Identification and functional characterization of a new sunflower germacrene A synthase (HaGAS3)

Sesquiterpenes and sesquiterpene lactones are major natural compounds found in linear and capitate glandular trichomes of sunflower, Helianthus annuus L. In addition to two recently identified germacrene A synthases HaGAS1 and HaGAS2, found in capitate trichome gland cells, reverse transcription-PCR experiments have now allowed identification of a third enzyme of this type, HaGAS3. Its cDNA sequence was established and its functional characterization as a germacrene A synthase was achieved through in vitro expression in engineered yeast, and by GC-MS experiments. PCR and RT-PCR experiments with cDNA from different plant organs revealed that the new enzyme is expressed independently from the other two. While these latter two were expressed in plant organs bearing capitate glandular trichomes and in roots, the new enzyme occurred in plant tissues not linked to the presence of specific trichomes (for example, cotyledons), and was absent in roots. The experiments show that independently regulated pathways for the first cyclic sesquiterpene, germacrene A, are present in sunflower.