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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.

Histological analyses of auxin-treated cuttings from the wild type and the rac mutant of tobacco (Nicotiana tabacum cv Xanthii) previously revealed that some rac phloem parenchyma or inner cortical parenchyma cells form callus in response to exogenous auxin treatment but these cells never undergo the organized divisions associated with adventitious root initiation in the wild type. Here we report the effect of the rac mutation on the temporal and spatial expression patterns of three genes previously shown to be associated with adventitious root meristems, HRGPnt3, iaa4/5, and gh3. Using histochemical staining analyses of HRGPnt3-GUS transformant cuttings, we determined that the rac mutation blocks auxin activation of the HRGPnt3 promoter. Thus, activation of the HRGPnt3 promoter occurs specifically during adventitious root initiation in tobacco cuttings. Histochemical staining analyses of iaa4/5-GUS and gh3-GUS transformant cuttings revealed that the rac mutation does not repress the auxin activation of the iaa4/5 and gh3 promoters. Based on our histochemical staining analyses, we conclude that differential gene expression occurs in response to auxin treatment during adventitious root initiation in the wild type compared with callus formation in rac cuttings. We also determined that HRGPnt3 mRNA accumulation occurs in response to components of our root-induction protocol other than auxin, indicating that HRGPnt3 expression is regulated both developmentally and environmentally.