We use flower development in Arabidopsis as a model to understand how cells in multicellular organisms assume their developmental fates and form distinct patterns. Our long-term goal is to identify and analyze the genetic networks and the underlying molecular mechanisms that lead to cell fate specification in the floral primordium. For a long time, three classes of floral homeotic transcription factors known as the A, B, and C genes have been the only players in floral organ identity specification. Our studies have identified a microRNA, miR172, as a translational represser of the class A gene APETALA2, indicating that posttranscriptional regulation also plays a role in flower development. In the proposed project, we will employ molecular genetic approaches to determine how miR172 fits in the regulatory circuitry governing flower development and to probe how miR172 regulates APETALA2 mRNA at the translational level. Homeotic genes that encode transcription factors act in cell fate specification in both animals and plants. Emerging evidence of microRNAs as regulators of homeotic genes in both animals and plants adds a new layer of regulation to the known transcriptional networks governing cell fate specification. The proposed research will undoubtedly provide insights into the integration of posttranscriptional and transcriptional mechanisms in developmental processes in multicellular organisms. The proposed research will also provide insights into the mechanism of microRNA-mediated translational repression of target mRNAs. Given the conserved actions of small RNAs in gene regulation in plants and animals and the potential of using small RNAs as therapeutic agents, this research will contribute to our understanding of small RNA biology in general and impact the uses of small RNAs to treat human diseases.