Some fungal pathogens can circumvent classical defense responses but the biochemical mechanisms behind these processes have been unclear. In August 22 Nature, K. Bouarab and colleagues at the Sainsbury Laboratory, John Innes Centre, Norwich, UK show that a fungal saponin- detoxifying enzyme mediates suppression of plant defenses in a two-step process (Nature 2002, 418:889-892).

Bouarab et al. used strains of the tomato leaf spot fungus Septoria lycopersici grown on Nicotiana benthamiana leaves. They observed that a fungal saponin-detoxifying enzyme hydrolyses plants antimicrobial saponins. The degradation product of this hydrolysis suppresses the induced defense responses by interfering with fundamental signal transduction processes in plants, leading to disease resistance.

"A better understanding of how plants and their diseases interact will eventually help scientists and breeders who are trying to breed plants with improved natural disease resistance. The next challenge for us is to find out how the saponin...

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