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PLANT PHYSIOLOGY , Vol 101, Issue 2 459-468, Copyright © 1993 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Fructan Synthesis in Excised Barley Leaves (Identification of Two Sucrose-Sucrose Fructosyltransferases Induced by Light and Their Separation from Constitutive Invertases)
U. Simmen, D. Obenland, T. Boller and A. Wiemken
Department of Botany, University of Basel, Hebelstrasse 1, CH-4056 Basel, Switzerland
Excised leaves of barley (Hordeum vulgare L.) exposed to continuous light
accumulate large amounts of soluble carbohydrates. Carbohydrates were
analyzed in deionized extracts by high-pressure liquid chromatography on an
anion exchange column coupled with pulsed amperometric detection. During
the first few hours of illumination, the main sugar to accumulate was
sucrose. The levels of glucose and fructans (oligofructosylsucroses)
increased later. The trisaccharide 1-kestose (1-kestotriose) predominated
initially among the fructans. Later, 6-kestose (6-kestotriose) and tetra-
and pentasaccharides accumulated also. Total extracts from barley leaves
were chromatographed on a MonoQ column, and each fraction was assayed for
enzymes of interest by incubation with 200 mM sucrose for 3 h, followed by
carbohydrate analysis. Freshly excised leaves yielded two peaks of
invertase, characterized by formation of fructose and glucose, but had
almost no trisaccharide-forming activities. In leaves exposed to continuous
light, two new enzyme activities appeared that generated fructan-related
trisaccharides and glucose from sucrose. One of them was a sucrose-sucrose
fructosyl-1-transferase (1-SST), producing 1-kestose exclusively: the peak
fractions of this activity contained almost no invertase. The other was a
sucrose-sucrose fructosyl-6-transferase (6-SST), producing 6-kestose. It
comigrated with one of the constitutive invertases on MonoQ but was
separated from it by subsequent chromatography on alkyl Superose.
Nevertheless, the preparation retained invertase activity, suggesting that
this enzyme may act both as fructosidase and fructosyltransferase. When
incubated with 1-kestose in addition to sucrose, this enzyme formed less
6-kestose but instead produced large amounts of the tetrasaccharide
bifurcose (1&6-kestotetraose), the main fructan tetrasaccharide
accumulating in vivo. These results suggest that two inducible enzymes,
1-SST and 6-SST, act in concert to initiate fructan accumulation in barley
leaves.
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