Reduced expression of the tomato ethylene receptor gene LeETR4 enhances the hypersensitive response to Xanthomonas campestris pv. vesicatoria

The hypersensitive response (HR) involves rapid death of cells at the site of pathogen infection and is thought to limit pathogen growth through the p lant. Ethylene regulates senescence and developmental programmed cell death , but its role in hypersensitive cell death is less clear. Expression of tw o ethylene receptor genes, NR and LeETR4, is induced in tomato (Lycopersico n esculentum cv,Mill) leaves during an HR to Xanthomonas campestris pv, ves icatoria, with the greatest increase observed in LeETR4, LeETR4 antisense p lants previously were shown to exhibit increased sensitivity to ethylene. T hese plants also exhibit greatly reduced induction of LeETR4 expression dur ing infection and an accelerated HR at inoculum concentrations ranging from 10(5) to 10(7) CPU/ml, Increases in ethylene synthesis and pathogenesis-re lated gene expression are greater and more rapid in infected LeETR4 antisen se plants, indicating an enhanced defense response. Populations of avirulen t X, campestris pv, vesicatoria decrease more quickly and to a lower level in the transgenic plants, indicating a greater resistance to this pathogen, Because the ethylene action inhibitor 1-methylcyclopropene alleviates the enhanced HR phenotype in LeETR4 antisense plants, these changes in pathogen response are a result of increased ethylene sensitivity.