Anatomy, histochemistry and biochemistry of glucovanillin, oleoresin and mucilage accumulation sites in green mature vanilla pod (Vanilla planifolia; Orchidaceae): a comprehensive and critical reexamination

Introduction. Mature green vanilla pods accumulate 4-O-(3-methoxy-benzaldehyde)-beta-D-glucoside (glucovanillin), which, upon hydrolysis by an endogenous beta-glucosidase, liberates vanillin, the major aroma component of vanilla. Sites of storage of glucovanillin in the pod have been controversially reported for decades; we aim, using precise and widely accepted technical terminology, to clarify this controversy by providing an anatomical, histochemical and biochemical evidence-based picture of glucovanillin accumulation sites. The pod also synthesizes an oleoresin and a mucilage of unknown constitutions; we report here their localization and structures. Materials and methods. The pod anatomy was examined by light and epifluorescence microscopy. A protocol was established allowing fine hand-dissection of diverse anatomical parts of the pod (mesocarp, placentae, trichomes, intralocular interstitial cell-free region and seeds). Glucovanillin and gamma-pyranones were extracted and analyzed by HPLC, while the structures of the mucilaginous polysaccharides were determined after permethylation. Results and discussion. Glucovanillin is essentially stored in the placentae (92%) and marginally in trichomes (7%); traces were measured in the mesocarp and intralocular interstitial cell-free medium. Trichomes store massive amounts of a fluorescing oleoresin (44%) rich in alkenylmethyldihydro-gamma-pyranones and synthesize a mucilage made of a glucomannan and a pectic polysaccharide carrying monomeric arabinose and galactose side-chains. Conclusion. To date, the physiological roles of glucovanillin, long-chain pyranones, and mucilage remain unknown.