Modelling of quinolizidine alkaloid net flows inLupinus albusand between L.albusand the parasiteCuscuta reflexa: new insights into the site of quinolizidine alkaloid synthesis

The present work reveals new and completely different conclusions about the alkaloid economy of symbiotically fed Lupinus albus and L. albus parasitized by Cuscuta reflexa in the study period of 43-55 d after sowing of lupin. Net flows of alkaloids within lupin and between host and parasite were calculated using the molar ratio of alkaloid nitrogen:total nitrogen combined with known net flows of nitrogen in the transport fluids and analysing alkaloid accumulation in plant organs by HRGC. In contrast to previous studies, quinolizidine alkaloids were predicted to be synthesized mainly in the root of L. albus and to be predominantly transported via xylem to the apical plant shoot organs. Parasitism by C. reflexa for 12 d induced a decline of alkaloid content in the host L. albus up to 53% compared to control plants and alkaloid synthesis was halved-apparently due to a shortage of the precursor lysine. In spite of an additional decrease in nitrogen levels at the second harvest, the host-parasite system showed a 1.3-fold higher alkaloid content than the control plants, 63% of the total alkaloids being attracted by Cuscuta. This indicates (a) restriction of catabolic processes within infected lupins, (b) a massive shift of nitrogen metabolism in the direction of alkaloids and (c) an enormous sink potential of Cuscuta for nitrogenous compounds. Although xylem was found to be the main translocation system for alkaloids, the modelling of alkaloid flows predicts Cuscuta to derive only 4.5% of its total alkaloid supply from the xylem and 95.5% from the phloem. By analogy with nitrogen flows, this finding requires xylem-phloem transfers which were assumed to occur within the stem axis of lupin. A similar proportion regarding the contribution of xylem and phloem to the supply of Cuscuta was obtained for the net flows of two selected alkaloids, lupanine and 13α-tigloyloxylupanine, and for the import of total nitrogen. This result points to similar uptake mechanisms for alkaloid nitrogen and other nitrogenous compounds excluding discrimination of otherwise toxic alkaloids. Cuscuta is likely to overcome the chemical barrier of alkaloids by accumulation and catabolic processes.