Pathway of malic acid synthesis in response to ion uptake in wheat and lupin roots: evidence from fixation of 13C [carbon isotope] and 14C [Triticum aestivum, Lupinus luteus]

• Published in: Plant physiology (Bethesda) Vol. 69; no. 6; pp. 1289 - 1292
• Main Authors: Popp, M, Osmond, C.B, Summons, R.E
• Format: Journal Article
• Language: English
• Published: American Society of Plant Physiologists 1982
• Subjects: Atoms; Barley; carbon; Carbon dioxide; isotopes; Lupinus luteus; Molecules; Organic acids; Plant roots; Plants; Radiocarbon; Random allocation; Salts; Triticum aestivum
• ISSN: 0032-0889; 1532-2548

Malate synthesis by CO2 fixation in wheat (Triticum aestivum L.) and lupin (Lupinus luteus) roots was investigated by labeling with $\text{NaH}^{13}\text{CO}_{3}$ as well as with $\text{NaH}^{14}\text{CO}_{3}$. The distribution of 14C label in the malate was examined, using enzymic degradation methods (malic enzyme, pyruvate decarboxylase) and, in the case of 13C, gas chromatography-mass spectrometry. In long-term experiments (2 to 12 hours), both methods showed that the [1-C] and [4-C] positions of malic acid are approximately equally labeled, in agreement with former findings. Short-term experiments (15, 30 seconds) showed that 14C is confined initially to the [4-C] position of malate but then is distributed quickly to the [1-C] atom. Neither labeling pattern nor rate of randomization was influenced by salt treatment. Analysis of malate from roots by gas chromatography-mass spectrometry, a procedure which was tested against in vitro-prepared [1-13C]-, [4-13C]-, and [1,4-13C] malate, gave strong evidence for the existence of only singly labeled malate molecules. These data suggest that only one carboxylation step, catalyzed by phosphoenolpyruvate carboxylase and/or phosphoenolpyruvate carboxykinase, is responsible for malic acid synthesis in roots and that malate label is randomized by a fumarase-like reaction, presumably in mitochondria.