Radiotracer Evidence Implicating Phosphoryl and Phosphatidyl Bases as Intermediates in Betaine Synthesis by Water-Stressed Barley Leaves

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Abstract
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.