Uptake of phloem-specific cardenolides by Cuscuta sp. growing on Digitalis lanata and Digitalis purpurea

Cuscuta europaea, C. platyloba and C. reflexa were cultivated on stems of Digitalis lanata und D. purpurea in the second year of vegetation as well as on petioles of rosettes of D. purpurea in the first year of vegetation. C. reflexa was most suitable as parasite because it formed relatively thick shoot axes and revealed the best rates of infection. Cardenolides were taken up by the parasitising Cuscuta sp. from the host plants. Within the parasite the cardenolide concentration decreased from the haustorial region to the shoot tip. The main cardenolides present in Cuscuta were identified as the monoglycosides strospeside (gitoxigenin digitaloside) and verodoxin (gitaloxigenin digitaloside). These cardenolides were found irrespective of growth of the parasite on D. lanata or D. purpurea though the bulk cardenolides of these species are tetraglycosides and the cardenolide profiles of D. lanata and D. purpurea differ from each other. However, strospeside and verodoxin are structurally related to digitalinum verum (gitoxigenin glucosyldigitaloside) and glucoverodoxin (gitaloxigenin glucosyldigitaloside) present in the phloem sap of D. lanata. They are obviously derived from these cardenolides by deglucosylation.