Zygomorphic flowers, with bilateral (dorsoventral) symmetry, are considered to have evolved several times independently in flowering plants. In Antirrhinum majus , floral dorsoventral symmetry depends on the activity of two TCP-box genes, CYCLOIDEA ( CYC ) and DICHOTOMA ( DICH ). To examine whether the same molecular mechanism of floral asymmetry operates in the distantly related Rosid clade of eudicots, in which asymmetric flowers are thought to have evolved independently, we investigated the function of a CYC homologue LjCYC2 in a papilionoid legume, Lotus japonicus . We showed a role for LjCYC2 in establishing dorsal identity by altering its expression in transgenic plants and analyzing its mutant allele squared standard 1 ( squ1 ). Furthermore, we identified a lateralizing factor, Keeled wings in Lotus 1 ( Kew1 ), which plays a key role in the control of lateral petal identity, and found LjCYC2 interacted with Kew1 , resulting in a double mutant that bore all petals with ventralized identity to some extents. Thus, we demonstrate that CYC homologues have been independently recruited as determinants of petal identities along the dorsoventral axis in two distant lineages of flowering plants, suggesting a common molecular origin for the mechanisms controlling floral zygomorphy.

The fragile X mental retardation protein (FMRP) is a RNA-binding protein proposed to post-transcriptionally regulate the expression of genes important for neuronal development and synaptic plasticity. We previously demonstrated that FMRP binds to its own FMR1 mRNA via a guanine-quartet (G-quartet) RNA motif. However, the functional effect of this binding on FMR1 expression was not established. In this work, we characterized the FMRP binding site (FBS) within the FMR1 mRNA by a site directed mutagenesis approach and we investigated its importance for FMR1 expression. We show that the FBS in the FMR1 mRNA adopts two alternative G-quartet structures to which FMRP can equally bind. While FMRP binding to mRNAs is generally proposed to induce translational regulation, we found that mutations in the FMR1 mRNA suppressing binding to FMRP do not affect its translation in cellular models. We show instead that the FBS is a potent exonic splicing enhancer in a minigene system. Furthermore, FMR1 alternative splicing is affected by the intracellular level of FMRP. These data suggest that the G-quartet motif present in the FMR1 mRNA can act as a control element of its alternative splicing in a negative autoregulatory loop.

Building a safe space for stem cells: The meristem, the collection of stem cells that builds plants, is resistant to viral infection. Wu et al. now show that WUSCHEL, a transcription factor that helps to sustain stem cell production in the shoot apical meristem of Arabidopsis , also protects that stem cell domain from viruses. WUSCHEL inhibited viral protein synthesis by repressing methyltransferases that regulate ribosomal RNA processing and ribosome stability. Science , this issue p. 227