Orotate leads to a specific increase in uridine nucleotide levels and a stimulation of sucrose degradation and starch synthesis in discs from growingpotato tubers

Freshly cut discs from growing potato tubers were incubated for 3 h with 10 mM orotate or 10 mM uridine. Control discs incubated without precursors sh owed a 30-40% decrease of uridine nucleotides, but not of adenine nucleotid es. Orotate- and uridine-feeding led to a 1.5- to 2-fold increase in the le vels of uridine nucleotides compared with control discs, and a 15-30% incre ase compared with the original values in intact tubers, but did not alter t he levels of adenine nucleotides. Between 70-80% of the uridine nucleotides were present as UDPglucose, 15-25% as UTP, and 2-3% as UDP. The increase o f uridine nucleotides involved a similar relative increase of UDPglucose, U TP and UDP. It was accompanied by a slight stimulation of the rate of [C-14 ]sucrose uptake, a 2-fold stimulation of the rate at which the [C-14]sucros e was subsequently metabolised, a small increase in the levels of hexose ph osphates, glycerate-3-phospate and ADPglucose, and a 30% shift in the alloc ation of the metabolised label in favour of starch synthesis, resulting in a 2.4-fold stimulation of the rate of starch synthesis. Orotate led to a si milar increase of uridine nucleotide levels in the presence of [C-14]glucos e, but did not significantly alter the rate of glucose uptake and metabolis m to starch, nor did it increase the rate of sucrose resynthesis. The level s of uridine nucleotides were high in tubers on 6 to 10-week-old potato pla nts, and declined in tubers on 12 to 15-week-old plants. Comparison with th e effect of the uridine nucleotide level in discs shows that the high level s of uridine nucleotides in tubers on young plants will play an important r ole in determining the rate at which sucrose can be converted to starch, an d that the level of uridine nucleotides is probably co-limiting for sucrose -starch conversions in tubers on older plants.