A signaling network controlling cell death and its consequences for Arabidopsis disease resistance

Plants have developed a complicated signaling pathway to sense pathogen attack, trigger downstream signaling circuitry and activate defense mechanisms such as systemic acquired resistance (SAR) and the hypersensitive response (HR). In this work, we used Arabidopsis as model plant and studied the circuitry controlling the hypersensitive response to avirulent Pseudomonas syringae. We developed quantitatively assays of the HR and associated signaling. By comparison of genetic mutants with wild type plants, we established a signaling pathway that includes one positive feedback and two negative feedback loops. Mathematical modeling was used to help refine placement of the "arrows" in signaling circuitry diagrams and to begin to address systems-level control issues. Our results also challenged longstanding dogma in several areas. We showed that high-level reactive oxygen production was a consequence, not the cause of the HR. We then established a new method to detect nitric oxide (NO·) accumulation after pathogen infection. In vivo time course experiments showed that NO· was not an early signal triggering the HR as was widely believed. Instead, it is produced extracellularly near the time of cell death and seems to be a signal contributing to cell-to-cell spread of the HR. We demonstrated that HR "triggering factors" were produced at different levels in response to different avr gene-encoded elicitors of defense signaling. We have also identified the first gene in this process as the gene over expressed in the hdn1 mutant. We have proven that this mutant over expresses a gene at a rate-limiting step in HR triggering. We also established the signaling circuitry downstream of the HR. We showed that there are two independent pathways contributing to SAR and PR gene expression, one of which was a direct consequence of the HR. Future attempts to engineer disease resistance in crop plants will depend upon our results showing how the HR is triggered and controlled and what the downstream consequences are.