Wild type accessions of Arabidopsis (Col-0, Nd-1 and Ws-3) were compared with phytoalexin-deficient mutants (Col-pad1, -pad2, -pad3 and -pad4) and the enhanced disease susceptibility mutant Ws-eds1 for camalexin accumulation following inoculation with virulent or avirulent isolates of Hyaloperonospora parasitica (downy mildew), or treatment with the abiotic elicitor AgNO3. Camalexin accumulation was greatest in Ws-3 with all induction treatments compared to the other wild type accessions, indicating that natural genetic variation exists in Arabidopsis for regulation of camalexin biosynthesis. In Col-0, more camalexin accumulated in plants inoculated with the virulent H. parasitica isolate (Maks9) than with the avirulent isolate (Hiks1, recognized by RPP7). In pad4, at least two-fold less camalexin accumulated compared with Col-0 following inoculation with the virulent isolate; and a similar reduction (but less than two-fold difference) was observed with pad2. No camalexin was detected in the biosynthesis mutant pad3 following pathogen inoculation; but traces were identified following AgNO3 treatment. Equivalent levels of camalexin accumulated in pad1 as in wild type with all treatments. Results from Ws-eds1 confirmed that the EDS1 protein is required for regulating both RPPI-mediated downy mildew resistance and camalexin production. The combined results suggest that camalexin biosynthesis is most likely a consequence of disease caused by H. parasitica, and does not therefore confer resistance to primary infection by this parasite. It also appears that signalling of camalexin biosynthesis in this case involves PAD2, PAD4 and EDS1 proteins but not necessarily PAD1. (C) 2003 Elsevier Ltd. All rights reserved.