1.1.5S
PATHOGEN RECOGNITION AND SIGNAL TRANSDUCTION EVENTS INVOLVED IN PLANT DISEASE RESISTANCE

GB MARTIN, R THILMONY, Y-T LOH, B RIELY, D HALTERMAN, G SESSA, M D'ASCENSO, A BOGDANOVE, Y GU, R KANTETY, T SUBRAHMANYAN, Y-J KIM, J ZHOU, X TANG and R FREDERICK

Department of Agronomy, Purdue University, West Lafayette, IN 47907-1150, USA

Background and objectives
The goal of our research is to understand the molecular basis of pathogen recognition and signal transduction events involved in plant disease resistance. Towards this goal, we study two experimental systems in tomato - resistance to Pseudomonas syringae pv. tomato (causative agent of bacterial speck disease) conferred by the Pto serine/threonine kinase, and sensitivity to the fenthion insecticide conferred by the closely related Fen scrine/threonine kinase. Resistance to bacterial speck disease in tomato is governed by a 'gene-for-gene' interaction in which the tomato Pto gene responds to the expression of the avrPto avirulence gene in the pathogen.

Results and conclusions
To understand the molecular basis of recognition specificity exhibited by Pto and Fen, we developed a large series of chimaeric constructs that contain different portions of the two kinases. Each chimaeric protein is being studied in the yeast two-hybrid system for its interaction with previously identified Pto-interacting and Fen-interacting proteins and for the ability to confer disease resistance or fenthion sensitivity in plants. In related work, we found that transient expression of an avrPto transgene in plant cells containing Pto induces a defence response, indicating that the AvrPto protein functions inside the plant cell. In the yeast two-hybrid system, the Pto kinase physically interacts with AvrPto and alterations of AvrPto or Pto that disrupt the interaction in yeast also abolish disease resistance in plants. We are currently determining the interaction domains of both AvrPto and Pto and testing specific models to understand how the physical interaction of these two proteins initiates disease resistance.

To understand the role of the Pto kinase in signal transduction, we used the yeast two-hybrid system to isolate genes encoding Pto-interacting (Pti) proteins. One of the these genes, Pti1, encodes another serine/threonine kinase that probably acts directly downstream of Pto. A second class of genes, Pti4/5/6, are putative transcription factors which are similar to the tobacco ethylene-responsive element binding proteins (EREBPs). Pti4/5/6 specifically recognize and bind a DNA sequence that is present in the promoter region of a large number of genes encoding 'pathogenesis-related' (PR) proteins. These observations establish a direct connection between a disease resistance gene and the specific activation of plant defence genes.

References
1. Scofield SR et al., 1996. Science 274, 2063-2065.
2. Tang X et al., 1996. Science 274, 2060-2063.
3. Zhou J et al., 1995. Cell 83, 925-935.
4. Zhou J et al., 1997. EMBO Journal 16, 3207-3218.