FUNCTIONAL EXPRESSION OF A FUNGAL AVIRULENCE GENE FROM A MODIFIED POTATO-VIRUS-X GENOME

The Cf-9 gene of tomato controls resistance to races of the fungal pat hogen Cladosporium fulvum that possess the avirulence gene Avr9, When AVR9 elicitor is injected into healthy leaves or cotyledons of Cf-9 co ntaining 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 ins erted into a modified potato virus X expression vector capable of infe cting many Solanaceous species, Progeny virus, derived from in vitro t ranscripts of the hybrid PVX:Avr9 construct were infectious on Nicotia na clevelandii plants and directed high level expression of Avr9 in sy stemically infected tissue, When near-isogenic tomato lines of the cv, Moneymaker either containing Cf-9 or lacking this gene (plants design ated Cf9 and Cf0, respectively) were infected with PVX:Avr9, small nec rotic flecks developed at the cotyledon inoculation site specifically on the Cf9 plants within 3 days, Subsequently, these necrotic areas ex panded, coalesced, spread to other plant organs and the shoot apex, an d eventually killed the Cf9 plants, On Cf0 plants only mild mosaic sym ptoms formed, These data indicate that AVR9 peptide is produced by vir us-infected plant cells and that its function and specificity of actio n is effectively retained, Although the interaction mediated by Cf-9 a nd Avr9 occurred throughout the entire infection this does not result in the permanent cessation of viral movement, Virus particles that exp ress Avr9 are not avirulent on Cf-9 tomato. As several explanations co uld account for the lack of viral containment, it cannot be concluded that Cf-9 mediated resistance responses were ineffective in restrictin g the unrelated microbe PVX, The striking and reliable necrotic sympto ms produced by PVX:Avr9 on Cf9 plants make this an ideal vector to per form a rapid structure/function analysis of the Avr9 sequence and to i dentify mutant plant loci that abolish viral multiplication and/or sys temic spread.