Abstract
Taiwan oil millet (TOM–Eccoilopus formosanus) is an orphan crop cultivated by the Taiwan indigenous people. It has conspicuous white powder covering its leaf sheath indicating the abundance of epicuticular waxes, thus may contribute to its resilience. Here, we characterized the epicuticular wax secretion in TOM leaf blade and leaf sheath using various microscopy techniques, as well as gas chromatography to determine its composition. Two kinds of waxes, platelet and filaments, were secreted in both the leaf blades and sheaths. The platelet wax is secreted ubiquitously by epidermal cells, whereas the filament wax is secreted by a specific epidermal cell called cork cells. The newly developed filament waxes were markedly re-synthesized by the cork cells through the papillae protrusions on the external periclinal cell wall. The predominant wax component was a C28 primary alcohol in leaf blade, and a C28 free fatty acid in the leaf sheath, pseudopetiole and midrib. The wax morphology present in distinct organ and abundance corresponds to the specific chemical composition: platelet wax composed of alcohols exists mainly in the leaf blade, whereas filament wax constituted mainly by the singular compound of C28 free fatty acids is present abundantly in leaf sheath. Our study clarifies the filament wax composition in comparison to previous study in sorghum. Both platelet and filament waxes comprise a protection barrier for TOM.