Induction of δ-Cadinene Synthase and Sesquiterpenoid Phytoalexins in Cotton by Verticillium dahliae

Phytoalexin biosynthesis occurred earlier in the resistant cotton cultivar Seabrook Sea Island 12B2 (SBSI) (Gossypium barbadense) than in the susceptible cotton cultivar Rowden (G. hirsutum) after inoculation with a defoliating isolate of the pathogen Verticillium dahliae. This was demonstrated by significantly higher levels of phytoalexins in SBSI 12 h after inoculation. Furthermore, by 48 h after inoculation of SBSI, the phytoalexins hemigossypol and desoxyhemigossypol achieved levels (23.9 and 10.5 microgram/g of fresh tissue, respectively) sufficient to completely inhibit conidial germination. Rowden required 96 h to attain comparable levels. Similarly, the activity of delta-cadinene synthase, a key enzyme required for the biosynthesis of the terpenoid phytoalexins, increased more rapidly in the resistant cotton cultivar than in the susceptible one. The changes in phytoalexin concentrations and enzyme activity are consistent with the hypothesis that phytoalexins are an essential component in protecting the plant from infection by V. dahliae.