The primary goal of the research is to obtain information that will expand
approaches for controlling fungal diseases of corn and sorghum and lead to
new and improved disease management practices. The principal research activities
in the lab focus on northern leaf blight of corn, caused by Setosphaeria
turcica, and gray leaf spot of corn, caused by Cercospora zeae-maydis and C.
zeina. Additional projects concern the biosynthesis and action of phytotoxic
metabolites produced by fungal pathogens and their role in pathogenesis.
The lab is located within the Crop Production and Pest Control Research Unit
of the USDA-Agricultural Research Service, in Lilly Hall on the Purdue University
campus in West Lafayette, Indiana. The research is a component of a CRIS
project on Molecular and Genetic Mechanisms of Fungal Disease Resistance
in Grain Crops.
Current research projects in the laboratory include:
Role of cercosporin in gray leaf spot of corn. Gray leaf
spot is caused by two genetically and taxonomically distinct fungi, Cercospora
zeae-maydis and C. zeina. Like many species of Cercospora, C.
zeae-maydis produces cercosporin, a light-induced, photoactivated perylenequinone
that is toxic to a diverse array of organisms. Cercospora zeina elicits
identical disease symptoms but does not produce cercosporin in culture, suggesting
that cercosporin is not essential for pathogenicity or virulence. To address
the role of cercosporin in pathogenesis, we initiated a project to identify,
characterize, and disrupt genes associated with cercosporin biosynthesis.
Analysis of a cDNA subtraction (SSH) library revealed transcripts that are
up-regulated during cercosporin synthesis. We focused on sequences with high
homology to genes involved in signal transduction. Northern analyses of gene
expression in cercosporin-inducing and cercosporin-suppressing media confirmed
the involvement of a MAP kinase kinase kinase (designated CZK3). Targeted
gene disruption of CZK3 generated a mutant that is unable to produce
cercosporin and does not produce conidia but is unaffected in vegetative
growth. Inoculation of corn leaves with mycelia of the czk3 mutant
resulted in small chlorotic lesions with minimal colonization by intercellular
hyphae following penetration through stomata. The results indicate that CZK3 regulates
functions that are important for pathogenesis and suggest that cercosporin
is required only after the early stages of infection for extensive colonization
and lesion expansion. Current efforts are directed toward understanding the
role of cercosporin by analyzing mutants affected in their ability to perceive
or transmit light signals and other environmental cues that influence cercosporin
synthesis.
Light-regulation of conidiation and pathogenesis in Cercospora
zeae-maydis and Setosphaeria turcica. Conidia
of the pathogens causing gray leaf spot and northern leaf blight are
the primary inoculum, inciting leaf lesions and initiating the disease
epidemic, and are the source of repeated infections via wind dispersal
to other plants and fields. Sporulation in these two pathogens is inhibited
by growth in constant light and regulated by nutritional factors. We
are investigating the physical and nutritional factors as well as the
biosynthetic activities that are essential for conidial development and
pathogenesis in these fungi. Only wavelengths in the blue light range
repress conidiation, which contrasts with numerous other fungal species
in which blue light induces or substantially enhances conidiation. In
a project led by ARS Research Associate, Dr. Burt Bluhm, our primary
objective is to characterize the gene(s) encoding blue-light photoreceptors
and identify light-regulated genes that influence conidiation and biosynthesis
of cercosporin in order to discover potential targets for disease control.
Preliminary results have indicated that mutants of C. zeae-maydis disrupted
in the CWC-1 gene encoding white collar-1, a putative blue light
receptor in fungi, are light blind in conidiation and cercosporin synthesis
but nevertheless are unable to cause disease symptoms beyond small chlorotic
spots at the site of penetration.
Molecular analysis of resistance of corn to Setosphaeria turcica. The
host-pathogen interaction in northern leaf blight is controlled by single
dominant genes in the corn host that condition resistance to specific races
of the pathogen. Thus, it presents a rare opportunity to study gene-for-gene
interactions in corn and critically evaluate the involvement of defense mechanisms
established in model plant species. The approach involves microarray analyses
of genes expressed during stages of disease development in near-isogenic
lines of corn differing in the presence of a specific gene for resistance.
In addition, this genetically defined pathosystem can be exploited to determine
the mechanism of virulence in the pathogen. Our project evaluates the potential
involvement of phytotoxins with genotype specificity or, alternatively, of
race-specific avirulence factors, e.g., extracellular peptides, that interact
directly or indirectly with the individual resistance gene products to ascertain
the phenotypic consequences of the host-pathogen interaction. ARS Postdoctoral
Research Associate, Dr. Burt Bluhm, is leading this project.