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PLANT PHYSIOLOGY , Vol 109, Issue 3 1085-1091, Copyright © 1995 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Enzymes of Choline Synthesis in Spinach (Response of Phospho-Base N-Methyltransferase Activities to Light and Salinity)
E. A. Weretilnyk, D. D. Smith, G. A. Wilch and P. S. Summers
Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1
In spinach (Spinacia oleracea L.), choline is synthesized by the sequential
N-methylation of phosphoethanolamine -> phosphomono- -> phosphodi-
-> phosphotrimethylethanolamine (i.e. phosphocholine) followed by
hydrolysis to release choline. Differential centrifugation of spinach leaf
extracts shows that enzymes catalyzing the three N-methylations are
cytosolic. These enzymes were assayed in leaf extracts prepared from plants
growing under various light/dark periods. Under a diurnal, 8-h light/16-h
dark photoperiod, the activity of the enzyme catalyzing the N-methylation
of phosphoethanolamine is highest at the end of the light period and lowest
following the dark period. Prolonged dark periods (exceeding 16 h) lead to
a further reduction in the activity of this enzyme, although activity is
restored when plants are reexposed to light. In contrast, the activity of
the enzyme(s) catalyzing the N-methylations of phosphomono- and
phosphodimethylethanolamine does not undergo comparable changes in response
to light/dark treatments. Salt shock of plants with 200 mM NaCl results in
a 2-fold increase in all three N-methylation activities relative to
nonsalinized controls but only in plants exposed to light. Thus, light is
required for the salt-responsive up-regulation of choline synthesis in
spinach.
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