An Interplay Between Small Regulatory RNAs Patterns Leaves

Adaxial/abaxial (dorsoventral) leaf polarity is a unique example of a developmental process in which early patterning decisions are determined by small regulatory RNAs. In maize, an abaxial gradient of the microRNA miR166 in the incipient leaf defines the localization of HD-ZIPIII transcripts that promote adaxial identity. Loss of leafbladeless1 (lbl1) function, necessary for the biogenesis of TAS3-derived trans-acting short-interfering RNAs (ta-siRNAs), leads to ectopic accumulation of miR166 throughout the initiating leaf. These findings indicate that the TAS3 ta-siRNA pathway specifies leaf polarity by spatially restricting miR166 to the abaxial side. Here, we briefly discuss possible mechanisms by which the TAS3 ta-siRNA pathway could regulate miR166 expression. Such regulatory mechanisms likely involve AUXIN RESPoNSE FACToRS and the phytohormone auxin. The convergence of small RNAs, and perhaps auxin, on key determinants of adaxial/abaxial organ polarity implicates them as candidate mobile signals that act at the plant apex to relay positional information from the meristem to the initiating leaf.