Involvement of Arabidopsis Hexokinase1 in Cell Death Mediated by Myo-Inositol Accumulation.

Programmed cell death (PCD) is essential for several aspects of plant life, including development and stress responses. We recently identified the mips1 mutant of Arabidopsis thaliana, which is deficient for the enzyme catalyzing the limiting step of myo-inositol (MI) synthesis. One of the most striking features of mips1 is the light-dependent formation of lesions on leaves due to salicylic acid (SA)-dependent PCD. Here, we identified a suppressor of PCD by screening for mutations that abolish the mips1 cell death phenotype. Our screen identified the hxk1 mutant, mutated in the gene encoding the hexokinase1 (HXK1) enzyme that catalyzes sugar phosphorylation and acts as a genuine glucose sensor. We show that HXK1 is required for lesion formation in mips1 due to alterations in MI content, via SA-dependant signaling. Using two catalytically inactive HXK1 mutants, we also show that hexokinase catalytic activity is necessary for the establishment of lesions in mips1. Gas chromatography-mass spectrometry analyses revealed a restoration of the MI content in mips1 hxk1 that it is due to the activity of the MIPS2 isoform, while MIPS3 is not involved. Our work defines a pathway of HXK1-mediated cell death in plants and demonstrates that two MIPS enzymes act cooperatively under a particular metabolic status, highlighting a novel checkpoint of MI homeostasis in plants.

Chloroplast Activity and 3'phosphadenosine 5'phosphate Signaling Regulate Programmed Cell Death in Arabidopsis.

Programmed cell death (PCD) is a crucial process both for plant development and responses to biotic and abiotic stress. There is accumulating evidence that chloroplasts may play a central role during plant PCD as for mitochondria in animal cells, but it is still unclear whether they participate in PCD onset, execution, or both. To tackle this question, we have analyzed the contribution of chloroplast function to the cell death phenotype of the myoinositol phosphate synthase1 (mips1) mutant that forms spontaneous lesions in a light-dependent manner. We show that photosynthetically active chloroplasts are required for PCD to occur in mips1, but this process is independent of the redox state of the chloroplast. Systematic genetic analyses with retrograde signaling mutants reveal that 3'-phosphoadenosine 5'-phosphate, a chloroplast retrograde signal that modulates nuclear gene expression in response to stress, can inhibit cell death and compromises plant innate immunity via inhibition of the RNA-processing 5'-3' exoribonucleases. Our results provide evidence for the role of chloroplast-derived signal and RNA metabolism in the control of cell death and biotic stress response.

The BAF60 subunit of the SWI/SNF chromatin-remodeling complex directly controls the formation of a gene loop at FLOWERING LOCUS C in Arabidopsis.

SWI/SNF complexes mediate ATP-dependent chromatin remodeling to regulate gene expression. Many components of these complexes are evolutionarily conserved, and several subunits of Arabidopsis thaliana SWI/SNF complexes are involved in the control of flowering, a process that depends on the floral repressor FLOWERING LOCUS C (FLC). BAF60 is a SWI/SNF subunit, and in this work, we show that BAF60, via a direct targeting of the floral repressor FLC, induces a change at the high-order chromatin level and represses the photoperiod flowering pathway in Arabidopsis. BAF60 accumulates in the nucleus and controls the formation of the FLC gene loop by modulation of histone density, composition, and posttranslational modification. Physiological analysis of BAF60 RNA interference mutant lines allowed us to propose that this chromatin-remodeling protein creates a repressive chromatin configuration at the FLC locus.

The Polyadenylation Factor Subunit CLEAVAGE AND POLYADENYLATION SPECIFICITY FACTOR30: A Key Factor of Programmed Cell Death and a Regulator of Immunity in Arabidopsis.

