Edgar Huitema and Sophien Kamoun

Department of Plant Pathology, The Ohio State University-OARDC, 1680 Madison Ave. Wooster, Ohio

Phytophthora infestans, a host-specific oomycete plant pathogen, causes the late blight disease on some solanaceous plants, such as potato and tomato. P. infestans, also known as the Irish famine fungus, remains a destructive pathogen with annual worldwide crop losses topping $3 billion. However, no sustainable source of genetic resistance is available. A number of plant species, including Arabidopsis, are fully resistant to all known strains of P. infestans. This form of resistance, known as nonhost resistance, could reveal novel defense genes and pathways, but has hardly been studied at the genetic level. The objective of our research is to develop Arabidopsis into a key model plant for understanding nonhost resistance to oomycete pathogens. We initiated a number of studies aimed at exploiting nonhost resistance of Arabidopsis to Phytophthora. Following inoculation of Arabidopsis rosette leaves with zoospores of P. infestans, penetration of epidermal cells occurred, followed by active defense responses, including a hypersensitive response (HR)-like cell death. In addition, induction of BGL2 gene expression, a marker for the salicylic acid (SA) mediated defense pathway, was recorded. However, inoculation of several known Arabidopsis mutant genotypes, compromised in their SA mediated defense response, did not result in a susceptible phenotype. To further investigate Arabidopsis response to P. infestans, we used DNA microarray analyses to determine expression profiles for ca. 8,000-10,000 genes during nonhost resistance (AFGC Microarray Facility, Michigan State University). Complete rosette-stage Arabidopsis (Col-0) were inoculated by spraying either a P. infestans zoospore suspension or water onto the leaves. Sixteen hours after inoculation, RNA was extracted from the leaves, labeled and used in microarray hybridizations (two replications). Gene expression data generated an overview of activated and repressed disease response pathways. Some of the notable up-regulated genes included resistance gene analogs, genes encoding components of signal transduction pathways, and a number of genes potentially involved in cell death. These results will provide a rationale for future functional assays of the identified pathways using transgenic knockouts and mutant analyses. These studies should lead to the exciting prospect of utilizing Arabidopsis as a source of resistance genes to economically important pathogens (Kamoun et al. Trends Plant Sci., 4:196, 1999).