Induction of delta-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 suscepti ble cotton cultivar Rowden (G. hirsutum) after inoculation with a defoliati ng isolate of the pathogen Verticillium dahliae. This was demonstrated by s ignificantly higher levels of phytoalexins in SBSI 12 h after inoculation. Furthermore, by 48 h after inoculation of SBSI, the phytoalexins hemigossyp ol and desoxyhemigossypol achieved levels (23.9 and 10.5 mu g/g of fresh ti ssue, respectively) sufficient to completely inhibit conidial germination. Rowden required 96 h to attain comparable levels. Similarly, the activity o f delta-cadinene synthase, a key enzyme required for the biosynthesis of th e terpenoid phytoalexins, increased more rapidly in the resistant cotton cu ltivar than in the susceptible one. The changes in phytoalexin concentratio ns and enzyme activity are consistent with the hypothesis that phytoalexins are an essential component in protecting the plant from infection by V, da hliae.