RESUMO
Small RNAs and their targets form complex regulatory networks that control cellular and developmental processes in multicellular organisms. In plants, dorsoventral (adaxial/abaxial) patterning provides a unique example of a developmental process in which early patterning decisions are determined by small RNAs. A gradient of microRNA166 on the abaxial/ventral side of the incipient leaf restricts the expression of adaxial/dorsal determinants. Another class of small RNAs, the TAS3-derivated trans-acting short-interfering RNAs (ta-siRNAs), are expressed adaxially and repress the activity of abaxial factors. Loss of maize leafbladeless1 (lbl1) function, a key component of the ta-siRNA biogenesis pathway, leads to misexpression of miR166 throughout the initiating leaf, implicating ta-siRNAs in the spatiotemporal regulation of miR166. The spatial restriction of tasiRNA biogenesis components suggests that this pathway may act non-cell-autonomously in the meristem and possibly contributes to the classic meristem-borne adaxializing Sussex signal. Here, we discuss the key participants in adaxial/abaxial patterning and point out the intriguing possibility that organ polarity in plants is established by the opposing action of specific ta-siRNAs and miRNAs.
