Plant cell wall imaging by metabolic click-mediated labelling of rhamnogalacturonan II using azido 3-deoxy- d -manno-oct-2-ulosonic acid

In plants, 3-deoxy-d-manno-oct-2-ulosonic acid (Kdo) is a monosaccharide that is only found in the cell wall pectin, rhamnogalacturonan-II (RG-II). Incubation of 4-day-old light-grown Arabidopsis seedlings or tobacco BY-2 cells with 8-azido 8-deoxy Kdo (Kdo-N₃) followed by coupling to an alkyne-containing fluorescent probe resulted in the specific in muro labelling of RG-II through a copper-catalysed azide-alkyne cycloaddition reaction. CMP-Kdo synthetase inhibition and competition assays showing that Kdo and D-Ara, a precursor of Kdo, but not L-Ara, inhibit incorporation of Kdo-N₃ demonstrated that incorporation of Kdo-N₃ occurs in RG-II through the endogenous biosynthetic machinery of the cell. Co-localisation of Kdo-N₃ labelling with the cellulose-binding dye calcofluor white demonstrated that RG-II exists throughout the primary cell wall. Additionally, after incubating plants with Kdo-N₃ and an alkynated derivative of L-fucose that incorporates into rhamnogalacturonan I, co-localised fluorescence was observed in the cell wall in the elongation zone of the root. Finally, pulse labelling experiments demonstrated that metabolic click-mediated labelling with Kdo-N₃ provides an efficient method to study the synthesis and redistribution of RG-II during root growth.