Role of MicroRNA miR319 in Plant Development Carla Schommer, Edgardo G. Bresso, Silvana V. Spinelli, and Javier F. Palatnik Abstract Originally identified in a genetic screen, microRNA miR319 regulates transcription factors of the TCP family. The balance between miR319 and its targets controls leaf morphogenesis and several other plant developmental pro- cesses. High levels of miR319 or low TCP activity causes an excess of cell proliferation that generates a crinkled simple leaf in Arabidopsis and snapdragon or supercompound organ in tomato. In contrast, reduced miR319 levels or high TCP activity reduces leaf and petal size, results in a simple tomato leaf, and is lethal in extreme cases. Insights into the gene networks that are controlled by the miR319- regulated TCPs demonstrate their participation in multiple biological pathways, from hormone biosynthesis and signaling to cell proliferation and differentiation. 1 Discovery of miR319 Through a Genetic Screen MicroRNAs (miRNAs) are a conspicuous group of small RNAs present in animals and plants, which are defined by their unique biogenesis as they are processed from an imperfect fold-back precursor (Meyers et al. 2008). In both plants and animals, miRNAs have been discovered by three methods: direct cloning and sequencing, genetic screenings, and bioinformatic predictions. The first plant miRNAs were identified by isolating, cloning, and sequencing small RNA populations (Reinhart et al. 2002; Llave et al. 2002; Park et al. 2002). This approach bursts recently with the development of deep-sequencing strategies [e.g., (Fahlgren et al. 2007; Rajagopalan et al. 2006; Lu et al. 2005, 2006)]. Cloning C. Schommer (*) • E.G. Bresso • S.V. Spinelli • J.F. Palatnik (*) Instituto de Biologı ´a Molecular y Celular de Rosario (IBR-CONICET), Facultad de Ciencias Bioquı ´micas y Farmace´uticas, Universidad Nacional de Rosario, Suipacha 531, 2000 Rosario, Argentina e-mail: schommer@ibr.gov.ar; palatnik@ibr.gov.ar R. Sunkar (ed.), MicroRNAs in Plant Development and Stress Responses, Signaling and Communication in Plants 15, DOI 10.1007/978-3-642-27384-1_2, # Springer-Verlag Berlin Heidelberg 2012 29