Molecular, physiological, and histological evaluations of secreted lipases of Fusarium graminearum during wheat colonisation and in culture
Final Report Abstract
Fusarium graminearum is one of most important fungal pathogens worldwide. It infects cereals, especially wheat, barley, oats, and maize. It was first described in England in 1884, called wheat scab, and later, tombstone disease, because of the chalky, lifeless appearance of the infected kernels. It diminishes harvest yield and poisons food and feed production by accumulating mycotoxins. Previous studies in our lab demonstrated that disruption of a gene coding for a secreted lipase (FGL1), led to strongly decreased lipolytic activity in early time points after wheat germ oil induction and the lipase FGL1 was found to be required for successful colonization during infection of wheat and maize. We showed that the MAP kinase Gpmk1 regulates pathogenicity of the fungus. The disruption of the Gpmk1 MAP kinase leads to an apathogenic phenotype on wheat. Determination of secreted lipolytic activity of the ∆gpmk1 mutants showed that Gpmk1 was responsible for the overall induction of secreted lipolytic activities in culture and regulates the onset of lipase gene FGL1 during infection. However, the mechanism by which the lipases function as virulence factors is not yet known. To better understand the role of lipases, Ms. Giang Le focused her study on the identification of transcription factors regulating lipase expression. In her study, she identified by sequence homology 8 proteins which are putative transcriptional regulators in the fungus F. graminearum. Their possible role in regulation of lipase expression was characterized by gene deletion. Ms. Le managed to disrupt the genes by transformation mediated targeted homologous integration. Subsequently, she characterized the mutants in vitro and during wheat infection. Among the putative transcription factors, she discovered that two genes code for regulators of fatty acid metabolism. She found that these two transcription factors control fatty acid catabolism by regulating expression of several enzymes, e.g. peroxisomal proteins. Additionally, these transcription factors participate in the regulation of expression of secreted lipases. Moreover, she showed that their expression is necessary for full virulence of the fungus. Ms. Le also discovered a lipase regulator; named LR1. She demonstrated that LR1 participates in regulating expression of several secreted lipolytic enzymes including cutinases. Interestingly, she demonstrated that LR1 contributes to the expression of FGL1 and other secreted enzymes in vitro and in planta. Deletion of LR1 resulted in a marked reduction of expression, but a substantial lipolytic activity remained and the mutants are fully virulent. During her studies of lipases, Ms. Le found that butyrate is a strong inhibitor of secreted lipases. She provided evidence of a potential application of this short chain fatty acid in antifungal treatment of Fusarium diseases. Additionally, Ms. Le was involved in a project about the importance and regulation of autophagy in F. graminearum. A gene, named FgATG15 was identified by sequence similarity to the yeast Saccharomyces cerevisiae. Ms. Le showed that this gene exhibits lipase activity when heterologously expressed in P. pastoris. The gene was disrupted in F. graminearum and mutants deficient in FgATG15 were reduced in storage lipid degradation under starvation conditions, implicating the involvement of FgATG15 in lipid turnover. FgATG15 disruptants showed strongly reduced growth of aerial hyphae, conidia production, germination, and aberrant conidia shapes. Moreover, wheat head infection was severely attenuated, indicating the involvement of FgATG15 in pathogenesis. Additionally, we found that the deoxynivalenol levels of FgATG15 disruptants were significantly decreased compared with the wild type strain.
Publications
- Autophagy-like lipase FgATG15 of Fusarium graminearum is important for lipid turnover and plant infection. Fungal Genetics and Biology,
Vol. 48. 2011, Issue 3, pp. 217–224.
Nguyen, N.L, Bormann, J., Le, G.T.T., Stärkel, C., Olsson, S., Nosanchuk, J.D., Giese, H., W. Schäfer
(See online at https://dx.doi.org/10.1016/j.fgb.2010.11.004)