Programmed cell death (PCD) is essential for several aspects of plant life, including development and stress responses. Indeed, incompatible plant-pathogen interactions are well known to induce the hypersensitive response, a localized cell death. Mutational analyses have identified several key PCD components, and we recently identified the mips1 mutant of Arabidopsis (Arabidopsis thaliana), which is deficient for the key enzyme catalyzing the limiting step of myoinositol synthesis. One of the most striking features of mips1 is the light-dependent formation of lesions on leaves due to salicylic acid (SA)-dependent PCD, revealing roles for myoinositol or inositol derivatives in the regulation of PCD. Here, we identified a regulator of plant PCD by screening for mutants that display transcriptomic profiles opposing that of the mips1 mutant. Our screen identified the oxt6 mutant, which has been described previously as being tolerant to oxidative stress. In the oxt6 mutant, a transfer DNA is inserted in the CLEAVAGE AND POLYADENYLATION SPECIFICITY FACTOR30 (CPSF30) gene, which encodes a polyadenylation factor subunit homolog. We show that CPSF30 is required for lesion formation in mips1 via SA-dependent signaling, that the prodeath function of CPSF30 is not mediated by changes in the glutathione status, and that CPSF30 activity is required for Pseudomonas syringae resistance. We also show that the oxt6 mutation suppresses cell death in other lesion-mimic mutants, including lesion-simulating disease1, mitogen-activated protein kinase4, constitutive expressor of pathogenesis-related genes5, and catalase2, suggesting that CPSF30 and, thus, the control of messenger RNA 3' end processing, through the regulation of SA production, is a key component of plant immune responses.

Dual function of MIPS1 as a metabolic enzyme and transcriptional regulator.

Because regulation of its activity is instrumental either to support cell proliferation and growth or to promote cell death, the universal myo-inositol phosphate synthase (MIPS), responsible for myo-inositol biosynthesis, is a critical enzyme of primary metabolism. Surprisingly, we found this enzyme to be imported in the nucleus and to interact with the histone methyltransferases ATXR5 and ATXR6, raising the question of whether MIPS1 has a function in transcriptional regulation. Here, we demonstrate that MIPS1 binds directly to its promoter to stimulate its own expression by locally inhibiting the spreading of ATXR5/6-dependent heterochromatin marks coming from a transposable element. Furthermore, on activation of pathogen response, MIPS1 expression is reduced epigenetically, providing evidence for a complex regulatory mechanism acting at the transcriptional level. Thus, in plants, MIPS1 appears to have evolved as a protein that connects cellular metabolism, pathogen response and chromatin remodeling.

Multiple functions of Kip-related protein5 connect endoreduplication and cell elongation.

Despite considerable progress in our knowledge regarding the cell cycle inhibitor of the Kip-related protein (KRP) family in plants, less is known about the coordination of endoreduplication and cell differentiation. In animals, the role of cyclin-dependent kinase (CDK) inhibitors as multifunctional factors coordinating cell cycle regulation and cell differentiation is well documented and involves not only the inhibition of CDK/cyclin complexes but also other mechanisms, among them the regulation of transcription. Interestingly, several plant KRPs have a punctuated distribution in the nucleus, suggesting that they are associated with heterochromatin. Here, one of these chromatin-bound KRPs, KRP5, has been studied in Arabidopsis (Arabidopsis thaliana). KRP5 is expressed in endoreduplicating cells, and loss of KRP5 function decreases endoreduplication, indicating that KRP5 is a positive regulator of endoreduplication. This regulation relies on several mechanisms: in addition to its role in cyclin/CDK kinase inhibition previously described, chromatin immunoprecipitation sequencing data combined with transcript quantification provide evidence that KRP5 regulates the transcription of genes involved in cell wall organization. Furthermore, KRP5 overexpression increases chromocenter decondensation and endoreduplication in the Arabidopsis trithorax-related protein5 (atxr5) atxr6 double mutant, which is deficient for the deposition of heterochromatin marks. Hence, KRP5 could bind chromatin to coordinately control endoreduplication and chromatin structure and allow the expression of genes required for cell elongation.

Chromatin meets the cell cycle.

The cell cycle is one of the most comprehensively studied biological processes, due primarily to its significance in growth and development, and its deregulation in many human disorders. Studies using a diverse set of model organisms, including yeast, worms, flies, frogs, mammals, and plants, have greatly expanded our knowledge of the cell cycle and have contributed to the universally accepted view of how the basic cell cycle machinery is regulated. In addition to the oscillating activity of various cyclin-dependent kinase (CDK)-cyclin complexes, a plethora of proteins affecting various aspects of chromatin dynamics has been shown to be essential for cell proliferation during plant development. Furthermore, it was reported recently that core cell cycle regulators control gene expression by modifying histone patterns. This review focuses on the intimate relationship between the cell cycle and chromatin. It describes the dynamics and functions of chromatin structures throughout cell cycle progression and discusses the role of heterochromatin as a barrier against re-replication and endoreduplication. It also proposes that core plant cell cycle regulators control gene expression in a manner similar to that described in mammals. At present, our challenge in plants is to define the complete set of effectors and actors that coordinate cell cycle progression and chromatin structure and to understand better the functional interplay between these two processes.

