Multicellular organisms depend on cell production, cell fate specification, and correct patterning to shape their adult body. In plants, auxin plays a prominent role in the timely coordination of these different cellular processes. A well-studied example is lateral root initiation, in which auxin triggers founder cell specification and cell cycle activation of xylem polepositioned pericycle cells. Here, we report that the E2Fa transcription factor of Arabidopsis thaliana is an essential component that regulates the asymmetric cell division marking lateral root initiation. Moreover, we demonstrate that E2Fa expression is regulated by the LATERAL ORGAN BOUNDARY DOMAIN18/LATERAL ORGAN BOUNDARY DOMAIN33 (LBD18/ LBD33) dimer that is, in turn, regulated by the auxin signaling pathway. LBD18/LBD33 mediates lateral root organogenesis through E2Fa transcriptional activation, whereas E2Fa expression under control of the LBD18 promoter eliminates the need for LBD18. Besides lateral root initiation, vascular patterning is disrupted in E2Fa knockout plants, similarly as it is affected in auxin signaling and lbd mutants, indicating that the transcriptional induction of E2Fa through LBDs represents a general mechanism for auxin-dependent cell cycle activation. Our data illustrate how a conserved mechanism driving cell cycle entry has been adapted evolutionarily to connect auxin signaling with control of processes determining plant architecture. INTRODUCTIONAs plants develop postembryonically, they produce continuously new structures in a flexible manner, allowing modifications in plant architecture in response to environmental conditions and specific needs. To model the body plan in accordance with external triggers, plant hormones, in particular auxin, play an important role (Friml, 2003;Tanaka et al., 2006;Vanneste and Friml, 2009). Auxin maxima can be found at organ initiation sites as well as in organs upon, for instance, gravity or light stimuli (Friml et al., 2002;Benková et al., 2003;Fuchs et al., 2003;Esmon et al., 2006;Traas and Moné ger, 2010). A well-studied example of hormone-driven morphogenesis is root architecture that is determined by the number and placement of lateral roots (Overvoorde et al., 2010). In Arabidopsis thaliana, lateral root initiation is preceded by an oscillating auxin response in the basal meristem, priming the xylem-pole pericycle (XPP) as founder cells of lateral root primordia (De Smet et al., 2007;De Rybel et al., 2010;Moreno-Risueno et al., 2010). As they mature, these cells have the potential to undergo an asymmetric cell division, initiating the formation of a new lateral root. The subsequent cell divisions follow a well-organized pattern, resulting in lateral root emergence (Pé ret et al., 2009).The molecular mechanism controlling lateral root initiation is based on the auxin-dependent degradation of INDOLE-ACETIC ACID INDUCED PROTEIN14 (IAA14)/SOLITARY ROOT (SLR), which leads to the derepression of AUXIN RESPONSE FACTOR7 (ARF7) and ARF19 (Fukaki et al., 2002;Okushima et al., 2005;Wilm...

C4 photosynthesis outperforms the ancestral C3 state in a wide range of natural and agro-ecosystems by affording higher water-use and nitrogen-use efficiencies. It therefore represents a prime target for engineering novel, high-yielding crops by introducing the trait into C3 backgrounds. However, the genetic architecture of C4 photosynthesis remains largely unknown. To define the divergence in gene expression modules between C3 and C4 photosynthesis during leaf ontogeny, we generated comprehensive transcriptome atlases of two Cleomaceae species, Gynandropsis gynandra (C4) and Tarenaya hassleriana (C3), by RNA sequencing. Overall, the gene expression profiles appear remarkably similar between the C3 and C4 species. We found that known C4 genes were recruited to photosynthesis from different expression domains in C3, including typical housekeeping gene expression patterns in various tissues as well as individual heterotrophic tissues. Furthermore, we identified a structure-related module recruited from the C3 root. Comparison of gene expression patterns with anatomy during leaf ontogeny provided insight into genetic features of Kranz anatomy. Altered expression of developmental factors and cell cycle genes is associated with a higher degree of endoreduplication in enlarged C4 bundle sheath cells. A delay in mesophyll differentiation apparent both in the leaf anatomy and the transcriptome allows for extended vein formation in the C4 leaf.

The anaphase-promoting complex/cyclosome (APC/C) is a multisubunit ubiquitin ligase that regulates progression through the cell cycle by marking key cell division proteins for destruction. To ensure correct cell cycle progression, accurate timing of APC/C activity is important, which is obtained through its association with both activating and inhibitory subunits. However, although the APC/C is highly conserved among eukaryotes, no APC/C inhibitors are known in plants. Recently, we have identified ULTRAVIOLET-B-INSENSITIVE4 (UVI4) as a plant-specific component of the APC/C. Here, we demonstrate that UVI4 uses conserved APC/C interaction motifs to counteract the activity of the CELL CYCLE SWITCH52 A1 (CCS52A1) activator subunit, inhibiting the turnover of the A-type cyclin CYCA2;3. UVI4 is expressed in an S phase-dependent fashion, likely through the action of E2F transcription factors. Correspondingly, uvi4 mutant plants failed to accumulate CYCA2;3 during the S phase and prematurely exited the cell cycle, triggering the onset of the endocycle. We conclude that UVI4 regulates the temporal inactivation of APC/C during DNA replication, allowing CYCA2;3 to accumulate above the level required for entering mitosis, and thereby regulates the meristem size and plant growth rate.