Characterization of Host Plant Defense Responses to Parasitization by <I>Orobanche aegyptiaca</I>
Orobanche (spp.) are parasitic plants that attack the roots of many important crops. O. aegyptiaca penetrates the host root (aided by digestive enzymes) and forms connections to the host vascular tissue, from which it will withdraw all of its water and nutrient requirements. In order to control this weed, it is important to understand the relationship between the host and the parasite. To investigate how parasitism effects host defense pathways, we are studying the patterns of expression of host genes known to be involved in various aspects of plant defense responses. With respect to local defense responses, two genes of the isoprenoid pathway were studied, one of which is expressed in wounded tissue (hmg1), and another that is induced in response to wounding yet repressed in response to pathogen elicitors (squalene synthase). Genes analyzed that are associated with systemic defense include PR-1, PR-2, and PR-5, all of which are induced in response to pathogen attack as part of the systemic acquired resistance (SAR) response. Plant gene expression was studied using transgenic tomato plants containing hmg1-GUS fusions, and northern hybridization analysis of tobacco and Arabidopsis roots using gene-specific probes. Results indicated that expression of hmg1 is induced, PR-2 and squalene synthase are repressed, and PR-1a, PR-1, and PR-5 are not affected in tissue parasitized by O. aegyptiaca. Together, these results indicate a complex response to the parasite. Whereas hmg1 induction is consistent with O. aegyptiaca inflicting a simple wound-like injury, the repression of squalene synthase is consistent with plant recognition of a pathogen attack. In contrast, the failure of Orobanche to induce SAR- related PR-1 in tobacco and PR-1, PR-2, or PR-5 in Arabidopsis indicates an ability to avoid or perhaps inhibit some defense-related pathways. By comparing the regulation of these defense genes in response to O. aegyptiaca attack, we are able to gain a greater understanding of the host plant response to parasitization and explore potential gene candidates for future engineering strategies to create Orobanche resistant crops.