Abstract
We assessed the biological function of CpSlt2, an ortholog of the cell wall integrity (CWI) MAPK of Saccharomyces cerevisiae, in the chestnut blight fungus Cryphonectria parasitica. The CpSlt2-null mutant exhibited marked changes in colonial growth, near absence of conidiation and aerial hyphae, and abnormal pigmentation. In addition, the CpSlt2-null mutant exhibited CWI-related phenotypic defects including hypersensitivity to cell wall-disturbing agents and other stresses. Electron microscopy revealed the presence of abnormal hyphae such as intrahyphal hyphae. In addition, virulence assays indicated that the CpSlt2 gene plays an important role in fungal pathogenesis. As cultivation of the mutant strains progressed, the majority of the colonies showed sporadic sectorization and mycelia from the sectored area stably maintained the sectored phenotype. Although mycelial growth was partially recovered, the sectored progeny had dramatically impaired virulence, confirming the CpSlt2 gene has a role in pathogenicity. Compared to a previous mutant of the CpBck1 gene, a MAPKKK gene in CWI pathway, the CpSlt2-null mutant showed similar, although not identical, phenotypic changes and most phenotypic changes were less severe than those of the CpBck1-null mutant. These results suggest that the unique sectorization is CWI pathway-specific, though the components in the same CWI pathway have common and specific functions.
Highlights
At the beginning of the 20th century in North America, the chestnut forests were devastated by Cryphonectria parasitica (Murrill) Barr that caused chestnut blight disease (Van Alfen, 1982)
The dark brown area was even smaller than that of the hypovirulent UEP1 strain. These results indicate that the production of phenoloxidase was greatly hampered by mutation of the CpSlt[2] gene, regardless of the mycelial growth defects, suggesting that phenoloxidase activity may play a role in maintaining fungal cell wall integrity (CWI) or, aside from CWI, this signaling pathway modulate phenoloxidase production resulting in new aspects of phenoloxidase-mediated biological function of the CWI pathway
In filamentous plant pathogenic fungi, MAPK pathways underlie the pheromone response, the CWI pathway, and the osmoregulation/stress response pathways[22]. Among these MAPK pathways, the CWI pathway of plant pathogenic fungi is primarily responsible for CWI maintenance and other species-specific processes such as virulence, sporulation, female sterility, hyphal growth and polarity, secondary metabolism, stress response, and surface hydrophobicity maintenance[37,38,39,40,41,42,43,44]
Summary
At the beginning of the 20th century in North America, the chestnut forests were devastated by Cryphonectria parasitica (Murrill) Barr that caused chestnut blight disease (Van Alfen, 1982). Since phenotypic changes in virus-infected strains are pleiotropic, a signal transduction pathway during viral symptom development was attributed as a coordinate and specific mechanism of virus-mediated fungal gene regulation. In C. parasitica, several genes in MAPK pathways and their downstream effectors are regulated by hypovirus, or are involved in pathogenicity[9, 16, 17, 23,24,25,26] Among these pathways, the CWI pathway has aroused interest because Cpkk[1], an ortholog of yeast Mkk[2] (a CWI MAPKK) was found to be modulated by hypovirus and was important for virulence in the chestnut tree[25, 27].
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