In eukaryotes, active import of large signaling molecules and proteins over 40 kD in the nucleus is mediated by aptly named “importin” proteins. As there are considerably fewer importins than there are cargos, determining how cargo-transporter specificity is mediated constitutes a challenging

The plant leaf epidermis constitutes a critical interface with the environment and consists of a single layer comprising three cell types: stomata, trichomes, and pavement cells. Stomata are pores that regulate gas exchange and transpiration. Trichomes are hair-like cells that can secrete

Contrary to animals, sex chromosomes in flowering plants (angiosperms) have evolved at least hundreds of times independently, providing an opportunity to unravel the mechanisms underlying their repeated evolution. In the XY system, the Y chromosome dominantly confers male identity: XY individuals

Gamete formation in sexually reproducing plants begins with formation of a “mother cell” that undergoes meiosis to generate haploid spores. Haploid spores further develop into gametophytes within which gametes differentiate. In flowering plants (angiosperms), including crop systems, the female

Meiosis produces haploid cells essential for sexual reproduction. In yeast, entry into meiosis activates transcription factors which trigger a transcriptional cascade that results in sequential co-expression of early, middle and late meiotic genes. However, these factors are not conserved, and the factors and regulatory mechanisms that ensure proper meiotic gene expression in multicellular eukaryotes are poorly understood. Here, we report that DUET/MMD1, a PHD finger protein essential for Arabidopsis male meiosis, functions as a transcriptional regulator in plant meiosis. We find that DUET-PHD binds H3K4me2 in vitro, and show that this interaction is critical for function during meiosis. We also show that DUET is required for proper microtubule organization during meiosis II, independently of its function in meiosis I. Remarkably, DUET protein shows stage-specific expression, confined to diplotene. We identify two genes TDM1 and JAS with critical functions in cell cycle transitions and spindle organization in male meiosis, as DUET targets, with TDM1 being a direct target. Thus, DUET is required to regulate microtubule organization and cell cycle transitions during male meiosis, and functions as a direct transcription activator of the meiotic gene TDM1. Expression profiling showed reduced expression of a subset comprising about 12% of a known set of meiosis preferred genes in the duet mutant. Our results reveal the action of DUET as a transcriptional regulator during male meiosis in plants, and suggest that transcription of meiotic genes is under stagewise control in plants as in yeast.

In flowering plants, meiocytes develop from subepidermal cells in anthers and ovules. The mechanisms that integrate gene-regulatory processes with meiotic programs during reproductive development remain poorly characterized. Here, we show that Arabidopsis thaliana plants deficient in ACTIN-RELATED PROTEIN6 (ARP6), a subunit of the SWR1 ATP-dependent chromatin-remodeling complex, exhibit defects in prophase I of female meiosis. We found that this meiotic defect is likely due to dysregulated expression of meiotic genes, particularly those involved in meiotic recombination, including DMC1 (DISRUPTED MEIOTIC cDNA1). Analysis of DMC1 expression in arp6 mutant plants indicated that ARP6 inhibits expression of DMC1 in the megasporocyte and surrounding nonsporogeneous ovule cells before meiosis. After cells enter meiosis, however, ARP6 activates DMC1 expression specifically in the megasporocyte even as it continues to inhibit DMC1 expression in the nonsporogenous ovule cells. We further show that deposition of the histone variant H2A.Z, mediated by the SWR1 chromatin-remodeling complex at the DMC1 gene body, requires ARP6. Therefore, ARP6 regulates female meiosis by determining the spatial and temporal patterns of gene expression required for proper meiosis during ovule development.

