Summary miRNAs are a class of versatile small RNAs that control gene expression post-transcriptionally, governing many facets of plant cell functions. They interact with their target mRNA at a site of sequence complementarity and modulate their expression levels. Here, we provide evidence, based on transient assays and stable transgenic lines, that the 3¢ UTR of the Arabidopsis SBP box gene SPL3 contains a functional miRNAresponsive element (MRE) that is complementary to miR156 and miRNA157. Seedlings of transgenic lines constitutively over-expressing an SPL3 transgene either carrying an unaltered or a disrupted MRE accumulate considerable levels of SPL3 transcripts. However, while the unaltered MRE UTR does not allow the expression of detectable levels of SPL3 protein, the altered MRE does. Translational inhibition thus provides an important mechanism for miRNA-mediated post-transcriptional repression of SPL3. As a consequence of precocious translation of the constitutively expressed SPL3 transgene, due to the absence of a functional MRE, plants exhibit very early flowering in addition to frequent morphological changes.

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The isolation and initial characterization of the Arabidopsis thaliana SPL3 gene are described. SPL3 belongs to a gene family encoding putative transcription factors characterized by a conserved DNA-binding domain, the SBP domain. SPL3 transcription is developmentally regulated and is localized mainly in vegetative and inflorescence apical meristems, floral meristems and in leaf and floral organ primordia. SPL3 recognizes a conserved sequence motif in the promoter region of the A. thaliana floral meristem identity gene AP1. Similarly to AP1, constitutive expression of SPL3 results in early flowering. However, constitutive expression of SPL3 in an ap1 mutant background showed that AP1 is not required for the early flowering phenotype of the SPL3 transgenic plants. The function of SPL3 during flowering as well as its possible functional redundancy are discussed.