Wood formation is a biological process of great economical importance. Genes active during the secondary cellwall formation of wood fibers from Populus tremulatremuloides were previously identified by expression profilingthrough microarray analyses. A number of these genes encode glycosyltransferases (GTs) with unknown substratespecificities. Here we report heterologous expression of one of these enzymes, PttGT43A, a putative IRREGULARXYLEM9 (IRX9) homologue. Expression trials in Pichia pastoris and insect cells revealed very low levels of accumulationof immunoreactive PttGT43A, whereas transient expression in Nicotiana benthamiana leaves by Agrobacterium infiltration(agroinfiltration) using a viral vector produced substantial amounts of protein that mostly precipitated in the crude pellet.Agroinfiltration induced weak endogenous xylosyltransferase activity in microsomal extracts, and transient PttGT43Aexpression further increased this activity, albeit only to low levels. PttGT43A may be inactive as an individual subunit,requiring complex formation with unknown partners to display enzymatic activity. Our results suggest that transient coexpressionin leaves of candidate subunit GTs may provide a viable approach for formation of an active xylanxylosyltransferase enzymatic complex.