The transcriptional coactivator ANGUSTIFOLIA3 (AN3) stimulates cell proliferation during Arabidopsis thaliana leaf development, but the molecular mechanism is largely unknown. Here, we show that inducible nuclear localization of AN3 during initial leaf growth results in differential expression of important transcriptional regulators, including GROWTH REGULATING FACTORs (GRFs). Chromatin purification further revealed the presence of AN3 at the loci of GRF5, GRF6, CYTOKININ RESPONSE FACTOR2, CONSTANS-LIKE5 (COL5), HECATE1 (HEC1), and ARABIDOPSIS RESPONSE REGULATOR4 (ARR4). Tandem affinity purification of protein complexes using AN3 as bait identified plant SWITCH/SUCROSE NONFERMENTING (SWI/SNF) chromatin remodeling complexes formed around the ATPases BRAHMA (BRM) or SPLAYED. Moreover, SWI/SNF ASSOCIATED PROTEIN 73B (SWP73B) is recruited by AN3 to the promoters of GRF5, GRF3, COL5, and ARR4, and both SWP73B and BRM occupy the HEC1 promoter. Furthermore, we show that AN3 and BRM genetically interact. The data indicate that AN3 associates with chromatin remodelers to regulate transcription. In addition, modification of SWI3C expression levels increases leaf size, underlining the importance of chromatin dynamics for growth regulation. Our results place the SWI/SNF-AN3 module as a major player at the transition from cell proliferation to cell differentiation in a developing leaf.

The survival of plants as sessile organisms depends on their ability to cope with environmental challenges. Of key importance in this regard is the phytohormone abscisic acid (ABA). ABA not only promotes seed dormancy but also triggers growth arrest in postgermination embryos that encounter water stress. This is accompanied by increased desiccation tolerance. Postgermination ABA responses in Arabidopsis thaliana are mediated in large part by the ABA-induced basic domain/leucine zipper transcription factor ABA INSENSITIVE5 (ABI5). Here, we show that loss of function of the SWI2/SNF2 chromatin remodeling ATPase BRAHMA (BRM) causes ABA hypersensitivity during postgermination growth arrest. ABI5 expression was derepressed in brm mutants in the absence of exogenous ABA and accumulated to high levels upon ABA sensing. This effect was likely direct; chromatin immunoprecipitation revealed BRM binding to the ABI5 locus. Moreover, loss of BRM activity led to destabilization of a nucleosome likely to repress ABI5 transcription. Finally, the abi5 null mutant was epistatic to BRM in postgermination growth arrest. In addition, vegetative growth defects typical of brm mutants in the absence of ABA treatment could be partially overcome by reduction of ABA responses, and brm mutants displayed increased drought tolerance. We propose a role for BRM in the balance between growth or stress responses.

