The PHANTASTICA (PHAN) MYB gene of Antirrhinum majus is involved in growth and adaxial-abaxial determination of lateral organs. Its activity is required early in the establishment of the proximal-distal axis

INTRODUCTION Leaves develop as lateral organs from the peripheral zone of a shoot apical meristem. Initially, a group of cells is patterned along the proximal-distal axis and then establishment of the adaxial-abaxial axis is crucial for further leaf development. Subsequent cell proliferation along the medial-lateral axis results in flat and mediolateral symmetric leaves (Steeves and Sussex, 1989; Waites et al., 1998; Hudson, 2000; Byrne et al., 2001; Semiarti et al., 2001; Tsukaya, 2006; Bowman and Floyd, 2008; Szakonyi et al., 2010; Nakata et al., 2012). The PHANTASTICA (PHAN) MYB gene of Antirrhinum majus is involved in growth and adaxial-abaxial determination of lateral organs. Its activity is required early in the establishment of the proximal-distal axis (Waites and Hudson, 1995; Waites et al., 1998). The class III HD-ZIP genes of Arabidopsis thaliana specify the adaxial identity of lateral organs (McConnell and Barton, 1998; McConnell et al., 2001; Emery et al., 2003; Bao et al., 2004; Mallory et al., 2004). Members of the KANADI (KAN) and FILAMENTOUS FLOWER (FIL) [also known as YABBY (YAB)] gene families have been identified as abaxial determinants (Bowman and Smyth, 1999; Eshed et al., 1999; Sawa et al., 1999; Siegfried et al., 1999; Eshed et al., 2001; Kerstetter et al., 2001; Bowman and Floyd, 2008; Goldshmidt et al., 2008; Sarojam et al., 2010). Other components determining abaxial cell identity include the AUXIN RESPONSE FACTOR3 (ARF3) [also known as ETTIN (ETT)] and AUXIN RESPONSE FACTOR4 (ARF4) genes (Sessions and Zambryski, 1995; Sessions et al., 1997; Pekker et al., 2005; Wu et al., 2008; Kelley et al., 2012). Expression of ETT is regulated by several developmental mechanisms at either transcription or translation levels (Nishimura et al., 2005; Ng et al., 2009; Inagaki et al., 2009). In addition, ETT and ARF4 are both targeted by a transacting small interfering RNA (ta-siRNA) called tasiR-ARF (Allen et al., 2005; Williams et al., 2005; Fahlgren et al., 2006; Hunter et al., 2006; Nogueira et al., 2006; Nogueira et al., 2007; Schwab et al., 2009; Chitwood et al., 2009) derived from non-coding TAS3 transcripts that are initially targeted for cleavage by miR390. ARGONAUTE7, RNA-DEPENDENT RNA POLYMERASE6 (RDR6) and DICER-LIKE4 are involved in the biogenesis of tasiRARF (Peragine et al., 2004; Allen et al., 2005; Gasciolli et al., 2005; Xie et al., 2005; Adenot et al., 2006; Montgomery et al., 2008). 1Plant Biology Research Center and 2Graduate School of Bioscience and Biotechnology, Chubu University, 1200 Matsumoto-cho, Kasugai, Aichi 487-8501, Japan. 3Graduate School of Horticulture, Chiba University, 648 Matsudo, Matsudoshi, Chiba 271-8510, Japan. 4Department of Biological Sciences, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043 Japan. 5Division of Biological Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan. 6Department of Plant Sciences, Weizmann Institute of Science, Rehovot, 76100, Israel. 7Temasek Life Sciences Laboratory, National University of Singapore, Singapore. 8Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Meguro, Tokyo 153-8902, Japan.