Post-transcriptional control, including mRNA transport, stability, and translation, plays a crucial role in plant growth and development (Choi et al., 2000; Okita & Choi, 2002). Most of these processes are inevitably achieved either directly or indirectly by RNA-binding proteins (RBPs) (Keene, 2001; Lorković & Barta, 2002). All earlier known RNA recognition motif (RRM)- and K homology (KH)-type RBPs involved in flowering, for example, Flowering Time Control protein A (FCA), FPA and FLOWERING LOCUS K (FLK), act as flowering activators (Macknight et al., 1997; Schomburg et al., 2001; Lim et al., 2004; Mockler et al., 2004). Of functionally identified RBPs, FCAs have been extensively characterized with respect to autoregulation by alternative splicing (Macknight et al., 2002; Quesada et al., 2003) and RNA-mediated FLC chromatin silencing (Baurle et al., 2007; Liu et al., 2007a, 2010; Manzano et al., 2009), thereby autonomously promoting flowering by the repression of FLC expression. It has been previously reported that AtBRUL-1 and AtBRUL-2 (hereafter AtBRN1 and AtBRN2, respectively) belong to the Bruno RBP family (Good et al., 2000). Following the first report of Bruno in flies, a number of Bruno homologs (also known as CELF, CUG-BP, and ETR) have been annotated on the basis of RRM structures in other organisms; for example, six in humans (Barreau et al., 2006), three, including FCA, in Arabidopsis (Good et al., 2000), at least three in Drosophila melanogaster, six in Xenopus laevis (Amato et al., 2005), and one in Caenorhabditis elegans (Good et al., 2000). Drosophila Bruno functions as a translation repressor of Oskar and Nanos mRNA before their localization to the posterior pole of the oocyte (Kim-Ha et al., 1995; Good et al., 2000; Chekulaeva et al., 2006). Bruno binds to cis-acting elements present in the 3′ UTR of Oskar mRNA called Bruno response elements (BREs) with CUP (Kim-Ha et al., 1995; Igreja & Izaurralde, 2011). In more recent studies, the Bruno–CUP–eIF4E complex inhibits the eIF4E–eIF4G interaction, resulting in the translation repression of Oskar mRNA (Wilhelm et al., 2003; Macdonald, 2004; Nakamura et al., 2004). Therefore, translational repression of Oskar mRNA is dependent on the interaction of the 5′ and 3′ UTRs, which is mediated by an eIF4E complex (Wilhelm et al., 2003; Macdonald, 2004). In another model, Bruno participates in forming 50–80S ribonucleoprotein particles (RNPs), which act as silencing particles by assembling on Oskar mRNA and consequently preventing access of the translational machinery (Chekulaeva et al., 2006).
Nevertheless, Bruno specifically disrupts the translation of BRE-containing mRNA, although its specificity may also be controlled by the spatiotemporal expression of proteins that interact with Bruno. For instance, the eIF4E2 transcript is predominantly accumulated in floral tissues and is distinct from eIF4E1, which is constitutively expressed in all organs (Good et al., 2000). FCAs may not have the capacity to bind to BRE or to directly regulate translation repression, because it functions as a component of poly(A) factor and/or is involved in RNA-mediated asymmetric methylation of single and low-copy genes at the chromatin level (Baurle et al., 2007). Based on the early- and late-flowering phenotype observed in atbrns double mutants and 35S::AtBRNs transgenic plants, the genes implicated in the control of flowering time were examined in this study. AtBRNs redundantly repress the activity of the SOC1 protein by interacting with the 3′ UTR of SOC1 mRNA. These results suggest that post-transcriptional coordination occurs in the control of flowering time in addition to previously known regulatory systems occurring at the transcriptional level.