Deficiency in phytoalexin production causes enhanced susceptibility of Arabidopsis thaliana to the fungus Alternaria brassicicola

The phytoalexin-deficient Arabidopsis mutant pad3-1, which is affected in t he production of the indole-type phytoalexin camalexin, has previously been shown not to display altered susceptibility to either the bacterium Pseudo monas syringae (Glazebrook and Ausubel, 1994; Proc. Natl. Acad. Sci. USA, 9 1: 8955-8959) or the biotrophic fungi Peronospora parasitica (Glazebrook et al., 1997; Genetics, 146: 381-392) and Erysiphe orontii (Reuber et al., 19 98; Plant J. 16: 473-485). We now show that this mutant is markedly more su sceptible than its wild-type parental line to infection by the necrotrophic fungus Alternaria brassicicola, but not to Botrytis cinerea. A strong cama lexin response was elicited in wild-type plants inoculated with either Alte rnaria brassicicola or Botrytis cinerea, whereas no camalexin could be dete cted in pad3-1 challenged with these fungi. Hence, PAD3 appears to be a key determinant in resistance to at least A. brassicicola. The induction of sa licylate-dependent and jasmonate/ethylene-dependent defense genes was not r educed in Alternaria-challenged pad3-1 plants compared to similarly treated wild-type plants. Camalexin production could not be triggered by exogenous application of either salicylate, ethylene or jasmonate and was not, or no t strongly, reduced in mutants with defects in perception of these defense- related signal molecules. Camalexin-production appears to be controlled by a pathway that exhibits little crosstalk with salicylate-, ethylene- and ja smonate-dependent signalling events.