MiRNA172 represses APETALA2 (AP2) expression in Arabidopsis flower development through translational inhibition. AP2 is a class A gene that specifies perianth identity, while miRNA172, which binds to AP2 mRNA, regulates its expression. Elevated miRNA172 levels lead to floral organ identity defects similar to those in apetala2 mutants. Mutant AP2 RNA with disrupted miRNA172 base pairing results in increased AP2 protein and severe floral patterning defects, indicating miRNA172 acts as a translational repressor. MicroRNAs (miRNAs) are small non-coding RNAs that regulate protein-coding RNAs by base pairing with target mRNAs. In Arabidopsis, miRNAs are processed by Dicer and require DCL1 and HEN1 for accumulation. miRNA172, highly complementary to AP2 mRNA, is a candidate miRNA in flower development. It binds to AP2 mRNA in the coding region, outside the conserved AP2 domain. miRNA172 regulates AP2 expression at the translational level, as shown by experiments with transgenic plants. Elevated miRNA172 levels cause floral homeotic phenotypes similar to ap2 loss-of-function mutants, suggesting miRNA172 represses AP2 expression. AP2 is regulated by miRNA172, which is expressed in floral meristems. miRNA172 is present in developing flowers and regulates AP2 expression during flower development. miRNA172 likely keeps AP2 expression low, ensuring proper development of reproductive organs and timely termination of floral stem cells. The regulation is mediated by direct sequence complementarity between AP2 mRNA and miRNA172, occurring at the level of translation rather than RNA stability. This study shows that plant miRNAs can regulate target mRNAs through translational inhibition or transcript cleavage.MiRNA172 represses APETALA2 (AP2) expression in Arabidopsis flower development through translational inhibition. AP2 is a class A gene that specifies perianth identity, while miRNA172, which binds to AP2 mRNA, regulates its expression. Elevated miRNA172 levels lead to floral organ identity defects similar to those in apetala2 mutants. Mutant AP2 RNA with disrupted miRNA172 base pairing results in increased AP2 protein and severe floral patterning defects, indicating miRNA172 acts as a translational repressor. MicroRNAs (miRNAs) are small non-coding RNAs that regulate protein-coding RNAs by base pairing with target mRNAs. In Arabidopsis, miRNAs are processed by Dicer and require DCL1 and HEN1 for accumulation. miRNA172, highly complementary to AP2 mRNA, is a candidate miRNA in flower development. It binds to AP2 mRNA in the coding region, outside the conserved AP2 domain. miRNA172 regulates AP2 expression at the translational level, as shown by experiments with transgenic plants. Elevated miRNA172 levels cause floral homeotic phenotypes similar to ap2 loss-of-function mutants, suggesting miRNA172 represses AP2 expression. AP2 is regulated by miRNA172, which is expressed in floral meristems. miRNA172 is present in developing flowers and regulates AP2 expression during flower development. miRNA172 likely keeps AP2 expression low, ensuring proper development of reproductive organs and timely termination of floral stem cells. The regulation is mediated by direct sequence complementarity between AP2 mRNA and miRNA172, occurring at the level of translation rather than RNA stability. This study shows that plant miRNAs can regulate target mRNAs through translational inhibition or transcript cleavage.