Plants respond to inoculation with pathogens with a range of defense-associated responses, which can include the accumulation of anti-microbial, low molecular weight compounds, termed phytoalexins. The significance of any one defense-associated response in disease resistance is unknown in most host/pathogen systems. Arabidopsis thaliana accumulates the toxic metabolite 3- thiazol-2'-yl-indole after inoculation with the bacterial pathogen Pseudomonas syringae pv. syringae. This compound, which has been given the trivial name camalexin, fits the classic definition of a phytoalexin. To determine the camalexin biosynthetic pathway and to address the role of camalexin in disease resistance, mutants unable to accumulate camalexin will be isolated. Arabidopsis is ideally suited for the isolation and genetic characterization of mutants. Both feeding studies with radiolabeled precursors and analyses of camalexin-deficient mutants will help construct a biosynthetic pathway for this secondary metabolite. To evaluate the role of camalexin in limiting pathogen growth in vivo disease development in one well-defined camalexin mutant will be compared to that occurring in wild-type plants. With these results, it will be possible to formulate a broad statement about the relative role of phytoalexins in resistance to both fungal and bacterial diseases of cruciferous plants.
