The Arabidopsis pad2-1 mutant belongs to a series of non-allelic camalexin-deficient mutants. It was originally described as showing enhanced susceptibility to virulent strains of Pseudomonas syringae and was later shown to be hyper-susceptible to the oomycete pathogen Phytophthora brassicae (formerly P. porri). Surprisingly, in both pathosystems, the disease susceptibility of pad2-1 was not caused by the camalexin deficiency, suggesting additional roles of PAD2 in disease resistance. The susceptibility of pad2-1 to P. brassicae was used to map the mutation to the gene At4g23100, which encodes gamma-glutamylcysteine synthetase (gamma-ECS, GSH1). GSH1 catalyzes the first committed step of glutathione (GSH) biosynthesis. The pad2-1 mutation caused an S to N transition at amino acid position 298 close to the active center. The conclusion that PAD2 encodes GSH1 is supported by several lines of evidence: (i) pad2-1 mutants contained only about 22% of wild-type amounts of GSH, (ii) genetic complementation of pad2-1 with wild-type GSH1 cDNA restored GSH production, accumulation of camalexin in response to P. syringae and resistance to P. brassicae and P. syringae, (iii) another GSH1 mutant, cad2-1, showed pad2-like phenotypes, and (iv) feeding of GSH to excised leaves of pad2-1 restored camalexin production and resistance to P. brassicae. Inoculation of Col-0 with P. brassicae caused a coordinated increase in the transcript abundance of GSH1 and GSH2, the gene encoding the second enzyme in GSH biosynthesis, and resulted in enhanced foliar GSH accumulation. The pad2-1 mutant showed enhanced susceptibility to additional pathogens, suggesting an important general role of GSH in disease resistance of Arabidopsis.