Understanding Striga-Host Interactions
Years of investigation have uncovered an elegant system of chemical signaling by which Striga recognizes a potential host plant and regulates its development in order to optimize its chances for survival. Germination, haustorial induction, attachment to and penetration of the host root vascular system are all critical events in the Striga life cycle. We are interested in what factors and processes are required for the establishment of compatible host-parasite interactions, and how resistant host and non-host plants avoid being parasitized.
Our model system is the interaction of Striga gesnerioides with cowpea, an important tropical legume. We know that S. gesnerioides is among the most variable of the witchweeds and that there is both host specificity and differential resistance responses within host species. There have been few attempts to analyze genetic variability of Striga species and the relationship between genetic variability of the parasite and its host range and virulence is not known at this time. We have identified at least 7 distinct races of S. gesnerioides that parasitize cowpea across West and Central Africa . We have also identified isolates of the parasite that are specific for different wild legume species as well as non-leguminous dicots such as tobacco.
Striga gesnerioides on cowpea in the field.
Careful observation has provided evidence for at least two different mechanisms of resistance to Striga parasitism in cowpea. One mechanism resembles the hypersensitive response (HR) observed in other plant-plant pathogen interactions and suggests the presence of a specific R gene-mediated response mechanism. The second type of resistance response involves arrested development of the parasite tubercle following attachment and attempted penetration of the host root cortex. We are interested in determining whether specific avirulence (Avr) genes exist in S. gesnerioides corresponding to the race-specific resistance (R) genes identified in cowpea, and how these and other heritable factors present in the parasite genome define its host range and specificity. In essence, we are testing whether the gene-for-gene mechanism of resistance operating in other plant-plant pathogen interactions is also functioning in Striga -host associations. We have identified a number of molecular markers that segregate with several of the race-specific resistance genes in cowpea and are currently pursuing the map-based cloning and characterization of these genes. We are also using microarray-based analyses to determine the global changes in gene expression associated with Striga parasitism of resistant and susceptible cowpea cultivars.