Plant microRNAs (miRNAs) show a high degree of sequence complementarity to, and are believed to guide the cleavage of, their target messenger RNAs. Here, I show that miRNA172, which can base-pair with the messenger RNA of a floral homeotic gene, APETALA2, regulates APETALA2 expression primarily through translational inhibition. Elevated miRNA172 accumulation results in floral organ identity defects similar to those in loss-of-function apetala2 mutants. Elevated levels of mutant APETALA2 RNA with disrupted miRNA172 base pairing, but not wild-type APETALA2 RNA, result in elevated levels of APETALA2 protein and severe floral patterning defects. Therefore, miRNA172 likely acts in cell-fate specification as a translational repressor of APETALA2 in Arabidopsis flower development.MicroRNAs (miRNAs), ~22-nucleotide non-coding RNAs that regulate protein-coding RNAs, are processed from longer hairpin transcripts by the enzyme Dicer [reviewed in (1, 2)]. In Arabidopsis, the accumulation of miRNAs requires a Dicer homolog, DCL1, and a novel protein, HEN1 [reviewed in (3)]. The two founding members of Caenorhabditis elegans miRNAs, lin-4 and let-7, inhibit the translation of their target mRNAs through imperfect base-pairing interactions with their targets (4, 5). Arabidopsis miRNAs show a higher degree of sequence complementarity to their potential targets, and, like short interfering RNAs, several plant miRNAs direct the cleavage of their target RNAs (6-9). Here, I report the role of an Arabidopsis miRNA, miRNA172, in the control of floral organ identity and floral stem cell proliferation as a potential translational repressor of a floral homeotic gene. Four organ types are specified in the floral meristem by the combinatorial actions of three classes of transcription factors, the floral A, B, and C genes [reviewed in (10)]. APETALA2 (AP2, a class A gene) and AGAMOUS (AG, a class C gene) specify the identities of the perianth and reproductive organs, respectively, and act antagonistically to restrict each other's activities to their proper domains of action within the floral meristem. We have identified HEN1 as a gene required for class C activity in the flower, because recessive hen1Supporting Online Material www.sciencemag.org/cgi/content/full/1088060/DC1 Materials and Methods Figs. S1 to S4 Table S1 HHS Public Access Author Manuscript mutations result in reproductive-to-perianth organ transformation in the hua1-1 hua2-1 background, which is partially compromised in class C activity (11,12). The requirement of HEN1 for the accumulation of miRNAs (13) suggests that the absence of a miRNA(s) is responsible for the floral homeotic transformation in hua1-1 hua2-1 hen1 flowers.

MicroRNAs (miRNAs) are ;21 nucleotide noncoding RNAs produced by Dicer-catalyzed excision from stem-loop precursors. Many plant miRNAs play critical roles in development, nutrient homeostasis, abiotic stress responses, and pathogen responses via interactions with specific target mRNAs. miRNAs are not the only Dicer-derived small RNAs produced by plants: A substantial amount of the total small RNA abundance and an overwhelming amount of small RNA sequence diversity is contributed by distinct classes of 21-to 24-nucleotide short interfering RNAs. This fact, coupled with the rapidly increasing rate of plant small RNA discovery, demands an increased rigor in miRNA annotations. Herein, we update the specific criteria required for the annotation of plant miRNAs, including experimental and computational data, as well as refinements to standard nomenclature.

MicroRNAs (miRNAs) are small noncoding RNA gene products about 22 nt long that are processed by Dicer from precursors with a characteristic hairpin secondary structure. Guidelines are presented for the identification and annotation of new miRNAs from diverse organisms, particularly so that miRNAs can be reliably distinguished from other RNAs such as small interfering RNAs. We describe specific criteria for the experimental verification of miRNAs, and conventions for naming miRNAs and miRNA genes. Finally, an online clearinghouse for miRNA gene name assignments is provided by the Rfam database of RNA families.

miRNAs are present in both plant and animal kingdoms. An evolutionarily conserved mechanism involving a protein, known as Dicer in animals and CAF in Arabidopsis, operates in miRNA metabolism. HEN1 is a new player in miRNA accumulation in Arabidopsis, and HEN1 homologs in metazoans may have a similar function. The developmental defects associated with caf-1 and hen1-1 mutations and the patterns of miRNA accumulation suggest that miRNAs play fundamental roles in plant development.

Methylation on the base or the ribose is prevalent in eukaryotic ribosomal RNAs (rRNAs) and is thought to be crucial for ribosome biogenesis and function. Artificially introduced 2'-O-methyl groups in small interfering RNAs (siRNAs) can stabilize siRNAs in serum without affecting their activities in RNA interference in mammalian cells. Here, we show that plant microRNAs (miRNAs) have a naturally occurring methyl group on the ribose of the last nucleotide. Whereas methylation of rRNAs depends on guide RNAs, the methyltransferase protein HEN1 is sufficient to methylate miRNA/miRNA* duplexes. Our studies uncover a new and crucial step in plant miRNA biogenesis and have profound implications in the function of miRNAs.