The plant Polycomb-group (Pc-G) protein CURLY LEAF (CLF) is required to repress targets such as AGAMOUS (AG) and SHOOTMERISTEMLESS (STM). Using chromatin immunoprecipitation, we identify AG and STM as direct targets for CLF and show that they carry a characteristic epigenetic signature of dispersed histone H3 lysine 27 trimethylation (H3K27me3) and localised H3K27me2 methylation. H3K27 methylation is present throughout leaf development and consistent with this, CLF is required persistently to silence AG. However, CLF is not itself an epigenetic mark as it is lost during mitosis. We suggest a model in which Pc-G proteins are recruited to localised regions of targets and then mediate dispersed H3K27me3. Analysis of transgenes carrying AG regulatory sequences confirms that H3K27me3 can spread to novel sequences in a CLF-dependent manner and further shows that H3K27me3 methylation is not sufficient for silencing of targets. We suggest that the spread of H3K27me3 contributes to the mitotic heritability of Pc-G silencing, and that the loss of silencing caused by transposon insertions at plant Pc-G targets reflects impaired spreading.

SummaryThe Spo11 protein is a eukaryotic homologue of the archaeal DNA topoisomerase VIA subunit (topo VIA). In archaea it is involved, together with its B subunit (topo VIB), in DNA replication. However, most eukaryotes, including yeasts, insects and vertebrates, instead have a single gene for Spo11/topo VIA and no homologues for topo VIB. In these organisms, Spo11 mediates DNA double-strand breaks that initiate meiotic recombination. Many plant species, in contrast to other eukaryotes, have three homologues for Spo11/topo VIA and one for topo VIB. The homologues in Arabidopsis, AtSPO11-1, AtSPO11-2 and AtSPO11-3, all share 20-30% sequence similarity with other Spo11/topo VIA proteins, but their functional relationship during meiosis or other processes is not well understood. Previous genetic evidence suggests that AtSPO11-1 is a true orthologue of Spo11 in other eukaryotes and is required for meiotic recombination, whereas AtSPO11-3 is involved in DNA endo-reduplication as a part of the topo VI complex. In this study, we show that plants homozygous for atspo11-2 exhibit a severe sterility phenotype. Both male and female meiosis are severely disrupted in the atspo11-2 mutant, and this is associated with severe defects in synapsis during the first meiotic division and reduced meiotic recombination. Further genetic analysis revealed that AtSPO11-1 and AtSPO11-2 genetically interact, i.e. plants heterozygous for both atspo11-1 and atspo11-2 are also sterile, suggesting that AtSPO11-1 and AtSPO11-2 have largely overlapping functions. Thus, the three Arabidopsis Spo11 homologues appear to function in two discrete processes, i.e. AtSPO11-1 and AtSPO11-2 in meiotic recombination and AtSPO11-3 in DNA replication.

SummaryControlled gene expression in time and space is a powerful tool for the analysis of gene function during plant development. Here, we report ethanol inducible gene expression in de®ned sub-domains of the shoot apical and¯oral meristems. For this, expression of an ethanol-regulated transcription factor, ALCR, is restricted to precise domains using speci®c promoters. Gene expression activation is followed using reporters under the control of the alcA promoter, which responds to ALCR only in the presence of the ethanol.We demonstrate that precise control of spatially limited gene expression can be achieved. The kinetics of reporter gene activation and inactivation following a pulse of ethanol induction shows that the system is dynamic and suitable for precise temporal control of expression. The system is both¯exible and robust, permitting simultaneous expression of two genes in a given domain or, conversely, the expression of a gene in two separate domains. We also show that this strategy can be applied to mis-express genes with developmental roles, by manipulating expression of the SHOOT MERISTEMLESS (STM) and CYCLIN D3;1 (CYCD3;1) genes during plant development.

How cells achieve their final sizes is a pervasive biological question. One strategy to increase cell size is for the cell to amplify its chromosomal DNA content through endoreduplication cycles. Although endoreduplication is widespread in eukaryotes, we know very little about its molecular mechanisms. Successful progression of the endoreduplication cycle in Arabidopsis requires a plant homologue of archaeal DNA topoisomerase (topo) VI. To further understand how DNA is endoreduplicated and how this process is regulated, we isolated a dwarf Arabidopsis mutant, hyp7 (hypocotyl 7), in which various large cell types that in the wild type normally endoreduplicate multiple times complete only the first two rounds of endoreduplication and stall at 8C. HYP7 encodes the RHL1 (ROOT HAIRLESS 1) protein, and sequence analysis reveals that RHL1 has similarity to the C-terminal domain of mammalian DNA topo II␣, another type II topo that shares little sequence homology with topo VI. RHL1 shows DNA binding activity in vitro, and we present both genetic and in vivo evidence that RHL1 forms a multiprotein complex with plant topo VI. We propose that RHL1 plays an essential role in the topo VI complex to modulate its function and that the two distantly related topos, topo II and topo VI, have evolved a common domain that extends their function. Our data suggest that plant topo II and topo VI play distinct but overlapping roles during the mitotic cell cycle and endoreduplication cycle.endoreduplication ͉ hypocotyl ͉ root hairless T he control of cell size is a highly regulated process with inputs from genetic, hormonal, and environmental cues. Yeast and mammalian cells usually only double their size during their development; therefore, a key question in their size control is how proliferating cells coordinate cell growth and cell division to maintain size homeostasis. Yeast and many mammalian cells have a cell size checkpoint mechanism in which cells divide only when they reach a critical size (1), whereas some mammalian cells may control their size through extracellular signals (2). Although plant cells also double their cell size during proliferation, they commonly undergo an additional, massive (sometimes Ͼ1,000-fold), postmitotic cell enlargement. Such a large increase in volume is driven by a combination of production of new cytoplasmic mass and cell expansion (driven by water uptake and vacuolar growth), but little is known about the underlying mechanisms involved. Recent genetic evidence strongly supports the classical ''karyoplasmic ratio'' theory that one mechanism to increase cell size is by increasing the ploidy level within a cell, for example through endoreduplication, defined as the amplification of chromosomal DNA without corresponding cell division (3). Several mutants and transgenic plants that have aberrant levels of endoreduplication have been isolated and have led to the identification of key regulators of the endoreduplication cycle or endocycle (3-7). How these regulators control downstream events, however, rem...

In a proteomics-based screen for proteins interacting with cyclin-dependent protein kinase (CDK), we have identi¢ed a novel CDK complex containing the eukaryotic translation initiation factor, eIF4A. Reciprocal immunoprecipitations using antibodies against eIF4A indicate that the interaction is speci¢c. The CDKA^eIF4A complex is abundant in actively proliferating and growing cells but is absent from cells that have ceased dividing. The CDKA^eIF4A complex contains kinase activity that is sensitive to the CDK-speci¢c inhibitor roscovitine. This interaction points to a possible molecular mechanism linking cell proliferation with translational control.