Molecular studies on the interaction between C. fulvum and tomato have been very successful in recent years. This success was based on profound biological knowledge of this gene-for-gene system, generated by both plant pathologists and plant breeders in the past. Near-isogenic lines of tomato carrying different Cf genes and many races of C. fulvum able to overcome those genes, were instrumental to this success.
An additional important factor is the strict apoplastic growth of this biotrophic fungus which allowed the study of the interaction in planta where important virulence and avirulence factors are specifically produced. Many of those factors are not, or hardly, produced when the fungus is grown in axenic culture. Four Cf genes and two Avr genes have been cloned. The cloned Cf genes represent major resistance genes that are members of gene families which are activated by stable A VR peptides. Many Cf homologues likely represent resistance genes with minor effects which are activated by AVR peptides that are probably more difficult to identify. Some activators appear crucial virulence factors, such as ECP2, recognized by the versatile surveillance system of tomato. The Cf-ECP2 gene activated by ECP2 probably represents a durable resistance gene. The Cf gene products are situated mainly extracellularly with a membrane anchor and are perfectly placed to bind peptide elicitors produced by an extracellular pathogen. High affinity membrane-localized binding sites have been identified which, however, do not seem to be the products of Cf genes. It is possible that Cf proteins are low affinity binding sites that can only be detected in binding studies using these proteins produced in vitro. Now that the Cf and Avr genes have been cloned, one can begin to study signal transduction pathways involved in pathogenicity and plant defence. In the future the C. fulvum-tomato interaction will remain a very versatile biological system in these types of studies. This pathosystem is unique in that it represents a model for a strictly extracellular pathogen in contrast to many other model pathogens which grow either partially or wholly in an intracellular fashion.
Acknowledgements
The author thanks Guy Honee, Matthieu H.A.J. Joosten, Richard Lauge and Ronelle Roth for critically reading the manuscript.
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Summary
HANS THORDAL-CHRISTENSENl,
PER L. GREGERSEN2 & DA VID B. COLLINGE2
J Plant Pathology and Biogeochemistry Department Risoe National Laboratory, DK-4000 Roskilde, Denmark ( hans. thordal@ risoe. dk)
2 Plant Pathology Section, Department of Plant Biology
The Royal Veterinary and Agricultural University, Thorvaldsensvej 40 DK-1871 Frederiksberg C, Copenhagen, Denmark
Barley leaves attacked by the powdery mildew fungus is a pathosystem well suited for studies of plant-pathogen interaction mechanisms. Nearly one hundred specific resistance genes have been identified, many of which are alleles at more or less complex loci.
The corresponding avirulence genes are, on the other hand, evenly distributed in the powdery mildew fungus genome. The fungus colonizes only the leaf surface, placing haustoria in the leaf epidermal cells to acquire nutrients in an obligately biotrophic manner. The fungus exhibits a highly synchronous development and the epidermal cells respond accordingly. Papillae, which are formed early, subjacent to fungal germ tubes, arrest a considerable fraction of the attempted penetrations in all combinations of pathogen and plant genotypes (i.e. both incompatible and compatible).
Papillae consist of several different components, and inhibitor studies have suggested that at least callose and phenylpropanoids are directly involved in arresting the growth of the penetration peg. In compatible interactions, the haustorial nutrient uptake is believed to be driven by fungal plasma membrane H+-ATPase activity. This is based on the apparent lack of H+-ATPase activity at the invaginated host plasma membrane.
In incompatible interactions, the hypersensitive response serves as a back-up resistance mechanism arresting the germlings which have managed to penetrate the papilla. While only limited information is available concerning the expression of pathogenicity genes in the powdery mildew fungus, significantly more is known in relation to host response gene expression. However, the biological role of the host response genes is poorly understood; data suggest that many of them are merely part of a general stress response with no direct involvement in defense.
A. Slusarenko, R.S.S. Fraser, and
I.e.
van Loon (eds), Mechanisms of Resistance to Plant Diseases, 77-100.© 2000 Kluwer Academic Publishers.