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Surface glandular trichomes distributed throughout the aerial parts of sweet basil (Ocimum basilicum) produce and store monoterpene, sesquiterpene, and phenylpropene volatiles. Three distinct basil chemotypes were used to examine the molecular mechanisms underlying the divergence in their monoterpene and sesquiterpene content. The relative levels of specific terpenes in the glandular trichomes of each cultivar were correlated with the levels of transcripts for eight genes encoding distinct terpene synthases. In a cultivar that produces mostly (R)-linalool, transcripts of (R)-linalool synthase (LIS) were the most abundant of these eight. In a cultivar that synthesizes mostly geraniol, transcripts of geraniol synthase were the most abundant, but the glands of this cultivar also contained a transcript of an (R)-LIS gene with a 1-base insertion that caused a frameshift mutation. A geraniol synthase-LIS hybrid gene was constructed and expressed in Escherichia coli, and the protein catalyzed the formation of both geraniol and (R)-linalool from geranyl diphosphate. The total amounts of terpenes were correlated with total levels of terpene synthase activities, and negatively correlated with levels of phenylpropanoids and phenylalanine ammonia lyase activity. The relative levels of geranyl diphosphate synthase and farnesyl diphosphate synthase activities did not correlate with the total amount of terpenes produced, but showed some correlation with the ratio of monoterpenes to sesquiterpenes.
Plants produce a large number of secondary metabolites that function in a variety of ecological contexts. Many specialized compounds are toxic and can therefore serve as defense agents against microbial pathogens and insect and animal herbivores (Wittstock and Gershenzon, 2002; Theis and Lerdau, 2003; Wink, 2003). Other compounds are volatile and serve to attract pollinators or even insects that prey on the plant's enemies or repel harmful organisms (Pare and Tumlinson, 1999; Kessler and Baldwin, 2001; Baldwin et al., 2002; Pichersky and Gershenzon, 2002).
Secondary compounds with roles in defense are often sequestered in specialized cells or structures, presumably to protect the plant itself from its own toxicity (Gershenzon et al., 1989; Pare and Tumlinson, 1997; Duke et al., 2000; Dussourd and Hoyle, 2000; Hallahan, 2000; Martin et al., 2002). A common mechanism of sequestration has been the evolution of anatomical structures, termed glandular trichomes, on the surface of the aerial parts of the plants. Such structures typically contain gland cells (or a...