COTYLEDON 2 (CUC2) facilitates the initiation of serrations by enabling PIN1 to reorientate within the membranes of margin cells to change the auxin flux from a leaf tip oriented flux into a segmented flux towards convergence points where the serrations

INTRODUCTION Plant leaves have a major role in the photosynthetic supply of nutrients, gas exchange, the distribution of nutrients and water transport (Tsukaya, 2006). To optimise these functions in different habitats, plants need to adapt leaf positioning, size and shape (Hasson et al., 2010; Nicotra et al., 2011). An important trait of leaf shape variation is the outline of the leaf margin, which can be smooth, serrated (toothed) or lobed. If the gaps between lobes reach the midvein, leaves are designated as compound leaves and the lobes are referred to as leaflets, which can reiterate a dissecting pattern. In the simple Arabidopsis thaliana (Arabidopsis) leaf, the degree of serration differs between accessions and is developmentally regulated: juvenile leaves are smooth and serration increases in later-produced leaves. Increased serration can be caused by an increased number of teeth or by an increased sinus (the region between the teeth) depth (Nikovics et al., 2006; Tsukaya, 2006). Leaf serration formation is mechanistically related to leaf primordia formation in the shoot apical meristem (SAM) (Scarpella et al., 2010). The auxin efflux carrier PIN-FORMED 1 (PIN1) establishes local auxin maxima that direct the outgrowth of the serrations (Scarpella et al., 2006). Accordingly, loss of PIN1 function results in smooth leaf margins (Hay and Tsiantis, 2006). It was shown that the transcription factor CUP-SHAPED COTYLEDON 2 (CUC2) facilitates the initiation of serrations by enabling PIN1 to reorientate within the membranes of margin cells to change the auxin flux from a leaf tip oriented flux into a segmented flux towards convergence points where the serrations develop (Bilsborough et al., 2011). Accordingly, CUC2 is essential for the formation of leaf serrations (Nikovics et al., 2006; Hasson et al., 2011). cuc2 mutants fail to initiate auxin convergence points at the leaf margin early in development and subsequently display a no longer discrete, but homogeneous, auxin distribution at their smooth leaf margins (Kawamura et al., 2010; Bilsborough et al., 2011; Hasson et al., 2011). Expression of CUC2 is observed along the leaf margin prior to the actual outgrowth of the teeth, which occurs at points of local CUC2 repression (Bilsborough et al., 2011). After the initiation of the teeth, CUC2 expression persists at the sinus region, where it functions, together with the partially redundant gene CUC3, in the maintenance of serration outgrowth (Hasson et al., 2011). CUC2 is post-transcriptionally downregulated in leaves by a microRNA (miRNA) encoded by the MIR164A locus. mir164a mutant plants display enhanced leaf serrations, whereas overexpression of MIR164A results in smooth leaf margins (Nikovics et al., 2006). In a feedback loop, auxin downregulates CUC2 both transcriptionally and posttranscriptionally by activation of MIR164A (Bilsborough et al., 2011). Chromatin-mediated gene repression through Polycomb group (PcG) proteins has an impact on leaf development (Katz et al., 2004). PcG proteins are structurally unrelated repressive proteins that assemble in several Polycomb repressive complexes (PRCs), and the components of PRCs are highly conserved in higher eukaryotes (Schwartz and Pirrotta, 2008). Genes to be repressed are methylated at lysine 27 of histone H3 (H3K27me3) by PRC2, and this mark is subsequently recognised by a chromodomain component of PRC1 (Kuzmichev et al., 2005; Schwartz and Pirrotta, 2008). PRC1 stays associated with the repressed locus and catalyses mono-ubiquitylation of lysine 119 in histone H2A (H2AK119ub1), leading to a compaction of chromatin that represses transcription (Morey and Helin, 2010). In plants, PRC2 is conserved and was shown to be involved in deposition of the H3K27me3 mark (Pien and Grossniklaus, 2007). The catalytic core of the sporophytic PRC2 comprises one of the partially redundant 1Department of Plant Developmental Biology, 2Plant Computational Biology, 3Max Planck Genome Centre Cologne, Max Planck Institute for Plant Breeding Research, Carl von Linné Weg 10, 50829 Köln, Germany.