Marc J. Orbach. Associate Professor of Plant Pathology. Ph.D., Stanford University. Molecular genetics of fungal cultivar specificity in the rice blast fungus, Magnaporthe grisea.

My research program uses Magnaporthe grisea, the fungal pathogen responsible for the rice blast disease, as a model system to study host-pathogen interactions at the molecular and biochemical level. This pathogen, like many plant pathogens interacts with its host in a gene-for-gene manner, where host resistance is induced when the plant contains a resistance gene and the pathogen, a corresponding avirulence gene. The main focus of our program is to understand what the signals between the pathogen and its host are, that dictate whether the host is able to mount a resistance response. Genetic analysis of M. grisea has identified several avirulence genes that determine, in a cultivar specific manner, whether disease resistance is induced. My laboratory is isolating and characterizing some of these fungal genes to determine what their products are and how these products interact with host plants to induce host defenses.

We are also interested in questions of genome stability and the generation of genetic variability in fungi. These questions are of significance in M. grisea because of the apparent ability of this pathogen to rapidly overcome host resistance in the field. We are addressing these questions by studying genome variation in M. grisea at the whole genome level using electrophoretic karyotyping methods. We are also specifically analyzing the role that a transposable element may play in genome variation and the high rate of mutation observed at some loci.

Orbach, M.J., Chumley, F.G., and Valent, B. 1996. Evidence for B chromosomes and translocations in the karyotypes of Magnaporthe grisea pathogens of diverse grasses. Mol. Plant-Microbe Interact.

Orbach, M.J.. 1994. A cosmid with a HyR marker for fungal library construction and screening. Gene 150:159-162.

Sweigard, J. A., B. Valent, M. J. Orbach, A. M. Walter, A. Rafalski, and F. G. Chumley. 1993. Genetic map of the rice blast fungus Magnaporthe grisea. in: Genetic Maps, 6th Edition Locus Maps of Complex Genomes. Book 3 - Lower Eukaryotes. Cold Spring Harbor Laboratory Press. Cold Spring Harbor, NY.

Hamer, J.E., L. Farrall, M.J. Orbach, B. Valent, and F.G. Chumley. 1989. Host Species-Specific Conservation of a Family of Repeated Sequences in the Genome of a Fungal Plant Pathogen. Proc. Natl. Acad. Sci. USA. 86:9981-9985.

Orbach, M.J., M.S. Sachs and C. Yanofsky. 1990. The Neurospora crassa arg-2 Locus: Structure and Expression of the Gene Encoding the Small Subunit of Arginine-Specific Carbamoyl Phosphate Synthetase. J. Biol. Chem. 265:10981-10987.