Response to Xanthomonas campestris pv.vesicatoria in Tomato Involves Regulation of Ethylene Receptor Gene Expression

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Abstract
Although ethylene regulates a wide range of defense-related genes, its role in plant defense varies greatly among different plant-microbe interactions. We compared ethylene's role in plant response to virulent and avirulent strains of Xanthomonas campestrispv. vesicatoria in tomato (Lycopersicon esculentum Mill.). The ethylene-insensitive Never ripe (Nr) mutant displays increased tolerance to the virulent strain, while maintaining resistance to the avirulent strain. Expression of the ethylene receptor genes NR andLeETR4 was induced by infection with both virulent and avirulent strains; however, the induction of LeETR4expression by the avirulent strain was blocked in the Nrmutant. To determine whether ethylene receptor levels affect symptom development, transgenic plants overexpressing a wild-typeNR cDNA were infected with virulent X. campestris pv. vesicatoria. Like theNr mutant, the NR overexpressors displayed greatly reduced necrosis in response to this pathogen.NR overexpression also reduced ethylene sensitivity in seedlings and mature plants, indicating that, like LeETR4, this receptor is a negative regulator of ethylene response. Therefore, pathogen-induced increases in ethylene receptors may limit the spread of necrosis by reducing ethylene sensitivity.