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Metabolism and Enzymology

Observation of Cytoplasmic and Vacuolar Malate in Maize Root Tips by ¹³C-NMR Spectroscopy ¹

Department of Biochemistry, University of California, Riverside, California 92521

The accumulation of malate by maize (Zea mays L.) root tips perfused with KH¹³CO₃ was followed by ¹³C nuclear magnetic resonance spectroscopy. In vivo nuclear magnetic resonance spectra contained distinct signals from two pools of malate in maize root tips, one at a pH 5.3 (assigned to the vacuole) and one at a pH > 6.5 (assigned to the cytoplasm). The ratio of cytoplasmic to vacuolar malate was lower in 12 millimeter long root tips than in 2 millimeter root tips. The relatively broad width of the signals from C1- and C4-labeled vacuolar malate indicated heterogeneity in vacuolar pH. During the 3 hour KH¹³CO₃ treatment, ¹³C-malate accumulated first primarily in the cytoplasm, increasing to a fairly constant level of 6 millimolar by 1 hour. After a lag, vacuolar malate increased throughout the experiment.

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