Research Accomplishment Reports 2009

Molecular Genetics of the Interaction between Corn and Corn Stalk Rot Fungi (Colletotrichum graminicola and Gibberella zeae)

L.J. Vaillancourt
Department of Plant Pathology

 

Non-Technical Summary

Stalk rot is one of the most economically important diseases of corn worldwide. Complete control is currently impossible, although damage can usually be reduced by use of hybrids with partial resistance to one or more of the causal fungi, and by minimizing plant stress. My long-term goal is to develop a detailed molecular and cellular understanding of the interactions between corn and corn stalk rot fungi so that we can manage stalk rot disease more effectively.

2009 Project Description

Industry estimates suggest that approximately 6 percent of the corn crop in the U.S. is lost to fungal stalk rot diseases annually. Resistance is available, but tends to break down during grain filling or under conditions that stress the plant. This makes management of this disease syndrome particularly challenging.

We are using a combination of molecular and cytological approaches to study and compare the relationship between pathogenicity of two important stalk rot fungi, Colletotrichum graminicola and Gibberella zeae and the nutritional and defense status of the host, and to understand more about how these fungi enter and colonize stalks. We have continued our studies of a novel nonpathogenic C. graminicola mutant that we identified during a large-scale screening experiment. 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 hypothesis we are testing is that the mutant fails to suppress resistance responses in the host because it cannot secrete a specific suppressor of the host resistance response. In our latest results we have been able to functionally demonstrate the existence of this suppressor: co-inoculation of the wild type up to 0.5 cm away from the mutant on maize leaf sheaths allows the mutant to develop normally in the host tissues. Having this bioassay will allow us to learn more about the nature of this suppressor.

We have also been pursuing an idea that the function of the CPR1 gene product is related to adaptation to secretion stress. Preliminary data have been supportive so far, in that we have been able to show that the mutant is more sensitive to treatments that induce secretion stress.

Meanwhile we have been working to develop F. graminearum as a model corn stalk rot pathosystem comparable to what we have already done for C. graminicola. We have developed protocols that work reliably in our hands for conducting crosses and for making knockout mutants, and for infection of corn stalks in the greenhouse with both fungal pathogens to facilitate future comparative analyses.

A genome sequence has been available for F. graminearum for several years. During the past year, we have been working to finish and release the genome of C. graminicola in collaboration with the Broad Institute. The sequence assembly was released several months ago, and the annotation phase is now underway. Having available genome sequences for these two organisms will greatly facilitate future comparative efforts.

2009 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, and we have developed a bioassay for that suppressor.

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. Together C. graminicola and G. zeae are the most aggressive and problematic of the stalk rot fungi in the U.S. Germplasm with resistance to one type generally does not provide resistance to the other. By developing these as model stalk rot pathosystems we have made it possible to conduct comparative analyses that may lead to more general understanding of factors important for stalk rots and in turn to more general therapies that will be effective against all stalk rot fungi.