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Sirodesmin PL is a secondary metabolite toxin produced by Leptosphaeria maculans, a fungus that causes blackleg disease of oilseed rape (Brassica napus). Biosynthesis of this epipolythiodioxopiperazine (ETP) toxin involves a cluster of 18 co-regulated genes. Putative ETP gene clusters have been found in 14 taxa discontinuously distributed amongst ascomycetes. Movement of entire clusters by horizontal gene transfer is the most parsimonious hypothesis to explain this discontinuous distribution. A mutant of L. maculans that does not produce sirodesmin PL forms similar sized lesions on cotyledons of B. napus to those formed by the wild type. However, it is half as effective in colonising stems, as indicated by reduced lesion size and reduced fungal biomass. This implicates sirodesmin PL as a virulence factor in B. napus stems. Expression patterns of two of the cluster genes is consistent with the distribution of sirodesmin PL in planta, as revealed by mass spectrometry experiments. The sirodesmin-deficient mutant has less antibacterial and antifungal activity than the wild type, suggesting that sirodesmin PL may play a role in protecting L.maculans against competition from other micro-organisms.

The fungus Cladosprium fulvum is a biotrophic leaf pathogen of tomato. The fungus develops in the intercellular space without forming specialized feeding structures and does not affect the leaf tissue. The outcome of the C. fulvum-tomato interaction can be described by the gene-for-gene model. Failure of infection is expressed by a hypersensitive response. Two fungal proteins, ECP1 and ECP2, have been isolated and their corresponding genes have been cloned. In a compatible interaction including many physiological races ECP1 and ECP2 are highly produced and a role in pathogenicity is suggestive. The ecp1 gene shows some homology with tumor necrosis factor receptors (TNFRs) while the ecp2 gene shows no homology with sequences known in data bases. However, disruption of one of the two genes showed no reduced pathogenicity of the fungus. Two race-specific elicitors, AVR4 and AVR9, have been isolated and their corresponding genes have been cloned. The avirulence genes Avr4 and Avr9 are only present in C. fulvum avirulent on Cf-4 and Cf-9 cultivars, respectively. The expression of these two genes is, like the expression of the ecp genes, highly induced when the fungus grows in planta. Disruption of the Avr9 gene in wild type avirulent races leads to virulence on tomato genotypes carrying the complementary resistance gene Cf-9. A single base-pair change in the avirulence gene Avr4 leads to virulence on tomato genotypes carrying the Cf-4 resistance gene. Isolation, characterization and possible function of ECP1, ECP2, AVR4, and AVR9 will be discussed.

Of the fungal tomato pathogen Cladosporium fulvum a number of in planta induced genes and their products have been isolated and characterized. They include: (i) putative pathogenicity genes of which two genes, (extracellular proteins) ecp1 and ecp2, have been cloned and sequenced; the function of those genes during pathogenesis is not known yet; effects of ecp gene disruption on pathogenicity are being studied, and: (ii) avirulence genes which encode race-specific elicitors interacting with the products of complementary resistance genes in the host plant, resulting in a hypersensitive response and other defense responses. Avirulence gene avr9 of C. fulvum is the first fungal avirulence gene that has been cloned. The regulation of this gene has been studied in vitro and in planta. In vitro, the gene is induced under low nitrogen conditions, whereas in planta the gene is highly expressed around the vascular tissue. Avirulent races carrying the avr9 gene become virulent on Cf9 genotypes of tomato after disruption of avr9.

Of the fungal tomato pathogen Cladosporium fulvum a number of avirulence genes and their products have been isolated and characterized. Avirulence genes which encode race-specific elicitors interact with the products of complementary resistance genes in the host plant resulting in a hypersensitive response and other defense responses. Avirulence gene avr9 of C. fulvum is the first fungal avirulence gene that has been cloned. The regulation of this gene has been studied in vitro and in plania. In vitro, the gene is induced under low nitrogen conditions, whereas in planta the gene is highly expressed around the vascular tissue. Avirulent races carrying the avr9 gene become virulent on Cf9 genotypes of tomato after disruption of avr9.