Ethylene and Pathogen Infection Responses in Tomato and Arabidopsis

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The interactions of plants and disease-causing pathogens generally fall into two types: compatible and incompatible. Incompatibility leads to resistance while compatibility leads to disease. In the absence of a resistant response, disease symptoms can spread, leading to severe damage or death of the plant. Analysis of mutants unable to synthesize ethylene, salicylic acid (SA) or jasmonic acid (JA), defined a multistage sequence of disease symptom development during a compatible tomato:pathogen interaction. Synthesis of all three phytohormones is essential for complete symptom development. The spread of disease is associated with an ethylene-dependent accumulation of SA. The position of JA within this cascade of events has yet to be determined. We intend to elucidate how these three hormones control symptom development. Experiments will define at a molecular genetic level the roles of ethylene, SA and JA in symptom development in tomato. Through a combination of hormone quantitation and add-backs, the order of action of the three hormones relative to one another will be determined. Microarray analyses will be performed to identify components of the progression from infection to cell death. These experiments will be complemented by work involving Arabidopsis:Xanthomonas interactions. In Arabidopsis, there is differential attenuation of disease symptoms with ethylene insensitive mutants in the various hormone receptors. This result suggests distinct functions for the five ethylene receptors. Molecular and genetic approaches will be used to define the differential responses. These experiments have the potential to identify unique functions for what has heretofore been considered a redundant receptor family.