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PLANT PHYSIOLOGY , Vol 109, Issue 3 991-998, Copyright © 1995 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Metabolism of the Raffinose Family Oligosaccharides in Leaves of Ajuga reptans L. (Inter- and Intracellular Compartmentation)
M. Bachmann and F. Keller
Institute of Plant Biology, University of Zurich, Zollikerstrasse 107, CH-8008 Zurich, Switzerland
We recently suggested that leaves of the frost-hardy species Ajuga reptans
L. (Lamiaceace) contain two pools of raffinose family oligosaccharides
(RFO): a large long-term storage pool in the mesophyll, possibly also
involved in frost resistance, and a transport pool in the phloem (M.
Bachmann, P. Matile, F. Keller [1994] Plant Physiol 105: 1335-1345). In the
present study, the inter- and intracellular compartmentation of anabolic
RFO metabolism was investigated by comparing whole-leaf tissue with
mesophyll protoplasts and vacuoles. The studies showed the mesophyll to be
the primary site of RFO synthesis in A. reptans. Mesophyll protoplasts were
capable of RFO formation upon in vitro 14CO2 photosynthesis.
Sucrose-phosphate synthase, galactinol synthase, and the
galactinol-independent galactosyltransferase, which is responsible for RFO
chain elongation, were located predominantly in the mesophyll protoplasts.
The percentage of stachyose synthase in the mesophyll changed greatly
during the cold-acclimation period (from 26% at the beginning to 88% after
20 d). The remainder was most probably in the intermediary cells of the
phloem. Compartmentation studies in which mesophyll protoplasts were
compared with vacuoles isolated from them showed that, of the components of
the RFO storage pool, galactinol synthase, stachyose synthase,
myo-inositol, galactinol, and sucrose were extravacuolar (most probably
cytosolic), whereas galactinol-independent galactosyltransferase and higher
RFO oligomers (with degree of polymerization 4) were vacuolar. Raffinose
was found in both locations and might serve as a cryoprotectant.
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