Research Accomplishment Reports 2007

Molecular Genetics of the Interaction between Corn and Corn Stalk Rot Fungi Colletotrichum graminicola and Fusarium graminearum

L.J. Vaillancourt
Department of Plant Pathology

 

Project Description

Between 5 and 10 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, almost nothing was known when this project began about fungal factors that are important for pathogenicity to corn stalks.  We did not even have a very good understanding of how the fungi invaded and moved through corn stalk tissues.  We used a combination of molecular and cytological approaches to study the relationship of the stalk rot fungi to the nutritional and defense status of the host, and to observe how these fungi enter and colonize stalks.

This research was 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 initiates the biotrophic phase of the disease normally, but does not switch to necrotrophic growth in leaves. Extensive cytological studies were carried out to compare fluorescently labeled mutant and wild type strains in order to understand how the fungi colonized and moved through stalk and leaf tissues.  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. The CPR1 cDNA was cloned and transformed into two different yeast signal peptidase mutants. However, CPR1 did not complement either mutant yeast strain, perhaps because CPR1 is too divergent from the homologue of this protein in yeast. To further elucidate the function of CPR1, a gene encoding a putative pSEC11 homologue of C. graminicola (Cgs11), was cloned and characterized. pSEC11 is one of four subunits of the signal peptidase in yeast, and has been shown to physically interact with pSPC3. The predicted protein encoded by the Cgs11 cDNA was 67 % similar to pSEC11. The Cgs11 cDNA complemented two different yeast sec11 mutants. Co-immunoprecipitation and co-localization experiments were initiated to determine the location and interaction of the Cgs11 and CPR1 proteins.  Additional research was focused on the potential role of CPR1 in quantitative and qualitative regulation of secretion. Results demonstrated that CPR1 has little effect on secretion when nutritional conditions are optimal, but that it is important for secretion of pectinase activity under inducing conditions.  Other experiments suggested that the pathogen fails to suppress resistance responses in the host because it cannot secrete a specific suppressor of the host resistance response.

Impact

Our investigations have dramatically increased our understanding of how stalk rot fungi cause decay symptoms in their host plants, and how those symptoms relate to fungal infection and colonization. We developed a reproducible greenhouse assay for infection of corn stalks that we have shared now with many other labs.  We have been able to show more parallels than previously suspected between infections of leaves and stalks by C. graminicola. We have identified a novel and previously unsuspected role for the protein secretion machinery, including the signal peptidase, in the establishment of biotrophic colonization and also in necrotrophic (rotting) development.  We have uncovered evidence that C. graminicola produces a secreted suppressor of the corn resistance response.  Our work has supported the U.S. hybrid corn industry by developing more effective screening tools, and by helping to identify and characterize, novel sources of resistance for this disease.

Publications

Venard, C., Vaillancourt L. (2007). Penetration and colonization of unwounded maize tissues by the maize anthracnose pathogen Colletotrichum graminicola, and by the related non-pathogen C. sublineolum. Mycologia 99: 368-377

Venard, C., Vaillancourt, L. (2007). Colonization of fiber cells by Colletotrichum graminicola in wounded maize stalks. Phytopathogy 97: 438-44

Du, M., Schardl., C.L., Nuckles, E.M., Vaillancourt, L.J. (2005). Using mating-type gene sequences for improved phylogenetic resolution of Colletotrichum species complexes. Mycologia 97: 641-658

Flowers, J.L., Vaillancourt, L.J. (2005). Parameters affecting the efficiency of Agrobacterium tumefaciens-mediated transformation of Colletotrichum graminicola. Current Genetics 48: 380-388

Mims, C.W., Vaillancourt, L.J. (2002). Ultrastructural characterization of infection and colonization of maize leaves by Colletotrichum graminicola, and by a C. graminicola pathogenicity mutant. Phytopathology 92: 803-812

Thon, M.R., Nuckles, E.M., Takach, J.E., and Vaillancourt, L.J. (2002). CPR1: A gene encoding a putative signal peptidase that functions in pathogenicity of Colletotrichum graminicola to maize. Molecular Plant-Microbe Interactions 15: 120-128.