Abstract
Arachidonic acid, a fungal elicitor from Phytophthora infestans, caused a 10-fold increase in sesquiterpene cyclase activity in potato tuber discs 36 h after application as compared to discs treated with water, whereas squalene synthetase activity increased 10-fold in discs treated with water 48 h after slicing. Arachidonic acid suppressed squalene synthetase activity in wounded potato tuber tissue 6 h after application, and the enzyme's activity remained low for the next 78 h. A similar elicitation of sesquiterpene cyclase activity and suppression of squalene synthetase activity was observed after inoculation with Helminthosporium carbonum, a non-pathogen of potato. There was a similar time course of induction of sesquiterpene cyclase activity after inoculation with either a compatible or incompatible race of Phytophthora infestans, whereas marked suppression of squalene synthetase activity occurred later after inoculation with a compatible race of P. infestans as compared to that which occurred after inoculation with an incompatible race. These results support the concept of co-ordinated regulation of sesquiterpene cyclase and squalene synthetase in potato during the elicitation of sesquiterpenoid phytoalexin accumulation.
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