Functional Expression of a Fungal Avirulence Gene from a Modified Potato Virus X Genome. Kim E. Hammond-Kosack . Sainsbury Laboratory, John Innes Centre, Colney Lane, Norwich, NR4 7UH, England. Brian J. Staskawicz(2), Jonathan D. G. Jones(1) and David C. Baulcombe(1). (1)Sainsbury Laboratory, John Innes Centre, Colney Lane, Norwich, NR4 7UH, England; (2)Department of Plant Biology, 111 Koshland Hall, University of California, Berkeley, California 94720, U.S.A. MPMI 8:181-185. Accepted 26 September 1994. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1995.

The Cf-9 gene of tomato controls resistance to races of the fungal pathogen Cladosporium fulvum that possess the avirulence gene Avr9. When AVR9 elicitor is injected into healthy leaves or cotyledons of Cf-9 containing genotypes, a gray necrotic response develops within 24 hr. To test whether expression of Avr9 from an unrelated pathogenic microbe would elicit a Cf-9 dependent necrotic response, the Avr9 gene was inserted into a modified potato virus X expression vector capable of infecting many Solanaceous species. Progeny virus, derived from in vitro transcripts of the hybrid PVX:Avr9 construct were infectious on Nicotiana clevelandii plants and directed high level expression of Avr9 in systemically infected tissue. When near-isogenic tomato lines of the cv. Moneymaker either containing Cf-9 or lacking this gene (plants designated Cf9 and Cf0, respectively) were infected with PVX:Avr9, small necrotic flecks developed at the cotyledon inoculation site specifically on the Cf9 plants within 3 days. Subsequently, these necrotic areas expanded, coalesced, spread to other plant organs and the shoot apex, and eventually killed the Cf9 plants. On Cf0 plants only mild mosaic symptoms formed. These data indicate that AVR9 peptide is produced by virus-infected plant cells and that its function and specificity of action is effectively retained. Although the interaction mediated by Cf-9 and Avr9 occurred throughout the entire infection this does not result in the permanent cessation of viral movement. Virus particles that express Avr9 are not avirulent on Cf-9 tomato. As several explanations could account for the lack of viral containment, it cannot be concluded that Cf-9 mediated resistance responses were ineffective in restricting the unrelated microbe PVX. The striking and reliable necrotic symptoms produced by PVX:Avr9 on Cf9 plants make this an ideal vector to perform a rapid structure/function analysis of the Avr9 sequence and to identify mutant plant loci that abolish viral multiplication and/or systemic spread.