SIGNAL RECOGNITION AND TRANSDUCTION IN BACTERIAL SPECK DISEASE RESISTANCE
Martin G., Zhou J., Tang X., Frederick R., Jia Y., Halterman D., Loh Y.-T., Thilmony R., Sessa G., Ascenzo M. D.
Department of Agronomy, Purdue University, Lilly Hall of Life Sciences, West Lafayette, IN 47907
The long term goal of our research is to understand the molecular basis of pathogen recognition and the subsequent 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 serine/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.
To understand the molecular basis of recognition-specificity exhibited by Pto and Fen we developed a series of 17 chimeric constructs that contain different portions of the two kinases. The ability of each construct to confer disease resistance or fenthion sensitivity is being determined in stable transformed tomato plants and each construct is also being studied in the yeast two-hybrid system for its interaction with Pto-interacting and Fen-interacting proteins.
In addition, we have isolated the recessive alleles of Pto and Fen and are using their divergent sequences to understand recognition-specificity. In related work, we found that transient expression of an avrPto transgene in plant cells induces a defense response indicating that the AvrPto protein functions in 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 testing specific models to understand how the physical interaction of Avrpto and Pto initiates disease resistance.
To further understand the role of the Pto kinase, we have 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 encode proteins with putative DNA-binding domains, acidic domains and nuclear localization motifs indicating they may function as transcription factors. Identification of the Pti4/5/6 factors establishes the first direct connection between a disease resistance gene and the specific activation of plant defense genes. We are now developing tomato transgenics that have antisensed or overexpressed copies of Pti1, Pti4, Pti5, or Pti6 to assess the role of these genes in disease resistance.
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