Fructan matabolism in cereals: induction in leaves and compartmentation in protoplasts and vacuoles

Leaf blades of Triticum aestivum L. cv Kolibri and Hordeum vulgare L. cv Gerbel were induced to accumulate fructan (polyfructosylsucrose of varying molecular size)in large amounts (~70 percent of dwt)by impeding the export of photosynthates. This was achieved by sub­ jecting plants to cold stress (5°C at night)or by continuous illumination of excised leaf blades. Concomitantly with the accumulation of fructan in the leaves the activity of a sucrose­ sucrose-fructosyltransferase (SST), probably the key-enzyme of fructan anabolism, increased several fold in cell-free extracts. Its pH optimum is 5.7. At 8°C its activity is still half of the activity at 28°C, the temperature optimum. This remarkably anomalous dependence on tem­perature is interesting with regard to fructan accumulation in grasses during the cold season. Protoplasts obtained from fructan-enriched barley leaves were employed for the isolation of vacuoles. All the fructans (~trisaccharide)as well as the SST activity were found to be asso­ ciated exclusively with the vacuoles, which therefore appear to play the central role in fructan storage and metabolism.