Genome Wide Transcription Studies on Infection Structure Formation and Function in Magnaporthe grisea

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Title: Genome Wide Transcription Studies on Infection Structure Formation and Function in Magnaporthe grisea
Author: Oh, Yeon Yee
Advisors: Dr. Ralph A. Dean, Committee Member
Dr. Gary A. Payne, Committee Member
Dr. David McK. Bird, Committee Member
Dr. Gregory C. Gibson, Committee Member
Abstract: Rice blast caused by the filamentous ascomycete fungi, Magnaporthe grisea (anamorph Pyricularia oryzae) is the most destructive disease of rice through out the world. To gain access to its host, M. grisea develops a specialized infection structure called an appressorium. To understand the mechanisms regulating formation and function of this structure, we performed microarray experiments using the M. grisea whole genome oligonucleotide array containing M. grisea and rice elements. The most dramatic change in gene expression occurred during spore germination where 21% showed differential expression with the vast majority being up-regulated. Approximately 3 % of the predicted genes were differentially expressed during appressorium formation in response to both a hydrophobic surface signal and exogenous cyclic AMP. Our data shows that germination stimulates a major transcriptional response characterized by a dramatic transcription of genes involved in metabolism and biosynthesis. In contrast, induction of appressorium formation triggered a significant decrease in this suite of genes, including the translational apparatus, with a coordinate increase in the expression of genes involved in protein and amino acid degradation, lipid metabolism, secondary metabolism and cellular transportation. Significantly, the set of upregulated genes was enriched for those encoding predicted secreted proteins. We identified 42 transcriptionally regulated transcription factors during appressorium formation, the majority of whose putative functions are regulation of secondary metabolism, nutrient assimilation and cell development. Functional characterization of differentially expressed genes using targeted gene disruption revealed novel pathogenicity factors, a subtilisin protease SPM1 and a NAD specific glutamate dehydrogenase Mgd1 in M. grisea. Our finding shows that protein turnover and amino acid metabolism are essential for proper appressorium formation and the infection process. Further, we found many differentially expressed genes, which included highly conserved transcription factors, were not required for appressorium formation and function. This may suggest that M. grisea employs a number of failsafe and backup systems, such as functional redundancy and compensatory processes in order to protect appressorium formation and to ensure the fungus can successfully invade its host. Genome wide transcriptional profiles followed by comprehensive functional studies provided broad and in depth insight into infection structure development in M. grisea. Our data will directly benefit efforts to find novel fungal pathogenecity factors and further to develop disease management systems.
Date: 2008-03-06
Degree: PhD
Discipline: Plant Pathology

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