Developmental changes of enzymes involved in conversion of sucrose to hexose-phosphate during early tuberisation of potato

Abstract

A highly synchronised in-vitro tuberisation system, based on single-node cuttings containing an axillary bud, was used to investigate the activity patterns of enzymes involved in the conversion of sucrose to hexose-phosphates during stolon-to-tuber transition of potato (Solanum tuberosum L.). Two dierent nontuberising systems were included to distinguish between changes that are or are not tuber-speci®c. At tuberisation the activity of soluble acid invertase decreased (13-fold) and of sucrose synthase increased (12-fold). The activity of both enzymes remained unchanged in the non-tuberising treatments. Based on the opposite patterns and large dierence in activity of these two sucrolytic enzymes, we conclude that sucrose synthase constitutes the predominant route of sucrose breakdown after tuber initiation. During the period before tuberisation, the activity of cell-wall-bound invertase and of hexokinase showed a highly positive correlation r 2 0X96 in all the three treatments, suggesting coordinated coarse control of both enzyme activities. After the onset of tuberisation cell-wall-bound invertase activity decreased to a very low level, a change not observed in the non-tuberising systems, indicating that cell-wall-bound invertase is presumably not involved in the unloading mechanism and/or short-distance transport of sucrose within the perimedulla of growing tubers. The overall activity of fructokinase and of hexokinase both showed a fourfold increase after tuber initiation, but remained unchanged in the non-tuberising systems. The increase of fructokinase suggests that the phosphorylation of fructose by fructokinase down-regulates the cytosolic fructose content in order to maintain a high sucrose-synthase-catalysed net¯ux of sucrose to phosphorylated hexoses during rapid tuber growth. The increase of total glucose-phosphorylating potential could be a response to the tuberisation-related starch accumulation process. The activity of UDP-glucose pyrophosphorylase showed no developmental change. The level of UDP-glucose pyrophosphorylase activity is very likely the result of metabolic regulation.Abbreviations: Susy = sucrose synthase; UDPGlc = uridine-5diphosphoglucose; UGPase = UDPGlc pyrophosphorylase Correspondence to: N. Appeldoorn: FAX

Alterations in Cytosolic Glucose-Phosphate Metabolism Affect Structural Features and Biochemical Properties of Starch-Related Heteroglycans

“…5). In growing tubers, source-derived Suc is imported into the parenchyma cells and subsequently cleaved, by the action of the cytosolic Suc synthase, into Fru and UDP-Glc (Appeldoorn et al, 1997;Viola et al, 2001). The latter is converted to Glc-1-P (plus UDP), while the former enters the Glc-P metabolism via a reaction sequence catalyzed by fructokinase and phosphoglucoisomerase.…”

Growth responses of potato (Solanum tuberosum) grown in a hot tropical lowland to applied paclobutrazol: 2. Tuber attributes

“…Booth & Lovell (1972) reported reduced sink strength of tubers resulting from high GA 3 accumulation in tuber tissue. Under conditions favourable for tuberisation, the activity of enzymes involved in starch biosynthesis such as ADPG-pyrophosphorylase, starch phosphorylase, and starch synthase increase (Visser et al 1994;Appeldoorn et al 1997). Mares et al (1981) observed that exogenous application of GA 3 to growing tubers substantially reduced the activity of ADPG-pyrophosphorylase, whereas the activity of starch phosphorylase remained more or less constant.…”

Differences in regulation of carbohydrate metabolism during early fruit development between domesticated tomato and two wild relatives

“…Total extracts for assays of enzymes were prepared by homogenizing 7-8 mg of the powdered, freeze-dried fruit tissue basically as described previously (Appeldoorn et al 1997(Appeldoorn et al , 1999. The activities of diVerent enzymes were determined in 300 l of assay medium at 20°C.…”