Role of the Pti4 and Pti5 Transcription Factors in Pto-Mediated Defense Gene Activation and Disease Resistance

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This project focuses on the molecular events that occur after a plant recognizes a bacterial pathogen and which result in increased expression of defense-related genes. To investigate these events, resistance in tomato to bacterial speck disease caused by Pseudomonas syringae pv. tomato is studied. Resistance to speck disease occurs in a "gene-for-gene" fashion when the tomato plant expresses the Pto resistance gene and the bacterium expresses the avrPto avirulence gene. Recognition of AvrPto protein by the product of the Pto gene, a serine-threonine kinase, leads to the rapid induction of many "pathogenesis-related" (PR) genes (among other defense responses). With previous NSF support, two transcription factors, Pti4 and Pti5, were identified that interact with the Pto kinase and which bind a DNA regulatory element present in the promoter of many PR genes. The Pti4 and Pti5 genes are induced in tomato leaves by Pseudomonas infection, and Pti5 expression is further increased during a Pseudomonas-tomato interaction involving AvrPto-Pto. Furthermore, the Pti4 protein is phosphorylated by the Pto kinase and its binding to the GCC box is thereby enhanced. Overexpression of Pti4 or Pti5 in plants leads to constitutive expression of certain PR genes. Thus, Pti4 and Pti5 appear to define a molecular link between the disease resistance gene Pto and activation of PR genes. Amino acids of Pti4 that are phosphorylated by Pto will be identified and the role of this phosphorylation in Pti4 activity will be determined. Genome-wide gene expression changes that occur in tomato and Arabidopsis plants overexpressing Pti4 and Pti5 will be elucidated, and these plants will be tested for alterations in disease resistance. To understand the Pto-mediated induction of Pti5, the Pseudomonas-responsive promoter elements of this gene will be determined and their cognate transcription factors will be identified. Finally, new plant loci that are required for Pti4-mediated expression of PR genes will be discovered. These studies will elucidate key steps in the regulation of transcription factors associated with plant disease resistance.