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Plant Physiology 68:814-822 (1981) © 1981 American Society of Plant Biologists Radiotracer Evidence Implicating Phosphoryl and Phosphatidyl Bases as Intermediates in Betaine Synthesis by Water-Stressed Barley Leaves 1,2MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, Michigan 48824
In barley, glycine betaine is a metabolic end product accumulated by wilted leaves; betaine accumulation involves acceleration of de novo synthesis from serine, via ethanolamine, N-methylethanolamines, choline, and betaine aldehyde (Hanson, Scott 1980 Plant Physiol 66: 342-348). Because in animals and microorganisms the N-methylation of ethanolamine involves phosphatide intermediates, and because in barley, wilting markedly increases the rate of methylation of ethanolamine to choline, the labeling of phosphatides was followed after supplying [14C]ethanolamine to attached leaf blades of turgid and wilted barley plants. The kinetics of labeling of phosphatidylcholine and betaine showed that phosphatidylcholine became labeled 2.5-fold faster in wilted than in turgid leaves, and that after short incubations, phosphatidylcholine was always more heavily labeled than betaine. In pulse-chase experiments with wilted leaves, label from [14C]ethanolamine continued to accumulate in betaine as it was being lost from phosphatidylcholine. When [14C]monomethylethanolamine was supplied to wilted leaves, phosphatidylcholine was initially more heavily labeled than betaine. These results are qualitatively consistent with a precursor-to-product relationship between phosphatidylcholine and betaine. The following experiments, in which tracer amounts of [14C]ethanolamine or [14C]formate were supplied to wilted barley leaves, implicated phosphoryl and phosphatidyl bases as intermediates in the methylation steps between ethanolamine and phosphatidylcholine. Label from both [14C]ethanolamine and [14C]formate entered phosphorylmonomethylethanolamine and phosphorylcholine very rapidly; these phosphoryl bases were the most heavily labeled products at 15 to 30 minutes after label addition and lost label rapidly as the fed 14C-labeled precursor was depleted. Phosphatidylmonomethylethanolamine and phosphatidylcholine were also significantly labeled from [14C]ethanolamine and [14C]formate at early times; the corresponding free bases and nucleotide bases were not. Addition of a trapping pool of phosphorylcholine reduced [14C]ethanolamine conversion to both phosphatidylcholine and betaine, and resulted in accumulation of label in the trap.
A computer model of the synthesis of betaine via phosphatidylcholine was developed from 14C kinetic data. The model indicates that about 20% of the total leaf phosphatidylcholine behaves as an intermediate in betaine biosynthesis and that a marked decrease (
1 Research conducted under Contract DE-AC02-76ER01338 from the United States Department of Energy. 2 Michigan Agricultural Experiment Station Journal Article 9785. This article has been cited by other articles:
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2-fold) in the half-life of this metabolically active phosphatidylcholine fraction accompanies wilting. Dual labeling experiments with [14C]choline and [3H]glycerol confirmed that the half-life of the choline portion of phosphatidylcholine falls by a factor of about 2 in wilted leaves. 
