Peltate glandular trichomes of Ocimum basilicum L. (sweet basil) contain high levels of enzymes involved in the biosynthesis of phenylpropenes

The plant defense compounds of the phenylpropene class (chavicol, eugenol and their derivatives) have been recognized since antiquity as important spices for human consumption, and plants that contain high concentrations of these compounds (e.g., cloves) have high economic value. Several cultivars of basil (Ocimum basilicum) have been developed that produce essential oils rich in specific phenylpropene compounds. However, our understanding of the biosynthetic pathway that produces these compounds in the plant has remained incomplete, with several enzymatic steps, as well as the site of synthesis and storage, still undetermined. Like other members of the Lamiaceae, basil leaves possess on their surface two types of glandular trichomes, termed peltate and capitate glands. Here we demonstrate that the essential oil constituents eugenol and methylchavicol accumulate in the peltate glands of, respectively, basil cultivars SW and EMX-1. Assays for phenylalanine ammonia lyase, the enzyme that catalyzes the first step in the biosynthesis of all phenylpropenes, and for chavicol O-methyltransferase, the enzyme that catalyzes the last step in the formation of methylchavicol, localized the corresponding enzyme activities almost exclusively to the peltate glands.