Sites of synthesis, translocation and accumulation of pyrrolizidine alkaloid N-oxides in Senecio vulgaris L.

(14)C-Labelled alkaloid precursors (arginine, putrescine, spermidine) fed to Senecio vulgaris plants via the root system were rapidly taken up and efficiently incorporated into the pyrrolizidine alkaloid senecionine N-oxide (sen-Nox) with total incorporations of 3-6%. Considerable amounts of labelled sen-Nox were translocated into the shoot and were directed mainly into the inflorescences, the major sites of pyrrolizidine-alkaloid accumulation. Detached shoots of S. vulgaris were unable to synthesize pyrrolizidine alkaloids, indicating that the roots are the site of their biosynthesis. Further evidence was obtained from studies with in-vitro systems established from S. vulgaris: root cultures were found to synthesize pyrrolizidine alkaloids but not cell-suspension cultures, tumor cultures or shoot-like teratomas obtained by transformation with Agrobacterium tumefaciens. Studies on transport of [(14)C]sen-Nox, which was fed either to detached shoots or to the root system of intact plants, indicate that the alkaloid N-oxide does not simply follow the transpiration stream but is specifically channelled to the target tissues such as epidermal stem tissue and flower heads. Exogenously applied [(14)C]senecionine is rapidly N-oxidized. If the phloem path along the stem is blocked by a "steam girdle" translocation of labelled sen-Nox is blocked as well. Root-derived sen-Nox accumulated below the girdle and only trace amounts were found in the tissues above. It is most likely that the root-to-shoot transport of sen-Nox occurs mainly if not exclusively via the phloem. In accordance with previous studies the polar, salt-like N-oxides, which are often considered to be artifacts, were found to be the real products of pyrrolizidine-alkaloid biosynthesis as well as the physiological forms for long-distance transport, tissue-specific distribution and cellular accumulation.