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Ethylene-Insensitive Tobacco Shows Differentially Altered Susceptibility to Different Pathogens

First, second, and sixth authors: Graduate School Experimental Plant Sciences, Faculty of Biology, Section Phytopathology, Utrecht University, P.O. Box 800.84, 3508 TB Utrecht, The Netherlands; third author: Department of Bioagricultural Science & Pest Management, Colorado State University, Fort Collins; and fourth and fifth authors: Laboratory of Phytopathology, Faculty of Agricultural and Applied Biological Sciences, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium

Transgenic tobacco plants (Tetr) expressing the mutant etr1-1 gene from Arabidopsis thaliana are insensitive to ethylene and develop symptoms of wilting and stem rot when grown in nonautoclaved soil. Several isolates of Fusarium, Thielaviopsis, and Pythium were recovered from stems of diseased Tetr plants. Inoculation with each of these isolates of 6-week-old plants growing in autoclaved soil caused disease in Tetr plants but not in nontransformed plants. Also, when 2-week-old seedlings were used, nontransformed tobacco appeared nonsusceptible to the Fusarium isolates, whereas Tetr seedlings did develop disease. Tetr seedlings were not susceptible to several nonhost Fusarium isolates. In contrast to results with Fusarium isolates, inoculation of 2-week-old seedlings with a Thielaviopsis isolate resulted in equal symptom development of nontransformed and Tetr tobacco. In order to explore the potential range of pathogens to which Tetr tobacco plants display enhanced susceptibility, the pathogenicity of several root and leaf pathogens was tested. Tetr plants were more susceptible to the necrotrophic fungi Botrytis cinerea and Cercospora nicotianae and the bacterium Erwinia carotovora, but only marginally more to the bacterium Ralstonia solanacearum. In contrast, the biotrophic fungus Oidium neolycopersici, the oomycete Peronospora tabacina, and Tobacco mosaic virus caused similar or less severe symptoms on Tetr plants than on nontransformed plants. Total peroxidase activity of Tetr plants was lower than that of nontransformed plants, suggesting a role for peroxidases in resistance against necrotrophic microorganisms. A comparable range of pathogens was examined on Arabidopsis and its ethylene-insensitive mutants etr1-1 and ein2-1. With the exception of one Fusarium isolate, ethylene insensitivity increased susceptibility of Arabidopsis plants to a similar spectrum of necrotizing pathogens as in tobacco. Thus, both ethylene-insensitive tobacco and Arabidopsis plants appear to be impaired in their resistance to necrotrophic pathogens.