Technical Abstract: Little is known concerning the mode of action of allelochemicals or plant defense responses mounted against them. Theoretically, changes in the expression of genes encoding the primary target or other proteins in the same pathway should occur soon after phytotoxin exposure. Defense responses, such as the induction of genes involved in chemical detoxification, may occur later, depending on the nature of the chemosensors which presumably exist in plant cells. We first used yeast (Saccharomyces cerevisiae) to test the concept of transcriptome profiling of toxicant modes of action. Characteristic gene induction profiles related to specific molecular target sites were verified with several fungicides. A battery of xenobiotic defense-associated genes were found to be dramatically induced in Arabidopsis following exposure to an array of structurally unrelated xenobiotics, including a herbicide, an allelochemical, and herbicide safeners. These genes are unlikely to be strongly linked to the mode of action of a specific phytotoxin, but rather constitute a coordinately-controlled xenobiotic defense gene network. Transcriptional profiling experiments using microarrays are being conducted to examine the effects of various herbicides and natural phytotoxins on the Arabidopsis tran-scriptome.