Uptake, Transport and Storage of Cardenolides in Foxglove. Cardenolide Sinks and Occurrence of Cardenolides in the Sieve Tubes of Digitalis lanata¹

Abstract

Cardiac glycoside transport was investigated on the organ and whole plant level. Uptake experiments were carried out with shoot and root cultures of Digitalis lanata. In both systems primary cardenolides, i.e., those with a terminal glucose in their oligosaccharide side chain, were taken up against their concentration gradient, whereas the glucose‐free secondary cardenolides were not. Active uptake of primary cardenolides was further evidenced by KCN inhibition of uptake. Using plantlets grown in vitro the long‐distance transport of primary cardenolides from the leaves to the roots was demonstrated. Cardenolides were also detected in etiolated leaves, induced on plants with green leaves, which are supposed to be unable to synthezise cardenolides de novo, providing further evidence for long‐distance transport. Several primary cardenolides were detected in the honeydew excreted by aphids fed on Digitalis lanata leaves, indicating that the phloem is a transporting tissue for cardenolides. On the other hand, the xylem sap obtained by applying the pressure‐chamber technique was cardenolide‐free. It was concluded that in Digitalis primary cardenolides serve as both the transport and the storage form of cardenolides. After their synthesis they are either stored in the vacuoles of the source tissue or loaded into the sieve tubes, from which they are unloaded at other sites where they are trapped in the vacuoles of the respective sink tissue.

Phloem Loading in Two Scrophulariaceae Species. What Can Drive Symplastic Flow via Plasmodesmata?

“…Therefore, the loading of both Suc and antirrhinoside into the phloem in A. barclaiana must be energized. This is consistent with the following observations: First, H 1 /Suc symporters from A. barclaiana were cloned and their function in Suc uptake confirmed by functional expression and complementation in yeast (Saccharomyces cerevisiae; Knop, 2001); second, transfer of glucosides via the plasma membrane against their concentration gradient has been demonstrated in another Scrophulariaceae species, Digitalis lanata (Christmann et al, 1993), and recently, the Suc transporter AtSUC2 was shown to accept a broad range of glucosides as substrates (Chandran et al, 2003); and third, antirrhinoside and Suc were both present in the apoplast and their apoplastic concentrations in leaves with blocked translocation increased 3-fold and 6-fold, respectively. Thus, in A. barclaiana, Suc and antirrhinoside are likely to be actively loaded into the phloem from the apoplast, perhaps mainly via TCs.…”

Glycosyltransferases in secondary plant metabolism: tranquilizers and stimulant controllers

“…Several secondary plant products, including cyanogenic glucosides (Mùller and Seigler 1998) and cardenolides (Christmann et al 1993), are only transported within the plant in a glycosylated form. In the case of the cyanogenic glucoside linamarin in Hevea brasiliensis, where the diglucoside but not the monoglucoside or aglycone is the transport form, the substrate speci®city of apoplastic beta-glucosidases restricts the extracellular transport of the monoglucoside (Gruhnert et al 1994).…”

Cardenolide Biosynthesis in Foxglove1

“…The long-distance transport of primary cardenolides from the leaves to the roots or to etiolated leaves was demonstrated. It was established that the phloem but not the xylem is a transporting tissue for cardenolides (84). To summarize, it seems as if primary cardenolides may serve as both the transport and the storage form of cardenolides.…”