Division of Plant Sciences, University of Missouri, Columbia 65211, U.S.A.
In this study, we screened 22 Nicotiana spp. for resistance to the tombusviruses Tomato bushy stunt virus (TBSV), Cucumber necrosis virus, and Cymbidium ringspot virus. Eighteen species were resistant, and resistance was manifested in at least two different categories. In all, 13 species responded with a hypersensitive response (HR)-type resistance, whereas another five were resistant but either had no visible response or responded with chlorotic lesions rather than necrotic lesions. Three different TBSV proteins were found to trigger HR in Nicotiana spp. in an agroinfiltration assay. The most common avirulence (avr) determinant was the TBSV coat protein P41, a protein that had not been previously recognized as an avr determinant. A mutational analysis confirmed that the coat protein rather than the viral RNA sequence was responsible for triggering HR, and it triggered HR in six species in the Alatae section. The TBSV P22 movement protein triggered HR in two species in section Undulatae (Nicotiana glutinosa and N. edwardsonii) and one species in section Alatae (N. forgetiana). The TBSV P19 RNA silencing suppressor protein triggered HR in sections Sylvestres (N. sylvestris), Nicotiana (N. tabacum), and Alatae (N. bonariensis). In general, Nicotiana spp. were capable of recognizing only one tombusvirus avirulence determinant, with the exceptions of N. bonariensis and N. forgetiana, which were each able to recognize P41, as well as P19 and P22, respectively. Agroinfiltration failed to detect the TBSV avr determinants responsible for triggering HR in N. arentsii, N. undulata, and N. rustica. This study illustrates the breadth and variety of resistance responses to tombusviruses that exists in the Nicotiana genus.