Glycosylation is the catalysed decoration of different molecules with sugars, providing them with new properties. Thanks to the large number of enzymes responsible for the transfer of activated sugars (glycosyltransferases), these reactions are very specific. Although these enzymes are well known, a rather neglected member of this class, apiosyltransferase, is active on the rare sugar apiose, a branched chain pentose involved in the formation of glycoconjugates and present in the plant cell wall. Apart for its presence in these complexes, little is known about apiose, and even less about apiosyltransferase. With this proposal, I will investigate the structural and biochemical features of apiosyltransferase, with an emphasis on substrate binding and on the adopted reaction mechanism. Understanding how apiosyltransferase accommodates and interacts with the pentose structure to functionalise its targets will provide a different perspective with regard to known glycosyltransferases, pinpointing divergence elements in sugar specificity. Also, testing the mechanism of apiosyltransferase I will study the largely unknown properties of its products, which range from polysaccharides involved in plants resistance to molecules active as signals and antioxidants.