Betaine synthesis in chenopods: Localization in chloroplasts

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Abstract
Plants from several families (Chenopodiaceae, Gramineae, Compositae) accumulate betaine (glycine betaine) in response to salt or water stress via the pathway: choline → betainal (betaine aldehyde) → betaine. Betaine accumulation is probably a metabolic adaptation to stress. Intact protoplasts from leaves of spinach ( Spinacia oleracea ) oxidized [ 14 C]choline to betainal and betaine, as did protoplast lysates. Upon differential centrifugation, the [ 14 C]choline-oxidizing activity of lysates sedimented with chloroplasts. Chloroplasts purified from protoplast lysates by a Percoll cushion procedure retained strong [ 14 C]choline-oxidizing activity (1-3 nmol/mg of chlorophyll per hr), although the proportion of the intermediate, [ 14 C]betainal, in the reaction products was usually higher than for protoplasts. Isolated chloroplasts also readily oxidized [ 14 C]betainal to betaine (20-100 nmol/mg of chlorophyll per hr). Light increased the oxidation of both [ 14 C]choline and [ 14 C]betainal by isolated chloroplasts ≈3-fold; this light-stimulation was abolished by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). Similar results were obtained with another chenopod ( Beta vulgaris ) but not with pea ( Pisum sativum ), a species that accumulates no betaine. The chloroplast site for betaine synthesis in chenopods contrasts with the mitochondrial site in mammals.