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The Role of Signal Peptidase in the Pathogenic Association of the Anthracnose Stalk Rot Fungus Colletotrichum graminicola With Maize
Department of Plant Pathology
Maize is economically the most important crop in the United States, worth 33.8 billion dollars in 2006. Fungal stalk rot is one of the most important and damaging diseases of maize worldwide, and is estimated to cause yield losses of between 5-10% annually. One of the most common causes of stalk rot is the anthracnose fungus Colletotrichum graminicola, the subject of this study.
Resistance to stalk-rot fungi is not expressed efficiently if the plant becomes drought or nutrient stressed, or once the plant reaches physiological maturity. The reduced tillage production methods now in use on much of the maize acreage in the U.S. can allow inoculum to build up to levels that are damaging even to resistant genotypes. Larger, higher yielding ears draw more photosynthate from the stalks and may weaken them, making them more vulnerable to stalk rot. Thus, the current common practice of breeding for resistance to fungal stalk rot by selecting for stalk strength and "standability" can reduce yielding capacity. Future management of anthracnose and other stalk rot diseases will depend on a better understanding of the disease cycle and the biology of the interaction between host and pathogen.
Recently, there has been a lot of interest in the roles of individual secreted effector proteins in phytopathogenic fungi. Our work is unique in having produced evidence that the control of secretion itself in C. graminicola is an important virulence factor. This research studies the role of endoplasmic-reticulum (ER) stress and the unfolded protein response (UPR), a ubiquitous adapatation to ER stress, in pathogenicity of C. graminicola to maize using a genetic approach.
Our work may have broader applications for other rot diseases, and for other fungal pathogens in general. We can study the role of ER stress and the UPR in both biotrophy and necrotrophy/rot in one system. In addition to helping to manage stalk rot diseases, our work could help to improve heterologous protein production in related industrial fungi, and perhaps lead to engineering of rot fungi for increased secretion of CWDE for use in biofuels production from lignocellulosic biomass.
2011 Project Description
Our research is focused on understanding the genetic basis for the biotrophic to necrotrophic transition in the hemibiotrophic fungus Colletotrichum graminicola, which causes anthracnose stalk rot disease of corn.
A novel nonpathogenic mutant was identified during a large-scale screening experiment. The mutant apparently initiates the biotrophic phase of the disease normally, but then ceases developing and never transitions to necrotrophic growth in leaves. The mutant is deficient in one component of the signal peptidase enzyme responsible for cleavage of signal peptides from proteins destined for transport through the endoplasmic reticulum system of the cell. The gene encoding this enzyme was named CPR1.
This project was based on the hypothesis that the pathogen fails to suppress resistance responses in the host because it cannot secrete a specific suppressor of the host resistance response, and that this effector secretion is regulated by signaling pathways involved with the adaptation to stress. We demonstrated the existence of a diffusible suppressor function in co-inoculation experiments with the mutant and wild type in corn leaf sheaths. When the wild type is inoculated up to half a centimeter away from the mutant, the mutant grows and develops normally in the host tissues. Culture filtrates of the wild type allow the mutant to colonize corn tissues normally. The mutant also grows normally in corn tissues that have been treated in various ways to compromise normal defense response.
Mutant infection sites are associated initially with much weaker oxidative burst responses (indicated by ROS staining) but later infections are associated with much stronger production of ROS than similar wild type infection sites, indicating higher levels of defensive activation.
We conducted a transcriptomics study of the mutant and wild type strains during infection of maize, and the mutant strain appears to express some stress response genes, particularly catalases, at higher levels than the wild type, supporting the idea that the mutant is dealing with greater levels of oxidative stress, possibly due to an inability to produce suppressors of the oxidative burst defense response. We found that the mutant strain is significantly more sensitive to oxidative stress in vitro.
The connection between the Cpr1 gene product and sensitivity to oxidative stress in vitro remains murky, but we have concluded on the basis of this study that the sensitivity to oxidative stress, combined with the reduced capability to suppress oxidative stress in planta, may explain the lack of pathogenicity of the mutant. Our next goal will be to understand the role of Cpr1 in oxidative stress response and secretion of effectors that suppress host defense.
Approximately 6 percent of the corn crop in the U.S. is lost to fungal stalk rot disease annually. In spite of the economic importance of this disease. We have identified a novel and previously unsuspected role for the protein secretion machinery, including adaptation to secretion stress, in the establishment of biotrophic colonization of corn by C. graminicola.
We have uncovered evidence that C. graminicola produces one or more secreted suppressors of the corn resistance response, and we have developed a bioassay for the suppressor function. If this suppressor(s) and its receptor(s) can be identified, it may be possible to engineer corn plants not to respond to the suppressor, thus making them resistant to this very devastating disease.