The onset of sucrose accumulation in cold‐stored potato tubers is caused by an increased rate of sucrose synthesis and coincides with low levels of hexose‐phosphates, an activation of sucrose phosphate synthase and the appearance of a new form of amylase

Abstract

These experiments investigate events involved in triggering sugar accumulation in the cold in tubers of Solanum tuberosum L. cv. Desirée. Sugar content, 14C‐glucose metabolism, metabolite levels and activities of sucrose phosphate synthase (SPS) and starch‐degrading enzymes were followed after transfer to 4°C. (i) Net sucrose accumulation began between 2 and 4 d. By 10 d, reducing sugars were also increasing. From 20 d onwards, sugar accumulation slowed. Sucrose fell, but reducing sugars continued to increase. (ii) To measure unidirectional sucrose synthesis, U‐[14C]glucose was injected into tubers after various times at 4°C. The tubers were then incubated for 6 h. After 1 d at 4°C, both the absolute and the relative (expressed as a percentage of the metabolized label) rates of sucrose synthesis decreased compared to those at 20°C. Between 2 and 4 d at 4°C, labelling of sucrose increased 3‐fold, to over 60% of the metabolized label. This high rate was maintained for up to 50 d in cold storage. When tissue slices were incubated with 2.5 mol m−3 U‐[14C]glucose, the rate of labelling of sucrose in slices from 6 d cold‐stored material was higher than in slices from warm‐stored material, irrespective of whether the incubation occurred at 4°C or at 20°C. (iii) Hexose‐phosphates increased during the first day after transfer to 4°C. Their levels fell during the next 3 d, as sucrose synthesis increased. They then rose (until 20 d) and fell, in parallel with the rise and decline of sucrose levels. UDPglucose remained unaltered during the first 4 d, and then increased and decreased in parallel with sucrose. (iv) SPS activity assayed in optimal conditions and the total amount of SPS protein did not change. However, when assayed in the presence of phosphate and limiting substrate concentrations, activity rose 3–5‐fold between 2 and 4 d. (v) Amylases and phosphorylases were investigated using zymograms to separate isoforms. Phosphorylases did not change. Between 2 and 4 d at 4°C, a new amylolytic activity appeared. (vi) Estimates of the specific activity of the phosphorylated intermediates and the absolute rate of sucrose synthesis (calculated from the 14C‐labelling data and metabolite analysis) showed that changed kinetic properties of SPS and decreased levels of hexose‐phosphate are accompanied by a 6–8‐fold stimulation of sucrose synthesis. They also show that the final level of sugar is partly determined by a cycle of sugar synthesis and degradation. (vii) It is concluded that the onset of sugar accumulation in cold‐stored tubers is initiated by a change in the kinetic properties of SPS and the appearance of a new amylolytic activity. It is discussed how other factors, including hexose‐phosphate levels and subcellular compartmentalization, could also influence the final levels of sugars by altering the balance of sugar synthesis and remobilization.

La temperatura de vernalización en bulbos reutilizados de lirio (Lilium sp.) afecta la distribución de materia seca y la producción de flor

“…La mayor conversión de almidón a azúcares solubles se debe a la acción de las giberelinas (GA) cuya síntesis puede ser estimulada por bajas temperaturas, estas sintetizan enzimas hidrolí-ticas, principalmente α-amilasa (Davies, 2004) isoamilasa, b-amilasa, a-glucosidasa y almidón fosforilasa (Cochrane et al, 1991;Cottrell et al, 1993;Claassen et al, 1993;Hill et al, 1996;Nielsen et al, 1997) encargadas de la degradación del almidón. Simultáneamente, los niveles de ABA disminuyen con el incremento en los niveles de frío y se ha encontrado que en bulbos de L. rubellum, almacenados a 4°C, la concentración de este inhibidor del crecimiento disminuye a medida que la vernalización aumenta (Rong-Yan et al, 2006) y también coincide con los estudios en Polianthes tuberosa (Nagar, 1995) y Allium wakegi ( Yamaz ak i et al, 2002).…”

A β-Amylase in Potato Tubers Is Induced by Storage at Low Temperature

“…Vol. 11 3 , 1997 Isherwood (1975) and Hill et al (1996) found no effect of storage temperature on phosphorylase activity, and Sowokinos et al (1985), who did report a selective increase in phosphorylase in the sweetening tuber tissues, found no changes in amylase activity. These inconsistencies might be due to differences between cultivars.…”

“…Cold sweetening is of commercial interest, since high sugar content will result in a poor-quality product when the tubers are used for fries and chips. Recently, it was shown that the accumulation of sugars is due to a specific activation of Suc synthesis by up-regulation of SUC-P synthase (Hill et al, 1996). Along with the activation of Suc synthesis, a new amylolytic activity also induced in the cold-stored tubers was discovered.…”

Mycorrhiza formation and elevated CO2 both increase the capacity for sucrose synthesis in source leaves of spruce and aspen

“…This could be sufficient for an increase in sucrose production in order to meet the elevated C demand of the mycorrhizal sink. An increase of the SPS activation state is mediated by reversible dephosphorylation of the enzyme protein (Huber & Huber, 1996) and occurs in a range of conditions such as low temperature (Hill et al, 1996), water and osmotic stress (Huber & Huber, 1996), and high light intensity (Stitt et al, 1988), all conditions which require increased sucrose production. The protein kinase involved in the modulation of SPS activation state, in turn, is also regulated, for example in spinach (Huber & Huber, 1996) as well as in spruce and aspen (A.…”