Small RNAs Turn Over a New Leaf as Morphogens Dana O. Robinson, Adrienne H.K. Roeder  Developmental Cell  Volume 43, Issue 3, Pages 253-254 (November 2017) DOI: 10.1016/j.devcel.2017.10.025 Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 1 Mobile Small RNA Gradients Generate Sharp, Stable Developmental Boundaries (A) Schematic of factors establishing top-bottom polarity in the leaf. Left: factors establishing adaxial (top) fate. Class III HD-ZIPs (dark green nuclei), which promote top fate, are expressed in the two uppermost cell layers. Their expression is limited to these cell layers by the action of miR166, a microRNA expressed in the bottom-most cell layer (light green nuclei) and diffusing upward (green gradient). Right: factors establishing abaxial (bottom) fate. The auxin response factors ARF3 and ARF4 (dark red nuclei) promote bottom fate and are expressed in the two bottom-most leaf layers. tasiARF, a small RNA, diffuses (pink gradient) from its expression domain in the two uppermost leaf layers (pink nuclei) to limit ARF3/ARF4 expression. (B) The diffusion gradient of tasiARF creates a binary threshold in the expression of ARF3. tasiARF levels are high in the two top-most cell layers, where it is produced, and decrease as tasiARF diffuses toward the leaf’s bottom face. ARF3 is transcribed throughout the leaf. In the upper and middle leaf cells, where tasiARF levels are high relative to ARF3, tasiARF silences ARF3. When the ratio of tasiARF to ARF3 drops below a binary threshold, ARF3 is expressed. tasiARF acts to buffer ARF3 expression below this binary threshold. Developmental Cell 2017 43, 253-254DOI: (10.1016/j.devcel.2017.10.025) Copyright © 2017 Elsevier Inc. Terms and Conditions