Disruption of the Avirulence Gene avr9 in Two Races of the Tomato Pathogen Cladosporium fulvum Causes Virulence on Tomato Genotypes with the Complementary Resistance Gene Cf9. Roland Marmeisse. Agricultural University Wageningen, Department of Genetics, Dreijenlaan 2, 6703 HA. Guido F. J. M. van den Ackerveken(1,2), Theo Goosen(1), Pierre J. G. M. de Wit(2), and Henk W. J. van den Broek(1). Agricultural University Wageningen, (1)Department of Genetics, Dreijenlaan 2, 6703 HA and (2)Department of Phytopathology, P. O. Box 8025, 6700 EE Wageningen, The Netherlands.. MPMI 6:412-417. Accepted 31 March 1993. Copyright 1993 The American Phytopathological Society.

To study the function of the avirulence gene avr9 of the tomato pathogen Cladosporium fulvum, we developed procedures for gene disruption experiments in two different races of the fungus both avirulent on tomato genotypes carrying the resistance gene Cf9. For this purpose we selected uridine auxotrophic strains amongst fluoroorotic acid resistant mutants. These mutants were transformed with a plasmid containing the avr9 genomic region in which the open reading frame was replaced by the pyrG gene from Aspergillus nidulans. For each of the two races used we selected one transformant in which the entire avr9 coding sequence was deleted as a result of a gene replacement event. The two transformants were able to successfully infect Cf9 tomato genotype, unlike their wild-type avr9⁺ parents, which induced hypersensitive responses on this genotype. We also demonstrated that these two transformants no longer produce the necrosis-inducing elicitor peptide specifically interacting with Cf9 tomato plants. These results confirm that the cloned avr9 sequence (and therefore the AVR9 peptide) is fully responsible for avirulence in wild-type avr9⁺ races of the fungus. These results also indicate that this gene is dispensible for normal vegetative growth and pathogenicity at least in a monocyclic process. These results are discussed in relation to the possible origin of avr9¯ strains and the long-standing resistance toward C. fulvum offered by the tomato Cf9 resistance gene.