Patterning of the floral organs is exquisitely controlled and executed by four classes of homeotic regulators. Among these, the class B and class C floral homeotic regulators are of central importance as they specify the male and female reproductive organs. Inappropriate induction of the class B gene APETALA3 (AP3) and the class C gene AGAMOUS (AG) causes reduced reproductive fitness and is prevented by polycomb repression. At the onset of flower patterning, polycomb repression needs to be overcome to allow induction of AP3 and AG and formation of the reproductive organs. We show that the SWI2/SNF2 chromatin-remodeling ATPases SPLAYED (SYD) and BRAHMA (BRM) are redundantly required for flower patterning and for the activation of AP3 and AG. The SWI2/SNF2 ATPases are recruited to the regulatory regions of AP3 and AG during flower development and physically interact with two direct transcriptional activators of class B and class C gene expression, LEAFY (LFY) and SEPALLATA3 (SEP3). SYD and LFY association with the AP3 and AG regulatory loci peaks at the same time during flower patterning, and SYD binding to these loci is compromised in lfy and lfy sep3 mutants. This suggests a mechanism for SWI2/SNF2 ATPase recruitment to these loci at the right stage and in the correct cells. SYD and BRM act as trithorax proteins, and the requirement for SYD and BRM in flower patterning can be overcome by partial loss of polycomb activity in curly leaf (clf) mutants, implicating the SWI2/SNF2 chromatin remodelers in reversal of polycomb repression. P lant development occurs largely postembryonically (1), and, as a consequence, many cell-fate choices do not take place until long after embryogenesis. One example is flower development; in the rapid-flowering winter annual Arabidopsis the first flowers are formed 1 mo to 1 y after germination (2). Precocious activation of the floral homeotic genes required for flower patterning results in pleiotropic defects including poor seed set and is prevented by chromatin repression, which is faithfully inherited throughout cell divisions until the first flowers are formed (3-7). The repressive chromatin needs to be erased for flower patterning to be initiated in flower primordia. For class B genes, such as APETALA3 (AP3), which are required for correct patterning of the showy petals and the male reproductive organs, activation of gene expression occurs in late stage 2 flower primordia in the cells that will give rise to whorls 2 and 3 of the flower (6). For class C gene AGAMOUS (AG), which is required for patterning both the male and the female reproductive organs, induction is observed in early stage 3 flowers in the cells that will give rise to whorls 3 and 4 (6).The mitotically heritable chromatin repression of AP3 and AG before flower formation is achieved by two polycomb complexes: polycomb repressive complex 1 (PRC1) and PRC2. PRC2 is responsible for trimethylation of lysine 27 of histone H3 (H3K27me3) (7,8). Two putative H3K27 methyltransferases and PRC2 complex components, SWINGER and ...

We analyzed 6,749 lines tagged by the gene trap vector pGA2707. This resulted in the isolation of 3,793 genomic sequences flanking the T-DNA. Among the insertions, 1,846 T-DNAs were integrated into genic regions, and 1,864 were located in intergenic regions. Frequencies were also higher at the beginning and end of the coding regions and upstream near the ATG start codon. The overall GC content at the insertion sites was close to that measured from the entire rice (Oryza sativa) genome. Functional classification of these 1,846 tagged genes showed a distribution similar to that observed for all the genes in the rice chromosomes. This indicates that T-DNA insertion is not biased toward a particular class of genes. There were 764, 327, and 346 T-DNA insertions in chromosomes 1, 4 and 10, respectively. Insertions were not evenly distributed; frequencies were higher at the ends of the chromosomes and lower near the centromere. At certain sites, the frequency was higher than in the surrounding regions. This sequence database will be valuable in identifying knockout mutants for elucidating gene function in rice. This resource is available to the scientific community at http://www.postech.ac.kr/life/pfg/risd.Insertional mutagenesis is one of the most useful methods for analyzing gene function. When foreign DNA is inserted into a gene, it not only creates a mutation but also tags the affected gene, facilitating its isolation and characterization (Azpiroz-Leehan and Feldmann, 1997). Transposons and T-DNA have been used most widely as an insertional mutagen (Mathur et al., 1998;Wisman et al., 1998; Krysan et al., 1999;Parinov et al., 1999;Speulman et al., 1999;Tissier et al., 1999). It is believed that T-DNA insertion is a random event and that the inserted sequences are stable through multiple generations (Azpiroz- Leehan and Feldmann, 1997;Parinov and Sundaresan, 2000). Insertional mutant pools have been constructed in Arabidopsis and used for functional analysis of a number of genes (Feldmann, 1991; Koncz et al., 1992; Azpiroz-Leehan and Feldmann, 1997; Bechtold and Pelletier, 1998; Krysan et al., 1999; Galbiati et al., 2000;Parinov and Sundaresan, 2000; Bouché and Bouchez, 2001;Sessions et al., 2002;Szabados et al., 2002). The procedure for T-DNA insertional mutagenesis has also been applied to rice (Oryza sativa) using the Agrobacterium tumefaciensmediated transformation method (Hiei et al., 1994). Jeon et al. (2000) have reported the construction of over 20,000 T-DNA-tagged rice lines. A T-DNA insertional mutagen can be modified to trap a gene by inserting a reporter gene, such as gus (␤-glucuronidase), next to the T-DNA border (Sundaresan et al., 1995; Jeon et al., 2000;Springer, 2000). Approximately 5% to 10% of the mutagenized lines are GUS positive, demonstrating the efficiency of this gene-trapping system (Chin et al., 1999; Jeon et al., 2000).Completion of the genome sequencing for both Arabidopsis and rice has provided new reverse genetic means for assigning biological functions to sequenced genes (Kumar...

