Trichome dynamics and artemisinin accumulation during development and senescence of Artemisia annua leaves.

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.

Role of cucurbitacin C in resistance to spider mite (Tetranychus urticae) in cucumber (Cucumis sativus L.).

Cucurbitacins are bitter triterpenoid compounds that are toxic to most organisms and occur widely in wild and cultivated Cucurbitaceae. The only cucurbitacin identified in Cucumis sativus is cucurbitacin C. The bitter taste of cucumber has been correlated with resistance to the spider mite Tetranychus urticae, but a quantitative relationship has not been established. We determined the spider mite resistance and cucurbitacin C content in the dihaploid progeny derived from the F1 generation of a cross between a bitter, spider-mite-resistant cucumber line and a bitter-free, spider-mite-susceptible line. The ratio of the number of bitter to bitter-free dihaploids conformed to the expected 1:1 ratio, based on a monogenic segregation pattern. Genetic analysis ascribed 69% of the variance of the difference in spider mite survival rate to the bitterness locus. Within the group of bitter dihaploids, cucurbitacin C content was significantly correlated with spider mite resistance. Thus, a quantitative relationship between cucurbitacin C content and spider mite resistance could be established.