HSFA1a modulates plant heat stress responses and alters the 3D chromatin organization of enhancer-promoter interactions.

The complex and dynamic three-dimensional organization of chromatin within the nucleus makes understanding the control of gene expression challenging, but also opens up possible ways to epigenetically modulate gene expression. Because plants are sessile, they evolved sophisticated ways to rapidly modulate gene expression in response to environmental stress, that are thought to be coordinated by changes in chromatin conformation to mediate specific cellular and physiological responses. However, to what extent and how stress induces dynamic changes in chromatin reorganization remains poorly understood. Here, we comprehensively investigated genome-wide chromatin changes associated with transcriptional reprogramming response to heat stress in tomato. Our data show that heat stress induces rapid changes in chromatin architecture, leading to the transient formation of promoter-enhancer contacts, likely driving the expression of heat-stress responsive genes. Furthermore, we demonstrate that chromatin spatial reorganization requires HSFA1a, a transcription factor (TF) essential for heat stress tolerance in tomato. In light of our findings, we propose that TFs play a key role in controlling dynamic transcriptional responses through 3D reconfiguration of promoter-enhancer contacts.

Involvement of Arabidopsis BIG protein in cell death mediated by Myo-inositol homeostasis.

Programmed cell death (PCD) is essential for several aspects of plant life. We previously identified the mips1 mutant of Arabidopsis thaliana, which is deficient for the enzyme catalysing myo-inositol synthesis, and that displays light-dependent formation of lesions on leaves due to Salicylic Acid (SA) over-accumulation. Rationale of this work was to identify novel regulators of plant PCD using a genetic approach. A screen for secondary mutations that abolish the mips1 PCD phenotype identified a mutation in the BIG gene, encoding a factor of unknown molecular function that was previously shown to play pleiotropic roles in plant development and defence. Physiological analyses showed that BIG is required for lesion formation in mips1 via SA-dependant signalling. big mutations partly rescued transcriptomic and metabolomics perturbations as stress-related phytohormones homeostasis. In addition, since loss of function of the ceramide synthase LOH2 was not able to abolish cell death induction in mips1, we show that PCD induction is not fully dependent of sphingolipid accumulation as previously suggested. Our results provide further insights into the role of the BIG protein in the control of MIPS1-dependent cell death and also into the impact of sphingolipid homeostasis in this pathway.

Characterization of a phosphatase 2C protein as an interacting partner of the histone acetyltransferase GCN5 in Arabidopsis.

Arabidopsis GCN5 is a major histone acetyltransferase. The mutation of the gene induces pleiotropic effects on plant development, and affects the expression of a large number of genes. The mechanism of action of this protein in controlling plant chromatin structure and genome expression is not understood. In this work, we report the identification of a number of potential protein interacting partners of GCN5 in Arabidopsis. In particular, GCN5 was shown to interact specifically with a phosphatase 2C protein (AtPP2C-6-6). GCN5 phosphorylated by activities in cellular extracts could be dephosphorylated by AtPP2C-6-6 in vitro. Analysis of T-DNA insertion mutants revealed a positive role of AtPP2C-6-6 in salt induction of stress-inducible genes, while the gcn5 mutation seemed to have no effect on the induction but showed up-regulation of a subset of the stress-inducible genes under non-induced conditions. In addition, the gcn5 mutation seriously reduced acetylation of histone H3K14 and H3K27, whereas the T-DNA insertions of the AtPP2C6-6 gene enhanced the acetylation of these lysine residues. Taken together, the present data suggest that AtPP2C-6-6 may function as a negative regulator of GCN5 activity in Arabidopsis.

The MYST histone acetyltransferases are essential for gametophyte development in Arabidopsis.

