MODELING OF QUINOLIZIDINE ALKALOID NET FLOWS IN LUPINUS-ALBUS AND BETWEEN LUPINUS-ALBUS AND THE PARASITE CUSCUTA-REFLEXA - NEW INSIGHTS INTO THE SITE OF QUINOLIZIDINE ALKALOID SYNTHESIS

The present work reveals new and completely different conclusions abou t the atkaloid economy of symbiotically fed Lupinus albus and L. albus parasitized by Cuscuta reflexa in the study period of 43-55 d after s owing of lupin. Net flows of alkaloids within lupin and between host a nd parasite were calculated using the molar ratio of alkaloid nitrogen :total nitrogen combined with known net flows of nitrogen in the trans port fluids and analysing alkaloid accumulation in plant organs by HRG C. In contrast to previous studies, quinolizidine alkaloids were predi cted to be synthesized mainly in the root of L. albus and to be predom inantly transported via xylem to the apical plant shoot organs. Parasi tism by C. reflexa for 12d induced a decline of alkaloid content in th e host L. albus up to 53% compared to control plants and alkaloid synt hesis was halved-apparently due to a shortage of the precursor lysine. In spite of an additional decrease in nitrogen levels at the second h arvest, the host-parasite system showed a 1.3-fold higher alkaloid con tent than the control plants, 63% of the total alkaloids being attract ed by Cuscuta. This indicates (a) restriction of catabolic processes w ithin infected lupins, (b) a massive shift of nitrogen metabolism in t he direction of alkaloids and (c) an enormous sink potential of Cuscut a for nitrogenous compounds. Although xylem was found to be the main t ranslocation system for alkaloids, the modelling of alkaloid flows pre dicts Cuscuta to derive only 4.5% of its total alkaloid supply from th e xylem and 95.5% from the phloem. By analogy with nitrogen flows, thi s finding requires xylem-phloem transfers which were assumed to occur within the stem axis of lupin. A similar proportion regarding the cont ribution of xylem and phloem to the supply of Cuscuta was obtained for the net flows of two selected alkaloids, lupanine and 13 alpha-tigloy loxylupanine, and for the import of total nitrogen. This result points to similar uptake mechanisms for alkaloid nitrogen and other nitrogen ous compounds excluding discrimination of otherwise toxic alkaloids. C uscuta is likely to overcome the chemical barrier of alkaloids by accu mulation and catabolic processes.