Trichome Dynamics and Artemisinin Accumulation during Development and Senescence ofArtemisia annuaLeaves

Abstract

Artemisinin is a sesquiterpene lactone endoperoxide and an important antimalarial drug produced in Artemisia annua. To unravel the diverse processes determining artemisinin yield in A. annua crops, artemisinin accumulation during the development of individual leaves was studied in two field experiments. During the life cycle of a leaf, artemisinin was always present. Quantities were low at leaf appearance and increased steadily. In leaves studied until after senescence, maximum quantities and concentrations were achieved after the leaf had turned brown. The total quantity of possible artemisinin precursors per leaf (dihydroartemisinic acid and other upstream precursors) was highest early in the leaf cycle when the leaf was still expanding. Dihydroartemisinic acid was more abundant than the other compounds and its quantity declined during leaf development whereas that of artemisinin increased. Dihydroartemisinic acid was not converted directly into artemisinin, because on a per leaf basis the decline in molar quantity of precursors in the earliest formed leaves was not compensated for by a simultaneous increase in artemisinin. Our results suggest that a (putative) intermediate such as dihydroartemisinic acid hydroperoxide temporarily may have accumulated in considerable quantities. The number of mature, capitate trichomes on the adaxial leaf side increased after leaf appearance until the end of leaf expansion, and then decreased, probably due to collapse of trichomes. Artemisinin production thus (also) occurred when trichomes were collapsing. Later formed leaves achieved higher concentrations of artemisinin than earlier formed leaves, because of a higher trichome density and a higher capacity per trichome.

Artemisia annua mutant impaired in artemisinin synthesis demonstrates importance of nonenzymatic conversion in terpenoid metabolism

“…These and other changes were largely restricted to the juvenile leaves (Datasets S1 and S3 and Fig. S5), which are relatively dense in glandular trichomes (29) and exhibit high expression levels of terpene synthases (30). Recent attempts to silence AMORPHA-4,11-DIENE SYNTHASE expression in self-pollinating varieties of Artemisia annua resulted in increased levels of two nonamorphadiene sesquiterpenes, caryophyllene and copaene, which may be due to an elevated pool of farnesyl diphosphate acting as substrate for other sesquiterpene synthases in the glandular trichomes (25).…”

“…Previous work had shown that early stage intermediates in the artemisinin pathway accumulate in young A. annua leaves, and, as they mature, artemisinin accumulates (25,29). To investigate the effects of the cyp71av1-1 mutation on artemisinin biosynthesis, we analyzed three leaf developmental stages: juvenile (leaves 1-5 as counted down from the apical meristem), expanding (leaves 7-9), and mature (leaves 11-13).…”

Modelling Processes Determining and Limiting the Production of Secondary Metabolites During Crop Growth: The Example of the Antimalarial Artemisinin Produced in Artemisia Annua

“…The relative importance of trichome densities or individual trichome productivity for explaining differences in artemisinin yield over crops is still largely unknown. No experimental data for studies on whole crops is available, but studies on individual leaves within a crop have revealed that trichome densities can partly explain the differences in artemisinin concentrations within a crop (Lommen et al, 2006). Genotype and environmental conditions also likely affect trichome densities.…”

“…The experimental work was done using either an open pollinated Vietnamese selection (Bouwmeester et al, 1999;Wallaart et al, 2000;Bertea et al, 2005;Lommen et al, 2006) or the hybrid cultivar Anamed A3 (cf. Lommen et al, 2006), and was carried out at two locations (Achterberg and Wageningen) in The Netherlands; experiments included different planting densities.…”

“…The crop was harvested at 112 days after transplanting into the field. Subsamples for dry matter determination and plant composition were dried at 70°C for a minimum of 24 h. Subsamples for determination of artemisinin and upstream precursors of artemisinin dihydroartemisinic acid, dihydroartemisinic aldehyde, artemisinic aldehyde and artemisinic alcohol were dried at 30°C for 24 h, ground and subjected to GC-MS analysis as described by Lommen et al (2006).…”