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-catalyzed 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 Kdo-N3 incorporation 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 II, co-localized 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.