Localization of enzymes of artemisinin biosynthesis to the apical cells of glandular secretory trichomes of Artemisia annua L

Abstract

A method based on the laser microdissection pressure catapulting technique has been developed for isolation of whole intact cells. Using a modified tissue preparation method, one outer pair of apical cells and two pairs of sub-apical, chloroplast-containing cells, were isolated from glandular secretory trichomes of Artemisia annua. A. annua is the source of the widely used antimalarial drug artemisinin. The biosynthesis of artemisinin has been proposed to be located to the glandular trichomes. The first committed steps in the conversion of FPP to artemisinin are conducted by amorpha-4,11-diene synthase, amorpha-4,11-diene hydroxylase, a cytochrome P450 monooxygenase (CYP71AV1) and artemisinic aldehyde Delta11(13) reductase. The expression of the three biosynthetic enzymes in the different cell types has been studied. In addition, the expression of farnesyldiphosphate synthase producing the precursor of artemisinin has been investigated. Our experiments showed expression of farnesyldiphosphate synthase in apical and sub-apical cells as well as in mesophyl cells while the three enzymes involved in artemisinin biosynthesis were expressed only in the apical cells. Elongation factor 1alpha was used as control and it was expressed in all cell types. We conclude that artemisinin biosynthesis is taking place in the two outer apical cells while the two pairs of chloroplast-containing cells have other functions in the overall metabolism of glandular trichomes.