BACKGROUND:The establishment of sister chromatid cohesion followed by its controlled release at the metaphase to anaphase transition is necessary for faithful segregation of chromosomes in mitosis and meiosis. Cohesion is established by the action of Ctf7/Eco1 on the cohesin complex during DNA replication following loading of cohesin onto chromatin by the Scc2-Scc4 complex. Ctf7 is also required for sister chromatid cohesion during repair of DNA double strand breaks. Ctf7 contains an acetyltransferase domain and a zinc finger motif and acetylates conserved lysine residues in the Smc3 subunit of cohesin. In Arabidopsis CTF7 is encoded by a single gene and mutations in AtCTF7 cause embryo lethality indicating that the gene is essential.RESULTS:To study the function of Ctf7 in plants and to determine its role in sister chromatid cohesion, we constructed a conditional allele of AtCTF7 in Arabidopsis using an inducible RNA interference (RNAi) strategy, so as to avoid the embryo lethality caused by mutations in AtCTF7. We found that induction of RNAi against AtCTF7 caused severe inhibition and defects in growth during vegetative and reproductive stages as well as sterility. AtCTF7-RNAi plants displayed chromosome fragmentation and loss of sister chromatid cohesion during meiosis. Immunostaining for the cohesion subunit AtSCC3 showed a marked reduction in association of cohesin with chromatin during meiosis in AtCTF7-RNAi plants.CONCLUSIONS:We find that AtCTF7 is essential for sister chromatid cohesion during meiosis in Arabidopsis and is required for association of cohesin with chromatin in prophase of meiosis.

SummaryAdherin plays an important role in loading the cohesin complex onto chromosomes, and is essential for the establishment of sister‐chromatid cohesion. We have identified and analyzed the Arabidopsis adherin homolog AtSCC2. Interestingly, the sequence analysis of AtSCC2 and of other putative plant adherin homologs revealed the presence of a PHD finger, which is not found in their fungal and animal counterparts. AtSCC2 is identical to EMB2773, and mutants show early embryo lethality and formation of giant endosperm nuclei. A role for AtSCC2 in sister‐chromatid cohesion was established by using conditional RNAi and examining meiotic chromosome organization. AtSCC2‐RNAi lines showed sterility, arising from the following defects in meiotic chromosome organization: failure of homologous pairing, loss of sister‐chromatid cohesion, mixed segregation of chromosomes and chromosome fragmentation. The mutant phenotype, which included defects in chromosome organization and cohesion in prophase I, is distinct from that of the Arabidopsis cohesin mutant Atrec8, which retains centromere cohesion up to anaphase I. Immunostaining experiments revealed the aberrant distribution of the cohesin subunit AtSCC3 on chromosomes, and defects in chromosomal axis formation, in the meiocytes of AtSCC2‐RNAi lines. These results demonstrate a role for AtSCC2 in sister‐chromatid cohesion and centromere organization, and show that the machinery responsible for the establishment of cohesion is conserved in plants.

Maternal effects are defined by mutations that affect the next generation when they are maternally inherited. To date, most indepth studies of maternal effects in plants have attributed their origin to genomic imprinting that restricts expression to the maternal allele. The DNA glycosylase DEMETER (DME) removes methylated cytosine residues, causing transcriptional activation of the maternal allele of imprinted genes. In this study, we show that loss-of-function of the major DNA LIGASE I (AtLIG1) in Arabidopsis thaliana causes maternal effects in the endosperm, which is the seed tissue that nurtures embryo development. AtLIG1 expression is not imprinted and has a limited impact on imprinted gene expression. Genetic interaction analyses further indicate that AtLIG1 acts downstream of DME. The removal of methylated cytosine residues by DME involves the creation of DNA single-strand breaks and our results suggest that AtLIG1 repairs these breaks.

