Uptake of Malate and Citrate into Plant Vacuoles
Of the different organic anions which are often present a high concentrations in plants, malate plays a central role. Plants exhibiting crassulacean acid metabolism (CAM) fix CO2 with the enzyme phosphoenolpyruvate carboxylase during the night and accumulate large amounts of malic acid. During the light period, malic acid is decarboxylated and the released CO2 is fixed in the Calvin cycle. C4 plants fix CO2 in the mesophyll in a similar reaction during the day, as CAM in the dark. In these plants, malate is transferred to the bundle sheaths, decarboxylated and the CO2 fixed in the photosynthetic reaction. This reaction enables the plant to fix CO2 more efficiently, since the affinity of phosphoenolpyruvate carboxylase to HCO3 - is much higher than that of ribulose-1,5- diphosphate carboxylase to CO2. Diurnal fluctuations of malate can also be observed in C3 plants. However, in these plants malate is accumulated during the day and used as an energy source for respiration in the dark (Winter, Usuda, Tsuzuki, Schmitt, Edwards, Thomas, and Evert, 1982; Gerhardt, Stitt, and Heldt, 1987). Malate metabolism and accumulation also play an important role during the opening of stomata since, in most plants, malate is used for balancing K+ (Schnabl and Kottmeier, 1984). Other prominent organic acids often accumulated at high concentrations in plants include shikimic acid, which is present mainly in gymnosperms and some woody angiosperms, as well as gallic, oxalic and citric acid.
KeywordsCitrate Respiration Lysine Polypeptide Histidine
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