A role for microRNAs (miRNAs) in the developmental biology of plants has been established in model annual species, but is poorly understood in perennials, particularly trees. We over-expressed in poplar (genus Populus) a unique miRNA gene from maize belonging to the MIR156 family called Corngrass1 (Cg1) under the control of the cauliflower mosaic virus 35S promoter. Transgenics had significantly greater axillary meristem outgrowth (branching), shorter internode length, and up to a 30% reduction in stem lignin content compared to stem lignin of the wild-type (trunk stem from shoot apex down to node 10). The severity of the phenotype was positively correlated with Cg1 expression level. In addition, the syringyl to guaiacyl ratio (S/G) was lower in 35S:Cg1 lines than in wild-type poplar or a control transgenic line with low abundance of Cg1 transcript. We have demonstrated that over-expression in poplar of a MIR156 class miRNA has dramatic effects on plant architecture, and demonstrated that miRNA over-expression represents a novel approach to altering lignin content and composition in poplar. These transgenic plants may provide a tool for investigating the potential role of MIR156 in regulating developmental processes in poplar. The 35S:Cg1 poplars may also have commercial value as a cellulosic feedstock for biofuel production and in the paper-manufacturing industry. Published by Elsevier Ltd.