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PLANT PHYSIOLOGY , Vol 105, Issue 2 659-670, Copyright © 1994 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Uridine Diphosphate Glucose Metabolism and Callose Synthesis in Cultured Pollen Tubes of Nicotiana alata Link et Otto
H. Schlupmann, A. Bacic and S. M. Read
Plant Cell Biology Research Centre, School of Botany, University of Melbourne, Parkville, Victoria 3052, Australia
Membrane preparations from cultured pollen tubes of Nicotiana alata Link et
Otto contain a Ca2+ -independent (1-3)-[beta]-D-glucan (callose) synthase
activity that has a low affinity for UDP-glucose, even when activated by
treatment with trypsin (H. Schlupmann, A. Basic, S.M. Read [1993] Planta
191: 470-481). Therefore, we investigated whether UDP-glucose was a likely
substrate for callose synthesis in actively growing pollen tubes.
Deposition of (1-3)-[beta]-glucan occurred at a constant rate, 1.4 to 1.7
nmol glucose min-1, in tubes from 1 mg of pollen from 3 h after
germination; however, the rate of incorporation of radioactivity from
exogenous [14C]-sucrose into wall polymers was not constant, but increased
until at least 8 h after germination, probably due to decreasing use of
internal reserves. UDP-glucose was a prominent ultraviolet-absorbing
metabolite in pollen-tube extracts, with 1.6 nmol present in tubes from 1
mg of pollen, giving a calculated cytoplasmic concentration of
approximately 3.5 mM. Radioactivity from [14C]-sucrose was rapidly
incorporated into sugar monophosphates and UDP-glucose by the growing
tubes, consistent with a turnover time for UDP-glucose of less than 1 min;
the specific radioactivity of extracted UDP-[14C]glucose was equal to that
calculated from the rate of incorporation of [14C]sucrose into wall
glucans. Large amounts of less metabolically active neutral sugars were
also present. The rate of synthesis of (1-3)-[beta]-glucan by
nontrypsin-treated pollen-tube membrane preparations incubated with 3.5 mM
UDP-glucose and a [beta]-glucoside activator was slightly greater than the
rate of deposition of (1-3)-[beta]-glucan by intact pollen tubes. These
data are used to assess the physiological significance of proteolytic
activation of pollen-tube callose synthase.
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