Abstract
Rice blast fungus, Magnaporthe oryzae, is the most destructive pathogen in the rice-growing area. This fungus has a biotrophic phase early in infection and later switches to a necrotrophic lifestyle. During the biotrophic phase, the fungus competes with its host for nutrients and oxygen. Continuous uptake of oxygen is essential for successful establishment of blast disease of this pathogen. Here, we report transcriptional responses of the fungus to oxygen limitation. Transcriptome analysis using RNA-Seq identified that 1,047 genes were up-regulated in response to hypoxia. Those genes are involved in mycelial development, sterol biosynthesis, and metal ion transport based on hierarchical GO terms, and are well-conserved among three fungal species. In addition, null mutants of two hypoxia-responsive genes were generated and their roles in fungal development and pathogenicity tested. The mutant for the sterol regulatory element-binding protein gene, MoSRE1, exhibited increased sensitivity to a hypoxia-mimicking agent, increased conidiation, and delayed invasive growth within host cells, which is suggestive of important roles in fungal development. However, such defects did not cause any significant decrease in disease severity. The other null mutant, for the alcohol dehydrogenase gene MoADH1, showed no defect in the hypoxia-mimicking condition (using cobalt chloride) and fungal development. Taken together, this comprehensive transcriptional profiling in response to a hypoxic condition with experimental validations would provide new insights into fungal development and pathogenicity in plant pathogenic fungi.
Highlights
Oxygen (O2) is an essential element for most eukaryotes due to its roles in the maintenance of many physiological processes such as the biosynthesis of heme, sterols, and fatty acids, and as a terminal electron acceptor in oxidative phosphorylation
To examine transcriptomic changes of the rice blast fungus in response to hypoxia, we incubated each of 10 plates separately under hypoxia (1% O2) and normoxia (20.9% O2) for 12 hours after pre-culture on complete medium (CM) for three days
Changes in transcriptomes and physiological observations in this study suggest that the hemibiotrophic rice blast fungus might encounter hypoxia in a living host cell during infection
Summary
Oxygen (O2) is an essential element for most eukaryotes due to its roles in the maintenance of many physiological processes such as the biosynthesis of heme, sterols, and fatty acids, and as a terminal electron acceptor in oxidative phosphorylation. Habitats lacking oxygen are known as anoxic, while those at the higher end of the oxygen range are known as aerobic [1]. Microbes living in those niches can be exposed to diverse oxygenic environments including hypoxia (a reduced oxygen level associated with physiological and pathophysiological processes) [2], and have adapted to microbial growth in their hosts. The exposure to oxygen depletion; i.e., hypoxia, dramatically influences various physiological processes [3,4,5] and global transcriptional regulation in eukaryotic cells [6, 7]. Microbes have evolved a set of cellular and molecular mechanisms to survive in low-oxygen environments
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