Eukaryotic E2Fs are conserved transcription factors playing crucial and antagonistic roles in several pathways related to cell division, DNA repair, and differentiation. In plants, these processes are strictly intermingled at the growing zone to produce postembryonic development in response to internal signals and environmental cues. Of the six AtE2F proteins found in Arabidopsis (Arabidopsis thaliana), only AtE2Fa and AtE2Fb have been clearly indicated as activators of E2F-responsive genes. AtE2Fa activity was shown to induce S phase and endoreduplication, whereas the function of AtE2Fb and the interrelationship between these two transcription factors was unclear. We have investigated the role played by the AtE2Fb gene during cell cycle and development performing in situ RNA hybridization, immunolocalization of the AtE2Fb protein in planta, and analysis of AtE2Fb promoter activity in transgenic plants. Overexpression of AtE2Fb in transgenic Arabidopsis plants led to striking modifications of the morphology of roots, cotyledons, and leaves that can be ascribed to stimulation of cell division. The accumulation of the AtE2Fb protein in these lines was paralleled by an increased expression of E2F-responsive G1/S and G2/M marker genes. These results suggest that AtE2Fa and AtE2Fb have specific expression patterns and play similar but distinct roles during cell cycle progression.The identification of various components of the plant cell cycle machinery has revealed remarkable similarities with the regulatory pathways found in animal cells, for which a key role is exerted by the E2F/DP family of transcription factors. The genome of the model plant Arabidopsis (Arabidopsis thaliana) contains eight genes of this family (six E2Fs and two DPs), whereas in mammalian cells 10 E2F/DP members have been discovered (eight E2Fs and two DPs; Attwooll et al., 2004;Christensen et al., 2005;Dimova and Dyson, 2005;Maiti et al., 2005). Most mammalian E2F proteins (E2F1-5) and three of the Arabidopsis members (AtE2Fa-c) show a similar domain organization, characterized by a highly conserved DNAbinding domain followed by a DP heterodimerization domain and a C-terminal transactivating domain, containing the pocket protein-binding region. The mammalian E2F6 lacks the carboxy-terminal transactivating region. Six mammalian E2Fs (E2F1-6) and three Arabidopsis E2F proteins (AtE2Fa-c) bind DNA by forming heterodimers with the distantly related DP proteins that contribute a second DNA-binding domain for binding to the consensus E2F cis-elements found in several E2F-responsive promoters. The remaining Arabidopsis E2Fs (AtE2Fd, e, and f/DEL2, 1, and 3) and the E2F7 and E2F8 proteins of mammalian cells only contain conserved duplicated DNA-binding domains. They cannot form heterodimers with DP proteins, but their duplicated DNA-binding domains allow autonomous binding to the consensus E2F sites (Mariconti et al

Shoot apical meristems produce organs in a highly stereotypic pattern that involves auxin. Auxin is supposed to be actively transported from cell to cell by influx (AUXIN/LIKE AUXIN proteins) and efflux (PIN-FORMED proteins) membrane carriers. Current hypotheses propose that, at the meristem surface, PIN proteins create patterns of auxin gradients that, in turn, create patterns of gene expression and morphogenesis. These hypotheses are entirely based on work in Arabidopsis (Arabidopsis thaliana). To verify whether these models also apply to other species, we studied the behavior of PIN proteins during maize (Zea mays) development. We identified two novel putative orthologs of AtPIN1 in maize and analyzed their expression pattern during development. The expression studies were complemented by immunolocalization studies using an anti-AtPIN1 antibody. Interestingly, the maize proteins visualized by this antibody are almost exclusively localized in subepidermal meristematic layers. Both tassel and ear were characterized by a compact group of cells, just below the surface, carrying PIN. In contrast to or to complement what was shown in Arabidopsis, these results point to the importance of internally localized cells in the patterning process. We chose the barren inflorescence2 (bif2) maize mutant to study the role of auxin polar fluxes in inflorescence development. In severe alleles of bif2, the tassel and the ear present altered ZmPIN1a and ZmPIN1b protein expression and localization patterns. In particular, the compact groups of cells in the tassel and ear of the mutant were missing. We conclude that BIF2 is important for PIN organization and could play a role in the establishment of polar auxin fluxes in maize inflorescence, indirectly modulating the process of axillary meristem formation and development.

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