular details will significantly contribute to our understanding of molecular mechanisms of F. graminearum-host interactions.
Highlights
Fusarium graminearum, a filamentous ascomycete, is a fungal pathogen that infects a wide range of cereal crops including wheat and barley, causing Fusarium head blight (FHB) (Bai and Shaner 2004; Goswami and Kistler 2004)
Colonisation and development of appressoria-like structures in F. graminearum To study the cytological events in living tissues of the host during F. graminearum infection, we used a live-cell confocal microscopy to observe wheat coleoptile cells invaded by Green fluorescent protein (GFP)-tagged F. graminearum (Fig. 1)
At early stages of the infection (5–8 h post inoculation, hpi), conidia of F. graminearum germinated into epiphytic runner hyphae on the extracellular surface or intercellular space of the wheat coleoptiles (Fig. 2)
Summary
A filamentous ascomycete, is a fungal pathogen that infects a wide range of cereal crops including wheat and barley, causing Fusarium head blight (FHB) (Bai and Shaner 2004; Goswami and Kistler 2004). Being one of the most destructive diseases of wheat, FHB attracts the attention of plant pathologists worldwide (Dean et al 2012; Figueroa et al 2018). FHB epidemics are recorded frequently in the United States, United Kingdom and China, the management of which requires billions of dollars (McMullen et al 1997; Dean et al 2012). A thorough investigation of the mechanisms involved in F. graminearum proliferation and (2019) 1:26 subsequent colonization of its host cells will help in devising effective means of controlling FHB
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