Modelling of quinolizidine alkaloid net flows in Lupinus albus and between L. albus and the parasite Cuscuta reflexa: new insights into the site of quinolizidine alkaloid synthesis

The present work reveals new and completely different conclusionsabout the alkaloid economy of symbiotically fed Lupinus albusand L. albus parasitized by Cuscuta reflexa in the study periodof 43–55 d after sowing of lupin. Net flows of alkaloidswithin lupin and between host and parasite were calculated usingthe molar ratio of alkaloid nitrogen: total nitrogen combinedwith known net flows of nitrogen in the transport fluids andanalysing alkaloid accumulation in plant organs by HRGC. Incontrast to previous studies, quinolizidine alkaloids were predictedto be synthesized mainly in the root of L. albus and to be predominantlytransported via xylem to the apical plant shoot organs. Parasitismby C. reflexa for 12 d induced a decline of alkaloid contentin the host L. albus up to 53% compared to control plants andalkaloid synthesis was halved—apparently due to a shortageof the precursor lysine. In spite of an additional decreasein nitrogen levels at the second harvest, the host-parasitesystem showed a1.3-fold higher alkaloid content than the controlplants, 63% of the total alkaloids being attracted by Cuscuta.This indicates (a) restriction of catabolic processes withininfected lupins, (b) a massive shift of nitrogen metabolismin the direction of alkaloids and (c) an enormous sink potentialof Cuscuta for nitrogenous compounds. Although xylem was foundto be the main translocation system for alkaloids, the modellingof alkaloid flows predicts Cuscuta to derive only 4.5% of itstotal alkaloid supply from the xylem and 95.5% from the phloem.By analogy with nitrogen flows, this finding requires xylemphloemtransfers which were assumed to occur within the stem axis oflupin. A similar proportion regarding the contribution of xylemand phloem to the supply of Cuscuta was obtained for the netflows of two selected alkaloids, lupanine and 13