Characterization of the defense response in rice against the blast pathogen Magnaporthe grisea

Tishina Bhargava, Purdue University

Abstract

Rice is one of the world's agronomically important cereal crops, providing food for more than one third of the global population. Magnaporthe grisea is an economically significant pathogen, best known as the causative agent of the devastating rice blast disease. These two organisms are experimentally tractable, with well defined genetics and molecular biology. The present study was undertaken to understand the host-pathogen interactions between rice and M. grisea. Rice cDNAs encoding transcripts exhibiting elevated expression in response to infection by the blast fungus, were isolated by differential screening from a M. grisea-infected rice leaf cDNA library (Talbot et al., 1993). Sequence analysis of these blast-induced transcripts (BITs) identified several defense-response homologues and novel genes. Among the defense genes identified were cDNAs encoding pathogenesis-related (PR) proteins. A genomic clone for rice PR-1 was identified and sequenced. BITs encoding PR-1, PR-10, and a novel chitinase were used as molecular probes to study their temporal regulation in nonhost, compatible and incompatible interactions. Transcriptional differences in the expression of these PR genes were observed by performing RNAse Protection Assays. These studies indicate that PR genes are differentially expressed in various host-pathogen interactions. The defense response in incompatible interactions was further studied using fungal mutants blocked at specific stages of penetration. Results suggest that defense genes are induced prior to pathogen penetration of the first rice cell. Experiments were performed to examine if resistance in rice against M. grisea can be induced biologically or chemically. The PR genes were induced by treatment with salicylic acid and jasmonic acid, substances known to concomitantly induce PR gene expression and resistance in dicots against a number of pathogens. However, these chemicals did not result in a reduction in blast disease symptoms, suggesting that they do not induct resistance in rice against M. grisea. On the other hand, inoculation of rice plants with a grass pathogen strain of M. grisea, prior to inoculation with a virulent rice strain, resulted in a dramatic reduction in disease symptoms. This indicates that resistance in rice against a virulent strain of M. grisea can be triggered by prior inoculation with a grass pathogen.

Degree

Ph.D.

Advisors

Hamer, Purdue University.

Subject Area

Molecular biology|Plant pathology|Botany

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