Abstract

Establishment of adaxial-abaxial (top/bottom) polarity is essential for the flattened outgrowth and patterning of leaves, and forms a key innovation in the evolution of land plants. We have shown that this polar axis is established through a cascade of opposing small RNAs in which miR390 triggers the biogenesis of a second class of small RNAs, the tasiR-ARFs, which in turn regulate the accumulation of miR166. Importantly, we showed that tasiR-ARF traffics from cell to cell. Movement of tasiR-ARF creates a gradient of accumulation across the leaf that dissipates towards the abaxial/bottom side. This gradient is interpreted into a sharp on-off domain of expression of the target and abaxial cell fate determinant ARF3 . Likewise, miR166 traffics from its site of biogenesis in the bottom epidermal layer to restrict expression of adaxial determinants, the HD-ZIPIII transcription factors, to the upper domain of the leaf. Mathematical modelling predicts that the opposing small RNA gradients generated through mobility are uniquely suited to create sharply defined and robust patterning boundaries. This model is supported by our recent observations, which will be presented. Our observations indicate that leaves are partitioned into top and bottom domains via a novel patterning mechanism involving opposing gradients of small RNAs that act as mobile morphogen-like instructive signals.