Small RNAs as Mobile Signals in Development

Abstract

Leaves of higher plants exhibit a varying degree of asymmetry along their dorsoventral axis. This asymmetry is specified via the polarized expression of class III homeodomain‐leucine zipper (hd‐zipIII) genes, which specify dorsal cell fate. Through detailed in situ hybridization analyses we were able to show that this polarized hd‐zipIII expression is set up by the graded expression pattern of a 21‐nucleotide microRNA, miR166. More recently, we have shown that this miR166 gradient is generated through the opposing activity of a different type of small regulatory RNA, the transacting siRNA tasiR‐ARF. A further twist to the story is added by the observation that tasiR‐ARF in turn is regulated by a third small RNA, miR390. Our observations identify a novel mechanism of pattern formation in which cell fates along a developmental axis are specified through the opposing activity of distinct small RNAs. tasiR‐ARF defines the dorsal side of the leaf by restricting the expression domain of miR166, which in turn delineates the ventral side by restricting expression of the hd‐zipIII transcription factors. Importantly, comparison of the expression patterns of small RNA precursor transcripts to those of the mature small RNA provides evidence that small regulatory RNAs can be mobile, and this offers the intriguing possibility that small RNAs may function as morphogen‐type signals in development.