Abiotic elicitation of coumarin phytoalexins in sunflower

Abstract

Detached leaves, leaf disks and cut stems and hypocotyls, but not roots of sunflower ( Helianthus annuus) plants accumulated the coumarin phytoalexins, scopoletin and ayapin, when treated with solutions of CuCl 2 or sucrose. Irradiation with shortware UV and exposure to Triton X-100 was also effective, while no effect was observed with longwave UV, salicylic acid, dichloroisonicotinic acid or glutathione. The elicitation of the coumarins was dependent on the type of elicitor used and the age of the plant tissues. Treatment with sucrose elicited similar amounts of scopoletin and ayapin while treatment with CuCl 2 favoured the formation of ayapin with low concentrations of scopoletin and none of the scopoletin 7- O-glucoside being observed. The selective induction of ayapin by CuCl 2 was associated with an inhibition in the activity of the O-glucosyltransferase responsible for glycosylating scopoletin, and an induction in a peroxidase which metabolized scopoletin but not ayapin. The abiotic elicitation of the phytoalexins increased in the leaves of sunflower as a function of age, with seedlings and immature plants being unable to synthesize scopoletin or ayapin. Inhibitor and precursor feeding studies were unable to elucidate the biosynthetic route to scopoletin although an elicitor-inducible methyltransferase activity which could methylate esculetin to produce scopoletin was identified.