Polyploidy, the state of having more than two sets of chromosomes, is common in flowering plants (angiosperms), including the major crops [1]. Indeed, it is estimated that 30%–80% of the angiosperms are polyploids [2], and most diploid plant species, including Arabidopsis thaliana, show evidence of genome duplication in their ancestry [3]. Polyploidy is accompanied by genome-wide changes in gene expression and epigenetic modifications, leading to phenotypic diversity and rapid adaptation [4]. Doubling of chromosome number can also rescue and stabilize interspecific hybrids that would otherwise show a high degree of sterility due to failures in meiosis. Thus, polyploidy is thought to be a driving force in plant evolution and speciation [5]. In this issue of PLoS Genetics, d'Erfurth and colleagues report on the molecular characterization of the AtPS1 gene and show that lesions in the gene result in unreduced gametes, which are believed to be an important step in polyploidization [6]. While polyploids can originate by an increase of chromosome number either during somatic growth or during meiosis, the major route is now considered to be via the formation of unreduced gametes [7]. The formation of 2n gametes resulting from failure of reduction during meiosis occurs in several plant species and can give rise to triploids that can serve as a bridge to the establishment of an even set of chromosomes in subsequent generations [8]. Unreduced gametes have also been used in breeding schemes for crop improvement wherein beneficial traits have been introduced from diploid relatives into cultivated polyploid species [9]. The paper by d'Erfurth et al. [6] provides the first molecular insight into high-efficiency unreduced pollen formation in plants based on analysis of mutants in the Arabidopsis gene AtPS1 that produce diploid pollen. A range of meiotic abnormalities can give rise to unreduced gamete formation. These can be broadly classified as first or second division restitution (FDR and SDR, respectively) [2]. The occurrence of parallel spindles at meiosis II, resulting in a single plane of cell division and formation of dyad spores, is a frequently found mechanism for 2n gamete formation and is genetically equivalent to FDR in that it results in retention of heterozygosity for centromeric markers (Figure 1). The parallel spindle 1 (ps1) mutant of potato, which produces unreduced pollen by such a mechanism, was described more than 80 years ago [10] and has been used in potato breeding for introgression of beneficial traits in cultivated strains [11]. While the occurrence of unreduced gamete formation in plants has long been recognized, the molecular mechanisms leading to diploid gamete formation have only recently begun to be uncovered [12],[13]. Figure 1 Formation of parallel spindles during meiosis II in atps1 gives unreduced spores. d'Erfurth et al. identified the AtPS1 gene using a novel bioinformatics approach. They looked for genes whose expression pattern in microarrays correlated with that of known meiotic genes. Subsequent analysis showed that T-DNA insertions in AtPS1 result in the production of diploid pollen, and triploidy in as many as 30% of progeny. The authors then show, by elegant genetic and cytological experiments, that the atps1 mutant phenotype results from the positioning of the spindles at metaphase II in a parallel orientation instead of in the normal tetrahedral configuration. In most angiosperms, including Arabidopsis, cytokinesis at the time of meiosis occurs only after the second round of chromosome segregation. The occurrence of parallel spindles at meiosis II in atps1 thus results in the formation of one plane of division, and gives rise to dyads containing diploid spores instead of a tetrad of haploid spores (Figure 1). Interestingly, AtPS1 is a plant-specific gene that encodes a protein containing an N-terminal Forkhead Associated (FHA) domain. FHA domains are phosphoprotein binding modules and are found in proteins that regulate signal transduction pathways in several different contexts: plant meristem homeostasis, DNA repair, and cell cycle control [14]. The identification of AtPS1 opens a new window on the control of meiosis II, particularly in relation to spindle orientation. The spindle pole body (SPB) and the centrosome are the yeast and animal microtubule organizing centers (MTOCs), respectively. MTOCs nucleate microtubules and orient the spindle during cell division [15]. Within the SPB and centrosome is a pair of centrioles that contain γ-tubulin, the site of nucleation of microtubules [16]. Although γ-tubulin is conserved in eukaryotes, land plants lack a conspicuous MTOC in the form of a centrosome. Nucleation of microtubules occurs in the cortical cytoplasm and on the nuclear envelope from sites along existing microtubules that are marked by γ-tubulin [17]. Microtubule nucleation in plants therefore appears to be spatially distributed within the cell. A number of reports point to γ-tubulin as the plant MTOC, but the precise mechanism underlying orientation of the spindle and its relation to spindle biogenesis during cell division in plants remains to be established [18]. The identification of AtPS1 and its role in controlling spindle orientation during meiosis II represents an important step in understanding the control of spindle positioning in plants, and determination of its mode of action would be of continued interest. Importantly, because unreduced gametes are of agronomic value, the discovery of AtPS1 may also benefit crop biotechnology.