Pathogen recognition and signal transduction by the pto kinase

In tomato, the disease resistance gene Pto confers resistance to bacterial speck disease by recognizing the expression of a corresponding avirulence gene, avrPto, in the pathogen Pseudomonas syringae pv. tomato (Martin et al. 1993). Similar gene-for-gene interactions occur in many plant-pathogen associations (Flor 1971). Such recognition events often lead to the activation in the plant of a variety of defense responses including a rapid induction of localized necrosis at the site of infection (the hypersensitive response, HR), increased expression of defense-related genes, production of antimicrobial compounds, lignin formation, and the oxidative burst (Lamb et al. 1989, Mehdy 1994). As a result, the pathogen is contained at the infection site and its growth is inhibited. Pto encodes a serine/threonine protein kinase and belongs to a clustered multigene family. Another member of the Pto family called Fen confers no known disease resistance, but mediates a hypersensitive-like reaction in the plant to the insecticide fenthion (Martin et al. 1994). We are interested in a number of fundamental questions concerning the Pto signaling pathways. What is the molecular basis of the Pto-avrPto gene-for-gene interaction? What are the components involved in the Promediated signal transduction chain? How does the Pto kinase activate complex defense responses? This paper summarizes our recent progress towards understanding these questions.