Among the regulatory targets of plant microRNAs are several auxin response factors (ARFs), which mediate plant responses to the phytohormone auxin. Mallory et al. (pages 1360–1375) show that regulation of ARF17 by the microRNA miR160 is necessary for normal development. Transgenic plants that express an ARF17 gene modified so that it is no longer repressed by miR160 have embryonic symmetry defects that lead to deeply lobed cotyledons (shown on cover) or extra cotyledons with extra emerging leaves. These plants also have vegetative and floral defects. This article, along with several others that provide additional details on the link between microRNAs and auxin signaling during development, is also highlighted in the In This Issue on pages 1335–1338