These authors contributed equally to this paper. SUMMARYChromatin remodeling ATPases and their associated complexes can alter the accessibility of the genome in the context of chromatin by using energy derived from the hydrolysis of ATP to change the positioning, occupancy and composition of nucleosomes. In animals and plants, these remodelers have been implicated in diverse processes ranging from stem cell maintenance and differentiation to developmental phase transitions and stress responses. Detailed investigation of their roles in individual processes has suggested a higher level of selectivity of chromatin remodeling ATPase activity than previously anticipated, and diverse mechanisms have been uncovered that can contribute to the selectivity. This review summarizes recent advances in understanding the roles and activities of chromatin remodeling ATPases in plants.

SUMMARYTo cope with a lifetime of exposure to a variety of pathogens, plants have developed exquisite and refined defense mechanisms that vary depending on the type of attacking pathogen. Defense-associated transcriptional reprogramming is a central part of plant defense mechanisms. Chromatin modification has recently been shown to be another layer of regulation for plant defense mechanisms. Here, we show that the RPD3/HDA1-class histone deacetylase HDA19 is involved in the repression of salicylic acid (SA)-mediated defense responses in Arabidopsis. Loss of HDA19 activity increased SA content and increased the expression of a group of genes required for accumulation of SA as well as pathogenesis related (PR) genes, resulting in enhanced resistance to Pseudomonas syringae. We found that HDA19 directly associates with and deacetylates histones at the PR1 and PR2 promoters. Thus, our study shows that HDA19, by modifying chromatin to a repressive state, ensures low basal expression of defense genes, such as PR1, under unchallenged conditions, as well as their proper induction without overstimulation during defense responses to pathogen attacks. Thus, the role of HDA19 might be critical in preventing unnecessary activation and self-destructive overstimulation of defense responses, allowing successful growth and development.

SummaryCREB-binding protein (CBP) and its homolog p300 possess histone acetyltransferase activity and function as key transcriptional co-activators in the regulation of gene expression that controls differentiation and development in animals. However, the role of CBP/p300-like genes in plants has not yet been elucidated. Here, we show that Arabidopsis CBP/p300-like genes promote flowering by affecting the expression of a major floral repressor FLOWERING LOCUS C (FLC). Although animal CBP and p300 generally function as co-activators, Arabidopsis CBP/p300-like proteins are required for the negative regulation of FLC. This CBP/p300-mediated FLC repression may involve reversible protein acetylation independent of histone modification within FLC chromatin.

Development of stomata, valves on the plant epidermis for optimal gas exchange and water control, is fine-tuned by multiple signaling peptides with unique, overlapping, or antagonistic activities. EPIDERMAL PATTERNING FACTOR1 (EPF1) is a founding member of the secreted peptide ligands enforcing stomatal patterning. Yet, its exact role remains unclear. Here, we report that EPF1 and its primary receptor ERECTA-LIKE1 (ERL1) target MUTE, a transcription factor specifying the proliferation-to-differentiation switch within the stomatal cell lineages. In turn, MUTE directly induces ERL1. The absolute co-expression of ERL1 and MUTE, with the co-presence of EPF1, triggers autocrine inhibition of stomatal fate. During normal stomatal development, this autocrine inhibition prevents extra symmetric divisions of stomatal precursors likely owing to excessive MUTE activity. Our study reveals the unexpected role of self-inhibition as a mechanism for ensuring proper stomatal development and suggests an intricate signal buffering mechanism underlying plant tissue patterning.DOI: http://dx.doi.org/10.7554/eLife.24102.001