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Keywords:

  • Clarkia breweri;
  • floral scent;
  • hawkmoths;
  • head space;
  • gas chromatography–mass spectrometry;
  • monoterpenoids;
  • Onagraceae;
  • terpene synthase

Abstract

The monoterpene alcohol, linalool, is present in the floral fragrance of diverse plant families and is attractive to a broad spectrum of pollinators, herbivores and parasitoids. Floral emission of linalool has evolved de novo in the fragrant, moth-pollinated annual Clarkia breweri (Gray) Greene (Onagraceae) through a combination of up-regulation and ectopic expression of its biosynthetic enzyme, linalool synthase (LIS), in conjunction with allometric size increases in all floral organs. Linalool synthase activity and linalool emissions are 1000-fold lower in a sibling species, C. concinna (Fischer & Meyer) Greene, that is diurnally pollinated. Linalool synthase expression is spatially and temporally regulated during C. breweri flower development, immediately precedes free linalool emission and is absent from nonfloral tissues. Its activity is highest in the style, but most of the linalool product appears to be converted to the pyranoid and furanoid linalool oxides. The LIS structural gene is a member of the terpene synthase gene family, sharing sequence identity with two discrete classes, represented by limonene synthase (LMS) and copalyl pyrophosphate synthase (CPS). Genetic crosses between C. breweri and C. concinna indicate that strong linalool emission segregates as a dominant mendelian trait, whereas the inheritance of linalool oxide formation is more complex, suggesting epistatic biosynthetic pathway interactions. We discuss areas for future research, including comparative studies of linalool biosynthesis in different plant families, entrainment of linalool emission to nocturnal circadian rhythms and the induction of vegetative linalool as an indirect herbivore defense.