Plant Golgi cell wall synthesis: from genes to enzyme activities.

T he cell wall provides mechanical support to the plant cells, and the surface wall, which is directly exposed to the extracellular environment, acts as a primary barrier against pathogen attack, mechanical injury, and other environmental stresses. Biophysical properties of the wall in conjunction with the cell turgor pressure determine the rate of cell expansion during primary growth (1). Polysaccharides, the most dominant fraction of the wall, consist of cellulose microfibrils that are embedded in a matrix of hemicellulose and pectin. Matrix polysaccharides in plants are made in the Golgi cisternae and then exported to the wall by exocytosis (2, 3). Cellulose is made at the plasma membrane by a cellulose synthase complex and directly deposited into the cell wall. Whereas the wall chemical composition has been well characterized and many of the enzyme activities for polysaccharide synthases have been biochemically assayed (4, 5), true to its name, the cell wall proved impregnable to the molecular understanding of its synthesis until relatively recently (6). In this issue of PNAS, Liepman et al. (7) present their work on overcoming this barrier by functionally expressing several of the plant genes encoding a wall matrix polysaccharide synthase in a non-plant eukaryotic host, cultured Drosophila cells. Several of the enzyme activities for the wall polysaccharide synthases, including the one the work of Liepman et al. (7) is based on, were identified nearly half a century ago by the pioneering work of Hassid’s group (4, 8–10). Preston used the term ‘‘stagnation’’ to describe the state of research on cell wall structure during the whole of the eighteenth century (11). Whereas the intervening period between the classic work of Hassid’s group and others (2, 3, 12) and the isolation of the first plant gene for cellulose synthase (CesA) was not quite like that with regard to the area of cell wall synthesis, the attempts at biochemical purification of polysaccharide synthases with the goal of obtaining protein sequence followed by gene cloning were met with limited success (13–15). Even in bacteria, it was not until 1990 that a breakthrough was achieved when the gene for CesA was isolated (16). A full 6 years elapsed until the isolation of the plant CesA gene, which was identified through EST analysis of developing cotton fibers with the help of conserved motifs that were common to the bacterial CesA and other polymerizing -glycosyltransferases (6, 17). Annotation of a large number of sequences in the public and private EST databases as being either CesA or CesA-like (Csl) followed once the cotton CesA sequence became available (18). Complete genome sequencing revealed the presence of 29 Csl genes in Arabidopsis and 37 in rice (18–20).