Biosynthesis of L-ascorbic acid and conversion of carbons 1 and 2 of L-ascorbic acid to oxalic acid occurs within individual calcium oxalate crystal idioblasts

L-Ascorbic acid (AsA) and its metabolic precursors give rise to oxalic acid (OxA) found in calcium oxalate crystals in specialized crystal idioblast c ells in plants; however, it is not known if AsA and OxA are synthesized wit hin the crystal idioblast cell or transported in from surrounding mesophyll cells. Isolated developing crystal idioblasts from Pistia stratiotes were used to study the pathway of OxA biosynthesis and to determine if idioblast s contain the entire path and are essentially independent in OxA synthesis. Idioblasts were supplied with various C-14-labeled compounds and examined by microautoradiography for incorporation of C-14 into calcium oxalate crys tals. [C-14]OxA gave heavy labeling of crystals, indicating the isolated id ioblasts are functional in crystal formation. Incubation with [1-C-14]AsA a lso Rave heavy labeling of crystals, whereas [6-C-14]AsA gave no labeling. Labeled precursors of AsA (L-[1-C-14]galactose; D-[1-C-14]mannose) also res ulted in crystal labeling, as did the ascorbic acid analog, D-[1-C-14]eryth orbic acid. Intensity of labeling of isolated idioblasts followed the patte rn OxA > AsA (erythorbic acid) > L-galactose > D-mannose. Our results demon strate that P, stratiotes crystal idioblasts synthesize the OxA used for cr ystal formation, the OxA is derived from the number 1 and 2 carbons of AsA, and the proposed pathway of ascorbic acid synthesis via D-mannose and L-ga lactose is operational in individual P, stratiotes crystal idioblasts. Thes e results are discussed with respect to fine control of calcium oxalate pre cipitation and the concept of crystal idioblasts as independent physiologic al compartments.