Sucrose phosphate synthase in leaves of mistletoe: its regulation in relation to host (Abies alba) and season

Sucrose-phosphate synthase (SPS; E.C. 2.4.1.14) was studied in 1-year-old leaves of the xylem-parasitic mistletoe (Viscum album L.), growing on Abies alba. Glucose-6-phosphate served as an allosteric activator of mistletoe SPS, increasing the affinity for both substrates, fructose-6-phosphate and UDP-glucose. The activation state of SPS, i.e. the ratio of substrate limited versus non-limited activity, showed two clear peaks between February and July which coincided with increased rates of net photosynthesis of the parasite. Periods of decreased SPS activity were accompanied by a transient accumulation of sucrose and starch. In samples exhibiting a high activation state, activity was decreased by incubation of the extract with ATP; however, ATP did not affect SPS activity in samples exhibiting a low activation state of SPS. In parallel to the first increase of the activation state in March, pool sizes of the positive effector glucose-6-phosphate were high, whereas pool sizes of fructose-2,6-bisphosphate, an inhibitor of sucroneogenesis, were low. The decline in the activation state in April occurred in parallel with increased rates of transpiration of the parasite. This could have increased the availability of host-derived sugars, although the xylem sap of A. alba showed rather consistent concentrations of total soluble sugars throughout the vegetation period (1.1–3.9 mM). We thus speculate that sugar availability in the host xylem controls carbohydrate metabolism in the parasite.