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Background: MicroRNAs (miRNAs) are ∼21 nucleotide (nt) RNAs that regulate gene expression in plants and animals. Most known plant miRNAs target transcription factors that influence cell fate determination, and biological functions of miRNA-directed regulation have been reported for four of 15 known microRNA gene families: miR172, miR159, miR165, and miR168. Here, we identify a developmental role for miR164-directed regulation of NAC-domain genes, which encode a family of transcription factors that includes CUP-SHAPED COTYLEDON1 (CUC1) and CUC2.

Results: Expression of a miR164-resistant version of CUC1 mRNA from the CUC1 promoter causes alterations in Arabidopsis embryonic, vegetative, and floral development, including cotyledon orientation defects, reduction of rosette leaf petioles, dramatically misshapen rosette leaves, one to four extra petals, and one or two missing sepals …