Abstract: Small RNAs as mobile, morphogen-like signals in development Damianos Skopelitis1, Anna Benkovics1, Aman Husbands1 and Marja Timmermans1,2 1) Cold Spring Harbor Laboratory, 1 Bungtown Rd, Cold Spring Harbor, NY 11724, USA 2) Center for Plant Molecular Biology, University of Tuebingen, Auf der Morgenstelle 32, 72076 Tuebingen, Germany Adaxial-abaxial (top-bottom) polarity drives the flattened outgrowth and patterning of leaves, and represents an important innovation in the evolution of land plants. Patterning of this axis is driven by an intricate gene regulatory network. Integral to this network are two sets of conserved transcription factors that promote either adaxial or abaxial fate, and are expressed in complementary domains on the top or bottom side of the leaf, respectively. The positional information needed to delineate these domains is provided in part by the small RNAs miR166 and tasiR-ARF. We have shown that these small RNAs move outside their defined domain of biogenesis and form opposing gradients across the leaf that polarize expression of key adaxial- and abaxial-promoting transcription factors, HD-ZIPIII and ARF3/4, respectively. Our observations, which will be presented, indicate that mobile small RNAs have the inherent capacity to generate sharp gene expression boundaries, and function as morphogen-like signals in development. Their patterning properties present small RNAs and their targets as highly portable regulatory modules through which to create pattern, and provides a compelling basis for the extensive conservation and repeated co-option of developmentally important small RNA-target modules.