Preferential Use of Acetate over Glucose Involves Acetate-Mediated Inhibition of Glucose Uptake during Diauxic Growth of Carrot Cells

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It has been reported that suspension-cultured rice cells grown on mixed carbon sources of glucose (Glc) and acetate exhibited diauxic growth in which acetate was the preferred carbon source (Lee and Lee 1996). Carrot (Daucus carota L.) suspension cells, showing a diauxic growth very similar to that of rice cells, were used to delineate the mechanisms underlying this preferential use of acetate over Glc. Uptakes of both Glc and 3-O-methylglucose (3-OMG), a non-metabolizable Glc analogue, were similarly inhibited when acetate or butylate, weak acids which are capable of transporting protons into the cytosol, were present in the uptake assay mixture containing cells harvested during the Glc-utilizing second growth phase. Inhibition of Glc uptake by these weak acids was similar when equivalent experiments were carried out with isolated plasma membranes. It was further shown that Glc uptake, which requires a proper proton gradient across the plasma membranes, was inhibited during the first growth phase by acetate-mediated alkalization of growth medium and/ or simultaneous acidification of cytosol. This study strongly suggests that Glc utilization in plant cells is inhibited by co-presenting carbon source(s) which can alter the proton gradient across the plasma membrane.
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