Abstract
Fusarium head blight (FHB) caused by Fusarium graminearum is a destructive disease of wheat and barley worldwide. In a previous study of systematic characterization of protein kinase genes in F. graminearum, mutants of three putative components of the osmoregulation MAP kinase pathway were found to have distinct colony morphology and hyphal growth defects on PDA plates. Because the osmoregulation pathway is not known to regulate aerial hyphal growth and branching, in this study we further characterized the functions of the FgHog1 pathway in growth, pathogenesis, and development. The Fghog1, Fgpbs2, and Fgssk2 mutants were all reduced in growth rate, aerial hyphal growth, and hyphal branching angle. These mutants were not only hypersensitive to osmotic stress but also had increased sensitivity to oxidative, cytoplasm membrane, and cell wall stresses. The activation of FgHog1 was blocked in the Fgpbs2 and Fgssk2 mutants, indicating the sequential activation of FgSsk2-FgPbs2-FgHog1 cascade. Interestingly, the FgHog1 MAPK pathway mutants appeared to be sensitive to certain compounds present in PDA. They were female sterile but retained male fertility. We also used the metabolomics profiling approach to identify compatible solutes that were accumulated in the wild type but not in the Fghog1 deletion mutant. Overall, our results indicate that the FgSsk2-FgPbs2-FgHog1 MAPK cascade is important for regulating hyphal growth, branching, plant infection, and hyperosmotic and general stress responses in F. graminearum.
Highlights
Fusarium graminearum is a causal agent of Fusarium head blight (FHB) or scab of wheat and barley [1,2]
In the complemented transformant HGC1, the production of these four compounds with or without NaCl treatment was similar to that of the wild type (Fig. 9C). These results indicate that the accumulation of glycerol, arabitol, mannitol, and possibly sucrose as compatible solutes in response to hyperosmotic stress is under the control of the FgHog1 mitogen-activated protein (MAP) kinase pathway in F. graminearum
Vegetative hyphae of these mutants had smaller branching angles and tended to grow in parallel in the extension zone of colonies formed on PDA plates, suggesting that the FgHog1 pathway may be involved in the negative autotropism of hyphal growth under certain growth conditions in F. graminearum
Summary
Fusarium graminearum is a causal agent of Fusarium head blight (FHB) or scab of wheat and barley [1,2]. Under favorable conditions, this pathogen can cause severe yield losses and contaminate infested grains with harmful mycotoxins such as deoxynivalenol (DON) and zearalenones. Sequential activation of the MEKK-MEK-MAPK cascade results in the expression of specific sets of genes in response to environmental stimuli. The high osmolarity glycerol (HOG) response pathway is required for growth under hyperosmotic conditions [5,6]. The yeast Hog MAPK is activated by the Pbs MEKK, which in turn is activated by two overlapping MEKKs, Ssk, and Ssk. Pbs can be activated by Ste via a putative membrane protein Sho1 [8] and a transmembrane mucin [9]
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