Technical Abstract: Cantharidin is a natural compound isolated from the blister beetle (Epicauta spp.). It is a very potent inhibitor of serine/threonine protein phosphatases PPP, especially PP2A and PP4. Protein phosphatases and kinases maintain a sensitive balance between phosphorylated and dephosphorylated forms of appropriate proteins, thereby playing important roles in signal transduction pathways and regulation of gene expression, cellular proliferation, cell differentiation, apoptosis, and other processes. The foliage of 12-day-old Arabidopsis thaliana seedlings were treated with 200 µM (IC25) of the PPP inhibitor cantharidin and the entire transcriptome profile was determined by microarray at 2, 10, and 24 h after treatment. The transcription of approximately 10% (2,577) of the 24,000 genes of Arabidopsis changed significantly after treatment. Inhibition of serine/threonine protein phosphatases reduced transcription of genes associated with auxin and light signaling and induced expression of genes involved in the hypersensitive response, and in flagellin and abscisic acid signaling. Various microarray results were verified at the protein level by 2D gel analysis. The great variety of up- and down-regulated genes in our microarray experiment implied that cantharidin interfered with the activities of serine/threonine protein phosphatases that interact directly or indirectly with receptors or are located near the beginning of signal transduction pathways. In many cases, serine/threonine protein phosphatases interact with protein complexes of various receptors such as ethylene or light sensors localized in different cell compartments. They function as negative regulators modifying receptor functions, thus altering signaling that influences transcriptional responses.