MicroRNA-mediated regulation of stomatal development in arabidopsis

MicroRNAs have important functions in the development of eukaryotes. In plants, highly
conserved miRNA families have been shown to regulate morphogenesis and organ identity,
primarily by targeting cleavage of mRNAs encoding transcription factors. Cloning identified a
21 nt Arabidopsis miRNA, miR824, conserved in Brassica species, but not in more distantly
related species. miR824 is encoded at a single genetic locus as a polyadenylated, primary
miRNA that is spliced and then processed via a precursor miRNA intermediate. miR824
mediates cleavage of the AGAMOUS-LIKE 16 (AGL16) mRNA. AGL16 is a member of the
MADS-box protein family. The plant-specific MADS-box protein family has many established
functions in regulating growth and development. Impairing the miR824-mediated repression
of AGL16 leads to leaf abnormalities and growth defects raising the possibility that AGL16
has pleiotropic function in leaf developmental programs. This study shows that the density
and development of stomatal complexes on the epidermis of Arabidopsis leaves depend, in
part, on microRNA-mediated regulation of AGL16 assigning a novel function for the MADSbox
protein family. Mutants deficient in AGL16 and transgenics overexpressing miR824 show
decrease in stomatal density and developed only primary stomatal complexes. Ectopic expression
of a miR824-resistant AGL16 mRNA, but not of the wild-type AGL16 mRNA in
transgenic plants, increases the stomatal density and the incidence of higher-order stomatal
complexes. These results and the localization of AGL16 mRNA and miR824 in mature
stomata and satellite meristemoids, respectively, leads to the conclusion that miR824/AGL16
pathway functions in stomatal development. The miR824-AGL16 regulatory pathway is restricted
to Brassica and might account for some Brassicaceae-specific taxonomic features
of stomatal organization.