Regulation of Fructan Metabolism in Leaves of Barley (Hordeum vulgare L. cv Gerbel)

Abstract

Excised primary leaf blades of barley (Hordeum vulgare L. cv Gerbel) rapidly synthesized large quantities of fructan in the light and, upon transfer to the dark, they rapidly degraded it again. In the course of such a light/dark cycle the activities of sucrose-sucrose-fructosyltransferase (SST), fructan hydrolase, and invertase were measured in cell-free extracts of the blades. SST activity increased 20-fold within 24 hours in the light and disappeared again upon transfer to the dark during a similar period of time. Cycloheximide inhibited the increase of SST activity in the light indicating de novo synthesis. The loss of SST activity in the dark, however, was unaffected by cycloheximide. No SST activity appeared in the light if photosynthesis was inhibited by lowering the CO2 concentration in the atmosphere. However, SST activity and fructan synthesis were induced even in the dark and at a low CO2 concentration when the leaf blades were immersed in a solution of sucrose. Several other sugars, maltose and fructose in particular, had the same effect. Trehalose induced SST activity but no fructan synthesis occurred. The activities of fructan hydrolase and invertase changed little during the light/dark cycle. It is suggested that the control of SST activity in conjunction with the supply of photosynthates plays a key role in the regulation of fructan metabolism.Fructan, a polyfructosylsucrose of varying molecular size, is the main carbohydrate reserve for intermediate and long-term storage in vegetative organs of perennial forage grasses and cereals, particularly in temperate and frigid climate zones (1,7,11,15). Whereas the diurnal difference between production, export, and consumption of photosynthates in the leaves is largely balanced by storage and mobilization of sucrose and starch (2, 16), fructan seems to be the reserve used preferentially for balancing seasonal differences due to changes in plant development and weather. Large quantities of fructan are usually stored in leaves and stems of grasses during autumn and winter. They are mobilized again during initial growth in early spring, during regrowth after mowin", and during the grain filling period (3,10,12,17). As a general rule fructan is accumulated when the conditions allow photosynthesis but are restricting growth or transport; fructans are depleted under opposite conditions.Fructan can be produced in enormous amounts which may exceed 70% of the dry weight of the leaves (13,18). Fructan and the fructan metabolizing enzymes were found to be located in the vacuoles of barley mesophyll cells (18,19). The same localization was found in the cells of the storage parenchyma of the tubers of Jerusalem artichokes (5).In spite of the central role of fructan metabolism in carbon partitioning in economically important grasses such as wheat and barley, very little work has been done on the enzymology and regulation of fructan synthesis and degradation (8,11,15). With the excised cereal leaf blades we had a system at hand in which fructan synthesis coul...

Isolation and Structural Analysis In Vivo of Newly Synthesized Fructooligosaccharides in Onion Bulbs Tissues (Allium cepa L.) during Storage

“…The synthesis carried out in vitro showed that the reaction is quite linear but faster during the first 100 hours slowing down after this (Figure 7). This pattern is regular and showed that reaction occurs without apparent inhibition, mainly caused by sucrose which is known to inhibit many enzymatic reactions when its concentration is high [16].…”

“…Although it was established that net fructan accumulation only occurs when the rate of synthesis exceeds the rate of degradation in the vacuole [16], an imbalance occurs during the postharvest life which lead to the exhausting of the FOS reserve in onion tissues. Beside these biosynthetic activities, up to this date, no work reported the biosynthesis of new FOS in onion bulbs during storage, or in any other liliaceous plants as well.…”

Transgenic potato tubers accumulate high levels of 1‐kestose and nystose: functional identification of a sucrose sucrose 1‐fructosyltransferase of artichoke (Cynara scolymus) blossom discs

“…A stimulating effect of pyridoxaI-HCI on fructosyltransferases was also observed by other authors (Koops and Jonker, 1996;Simmen etal., 1993;Wagner and Wiemken, 1986).…”

Fructan content of wheat seedlings(Triticum aestivumL.) under hypoxia and following re‐aeration

“…The lack of phosphorylated substrate causes a retarded glycolysis in spite of Numerous studies provide direct or indirect evidence linking fructan metabolism to environmental stress (Archbold, 1938;Hendry, 1987;Pollock & Cairns, 1991). The relationship between fructans and the response of plants to low temperature has often been described (Wagner, Wiemken & Matile, 1986;Tognetti et al, 1990). However, whether fructans are directly involved in the mechanism of cold adaptation is still debatable because their accumulation may be the result of an increased content of sucrose for very difTerent reasons (Wagner, Keller & Wiemken, 1983;Cairns & Pollock, 1988).…”