Illuminating the wall: using click chemistry to image pectins in Arabidopsis cell walls

Plant cell walls are the most abundant biomaterials on Earth and serve a multitude of purposes in human society. These complex extracellular matrices are mainly composed of polysaccharides, including cellulose, hemicelluloses, and pectins, which cannot be cytologically examined using conventional techniques. Click chemistry, which exploits a bio-orthogonal cycloaddition reaction between alkynyl and azido groups, has proven to be useful for the metabolic incorporation and detection of modified sugars in polysaccharides in animals, fungi, and bacteria, but its use to interrogate the biosynthesis or dynamics of plant cell walls has not been previously reported. Recently, we found that an alkynylated analog of fucose can be metabolically incorporated into Arabidopsis thaliana cell walls and click labeled with fluorescent probes, facilitating imaging of cell wall carbohydrates. Despite the presence of fucose in several classes of wall polysaccharides, fucose-alkyne was primarily incorporated into rhamnogalacturonan-I, a type of pectin. Using timecourse and pulse-labeling experiments, we observed the dynamics of pectin delivery and reorganization in expanding cell walls. The use of click chemistry to investigate plant cell wall architecture should help bridge the gap between biochemical characterization of isolated cell wall components and an understanding of how those components interact in intact cell walls.