219 Glycosyltransferases are involved in the biosyntheses of cell-wall polysaccharides, the addition of N-linked glycans to glycoproteins, and the attachment of sugar moieties to various small molecules such as hormones and flavonoids. In the past two years, substantial progress has been made in the identification and cloning of genes that encode glycosyltransferases. Moreover, analysis of the recently completed Arabidopsis genome sequence indicates the existence of several hundred additional genes encoding putative glycosyltransferases. Addresses MSU-DOE Plant Research Laboratory and Department of Biochemistry, Michigan State University, East Lansing, Michigan 48824, USA *e-mail: keegstra@msu.edu † e-mail: nraikhel@msu.edu Current Opinion in Plant Biology 2001, 4:219–224 1369/5266/01/$ —see front matter © 2001 Elsevier Science Ltd. All rights reserved. Abbreviations FT fucosyltransferase GlcNAc N-acetylglucosamine Man mannose Introduction Glycosyltransferases are enzymes that attach a sugar mole- cule to a specific acceptor, thereby creating a glycosidic bond. These enzymes are found in most living organisms but are particularly important in plants, which convert the products of photosynthesis into disaccharides, oligosaccha- rides, and polysaccharides. In addition, glycosyltransferases produce other important molecules including cell-wall polysaccharides, glycoproteins, and many different types of small molecules that have sugars attached to them. Glycosyltransferases have been classified into different families on the basis of the activated molecule that donates the sugar (usually a nucleotide-diphospho-sugar), the type of sugar that they transfer, and whether the enzyme forms an α- or β-glycosidic linkage. Many glycosyltransferases have been identified and studied in plant systems, but knowledge from bacterial, fungal, or animal systems is more advanced and therefore enhances studies in plant systems. It has been estimated that more than 100 distinct glyco- sidic linkages are present in the glycoconjugate repertoire of a typical multicellular organism. Because most glycosyl- transferases are very specific, it is likely that each different linkage requires the action of a distinct glycosyltransferase, leading to the prediction that multicellular organisms con- tain hundreds of different glycosyltransferases [1]. The availability of genomic-sequence information has allowed tentative confirmation of this prediction; for example, hundreds of putative glycosyltransferase genes have been identified in the Arabidopsis genome [2 •• ]. Using the sequence-based classification scheme described by Henrissat and colleagues [3 •• ], a summary updated on December 21, 2000, listed 49 families of glycosyl- transferases (URL http://afmb.cnrs-mrs.fr/~pedro/CAZY/ gtf.html). Twenty-five of these families contained repre- sentatives from Arabidopsis, which had a total of 351 putative glycosyltransferase genes. This accounting is only approximate and the number of Arabidopsis genes encod- ing glycosyltransferases will change as annotation of the genome is refined and as the ability to identify glycosyl- transferases on the basis of sequence information is improved. Of these hundreds of putative glycosyltransferase genes, the biochemical activity of the gene product has been con- firmed for only a handful. The extreme specificity of many glycosyltransferases increases the difficulty of assessing the biochemical function of the enzymes encoded by these genes. For example, the enzyme that adds fucose to xyloglucan (discussed below) has been utilized to identify a group of nine additional gene products with which it shares extensive sequence similarity, that is, family 37 in Henrissat’s scheme [3 •• ]. Preliminary analysis indicates, however, that none of these putative fucosyltransferases use xyloglucan as an acceptor (R Sarria-Milan, A Faik, K Keegstra, N Raikhel, unpublished data). If correct, this raises many important questions about their acceptor specificity as well as their biological functions. Considerable effort will be required to investigate the biochemical and biological function of each of the putative glycosyltransferase genes identified in the Arabidopsis genome. This review highlights some of the advances made over the past two years in our rapidly increasing understanding of plant glycosyltransferases. The glycosyltransferases involved in glycolipid [4,5], starch [6] and sucrose biosyn- thesis [7] are not considered in this review because of space limitations. Rather, we begin with a consideration of the Golgi enzymes responsible for the synthesis of plant cell-wall polysaccharides, then consider the enzymes in the endoplasmic reticulum and Golgi that are responsible for the modification of plant glycoproteins, before consid- ering the soluble enzymes that add sugars to a wide variety of small molecules. Glycosyltransferases involved in the biosynthesis of cell-wall polysaccharides A major feature of plant cells is the presence of a complex wall surrounding virtually every cell. These walls play a crucial role in a multiplicity of processes encompassing growth and development, signal transduction, and cellular responses to environmental factors including pathogens Plant glycosyltransferases Kenneth Keegstra* and Natasha Raikhel †