Recently two (+)D-cadinene synthases have been cloned from G. arboreum L. cv. Nangking cells and heterologously expressed. Both enzymes catalyze the same reaction but differ in amino acid sequence (80% identity), pH optimum and kinetic parameters. To answer the question why two identical enzymes are required for the catalysis of the rate limiting reaction in the biosynthetic pathway(s) leading to cotton tissue phytoalexins, the differential induction of the mRNA for the two enzymes was measured after elicitation, using quantitative reverse transcription polymerase chain reactions. The two enzymes, CAD A and CAD C, are differentially expressed in cotton roots and leaves. Transcription of CAD A is preferentially induced by the phytopathogenic fungus Verticillium dahliae in root tissue of four weeks old G. arboreum seedlings. Whereas both CAD A and C expression is induced in leaf tissue by the phytopathogenic bacterium Xanthomonas campestris pv. malvacearum.