Malate content of picoliter samples of Raphanus sativus cytoplasm

Malate, which plays many essential roles in plant metabolism, is a potent in vitro inhibitor of the cytosolic enzyme phosphoenolpyruvate carboxylase (PEPC). Because PEPC activity leads to malate biosynthesis, malate is assumed to attenuate its own synthesis in situ. To test this hypothesis, we measured directly the malate content of picoliter samples of Raphanus root-hair cytoplasm using quantitative histochemical techniques. We also obtained an estimate for malate accumulation in these cells. These values were compared with the PEPC activity of individual root hairs (less than 2 ng). The results indicate that high cytoplasmic malate concentration does not severely inhibit PEPC in situ. We suggest that the focus for studies on the regulation of organic anion accumulation be on the interactive effects of malate and other PEPC effectors.