BUILDING A DIVERSE PLANT CELL WALL: HOW PLANT CELLS USE AN ARRAY OF EXPORT STRATEGIES TO PRODUCE THEIR EXTRACELLULAR MATRIX.
All plant growth, including agricultural and forestry production, requires cell wall synthesis. Despite its critical importance, there is a major gap in our understanding of cell wall biosynthesis: how are cell wall components, which are made inside the cell, exported to the outside of the cell to build a functional wall? For the polysaccharide components of the cell wall, the hundreds of Golgi stacks in the cell package and secrete the wall matrix. However, the plant cell wall also contains specialized functional domains where the polysaccharides are impregnated with other macromolecules, such as lignin in wood, or the waxy cuticle coating the plant surface. The export of lignin precursors to the secondary cell wall during wood formation can be tracked with TEM autoradiography, demonstrating a non-vesicular export mechanism. For export of lipids to the cuticle, Arabidopsis mutant analysis has identified the first components of the wax export system, two ATP-binding cassette (ABC) transporters of the ABCG subclass, as well as a GPI-anchored lipid transfer protein, that are required for wax export from the epidermis to the cell wall.