Sequenced libraries of cDNAs from Phytophthora are being systematically tested for their ability to induce resistance or disease symptoms in tobacco and potato by transient expression using potato virus X (PVX) and A. tumefuciens T-DNA vectors as delivery systems (Takken et al. 2000; Torto et al. 2002). Agroinfiltration has been used in several plant systems to study the interaction phenotype of products of avirulence genes and specific resistance genes (van der Hoorn et al. 2000; Tai et al. 1999). Often plants are scored simply for formation of a HR or necrosis. Whether this is truly representative of the natural response to infection is unclear. It will be interesting to see what kinds of the candidate genes are found and whether their disruption or silencing corroborates the heterologous expression tests described here.
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