The phenylpropanoid pathway in plants leads to the synthesis of a wide range of soluble secondary metabolites, many of which accumulate as glycosides. In Arabidopsis, a small cluster of three closely related genes, UGT72E1-E3, encode glycosyltransferases shown to glucosylate several phenylpropanoids in vitro, including monolignols, hydroxycinnamic acids and hydroxycinnamic aldehydes. The role of these genes in planta has now been investigated through genetically downregulating the expression of individual genes or silencing the entire cluster. Analysis of these transgenic Arabidopsis plants showed that the levels of coniferyl and sinapyl alcohol 4-O-glucosides that accumulate in light-grown roots were significantly reduced. A 50% reduction in both glucosides was observed in plants in which UGT72E2 was downregulated, whereas silencing the three genes led to a 90% reduction, suggesting some redundancy of function within the cluster. The gene encoding UGT72E2 was constitutively overexpressed in transgenic Arabidopsis to determine whether increased glucosylation of monolignols could influence flux through the soluble phenylpropanoid pathway. Elevated expression of UGT72E2 led to increased accumulation of monolignol glucosides in root tissues and also the appearance of these glucosides in leaves. In particular, coniferyl alcohol 4-O-glucoside accumulated to massive amounts (10 micromol g(-1) FW) in root tissues of these plants. Increased glucosylation of other phenylpropanoids also occurred in plants overexpressing this glycosyltransferase. Significantly changing the pattern of glycosides in the leaves also led to a pronounced change in accumulation of the hydroxycinnamic ester sinapoyl malate. The data demonstrate the plasticity of phenylpropanoid metabolism and the important role that glucosylation of secondary metabolites can play in cellular homeostasis.