The folate precursor p-aminobenzoate is reversibly converted to its glucose ester in the plant cytosol.

Plants synthesize p-aminobenzoate (pABA) in chloroplasts and use it for folate synthesis in mitochondria. It has generally been supposed that pABA exists as the free acid in plant cells and that it moves between organelles in this form. Here we show that fruits and leaves of tomato and leaves of a diverse range of other plants have a high capacity to convert exogenously supplied pABA to its beta-D-glucopyranosyl ester (pABA-Glc), whereas yeast and Escherichia coli do not. High performance liquid chromatography analysis indicated that much of the endogenous pABA in fruit and leaf tissues is esterified and that the total pool of pABA (free plus esterified) varies greatly between tissues (from 0.2 to 11 nmol g-1 of fresh weight). UDP-glucose:pABA glucosyltransferase activity was readily detected in fruit and leaf extracts, and the reaction was found to be freely reversible. p-Aminobenzoic acid beta-D-glucopyranosyl ester esterase activity was also detected in extracts. Subcellular fractionation indicated that the glucosyltransferase and esterase activities are predominantly if not solely cytosolic. Taken together, these results show that reversible formation of pABA-Glc in the cytosol is interposed between pABA production in chloroplasts and pABA consumption in mitochondria. As pABA is a hydrophobic weak acid, its uncharged form is membrane-permeant, and its anion is consequently prone to distribute itself spontaneously among subcellular compartments according to their pH. Esterification of pABA may eliminate such errant behavior and provide a readily reclaimable storage form of pABA as well as a substrate for membrane transporters.