BACKGROUND: Histone acetyltransferases (HATs) play critical roles in the regulation of chromatin structure and gene expression. Arabidopsis genome contains 12 HAT genes, but the biological functions of many of them are still unknown. In this work, we studied the evolutionary relationship and cellular functions of the two Arabidopsis HAT genes homologous to the MYST family members. RESULTS: An extensive phylogenetic analysis of 105 MYST proteins revealed that they can be divided into 5 classes, each of which contains a specific combination of protein modules. The two Arabidopsis MYST proteins, HAM1 and HAM2, belong to a "green clade", clearly separated from other families of HATs. Using a reverse genetic approach, we show that HAM1 and HAM2 are a functionally redundant pair of genes, as single Arabidopsis ham1 and ham2 mutants displayed a wild-type phenotype, while no double mutant seedling could be recovered. Genetic analysis and cytological study revealed that ham1ham2 double mutation induced severe defects in the formation of male and female gametophyte, resulting in an arrest of mitotic cell cycle at early stages of gametogenesis. RT-PCR experiments and the analysis of transgenic plants expressing the GUS reporter gene under the HAM1 or the HAM2 promoter showed that both genes displayed an overlapping expression pattern, mainly in growing organs such as shoots and flower buds. CONCLUSION: The work presented here reveals novel properties for MYST HATs in Arabidopsis. In addition to providing an evolutionary relationship of this large protein family, we show the evidence of a link between MYST and gamete formation as previously suggested in mammalian cells. A possible function of the Arabidopsis MYST protein-mediated histone acetylation during cell division is suggested.

Regulation of meristem activity by chromatin remodelling.

The continuity and plasticity of plant development rely on the regulation of meristem activity in response to endogenous and environmental signals. Many plant development regulators involved in meristem function are transcription factors or signalling molecules. In the past few years, the role of chromatin remodelling in programming, maintaining or resetting specific gene expression profiles in subsequent cell generations has been shown to be crucial in plant development. Here, we summarize plant chromatin-remodelling factors required to regulate shoot apical meristem activity, particularly its maintenance during organogenesis and transitions between distinct developmental phases.

To die or not to die? Lessons from lesion mimic mutants.

Programmed cell death (PCD) is a ubiquitous genetically regulated process consisting in an activation of finely controlled signaling pathways that lead to cellular suicide. Although some aspects of PCD control appear evolutionary conserved between plants, animals and fungi, the extent of conservation remains controversial. Over the last decades, identification and characterization of several lesion mimic mutants (LMM) has been a powerful tool in the quest to unravel PCD pathways in plants. Thanks to progress in molecular genetics, mutations causing the phenotype of a large number of LMM and their related suppressors were mapped, and the identification of the mutated genes shed light on major pathways in the onset of plant PCD such as (i) the involvements of chloroplasts and light energy, (ii) the roles of sphingolipids and fatty acids, (iii) a signal perception at the plasma membrane that requires efficient membrane trafficking, (iv) secondary messengers such as ion fluxes and ROS and (v) the control of gene expression as the last integrator of the signaling pathways.

The Arabidopsis ortholog of the YEATS domain containing protein YAF9a regulates flowering by controlling H4 acetylation levels at the FLC locus.

Histone acetylation and complexes associated with this process are directly involved in chromatin regulation and gene expression. Among these, NuA4 complex is directly involved in acetylation of histone H4, H2A and H2A.Z. In yeast, the NuA4 complex contains the catalytic subunit, the histone acetyltransferase ESA1, and several associated components including YAF9. In this report we explored the biological role of YAF9a in Arabidopsis thaliana. Homozygous yaf9a-1 and yaf9a-3 mutants show early flowering phenotypes. Moreover, yaf9a-1 mutants displayed reduced expression of the flowering repressor FLC, whereas the expression of the flowering activators FT and SOC1 was induced in comparison to wild-type plants. Using chromatin immunoprecipitation assays with H4 tetra-acetylated antibodies we observed a positive correlation with gene expression profile of FLC and FT in yaf9a-1 mutants under long days. We therefore conclude that YAF9a in Arabidopsis is a negative regulator of flowering by controlling the H4 acetylation levels in the FLC and FT chromatin.

Histone acetyltransferase GCN5 interferes with the miRNA pathway in Arabidopsis.

MicroRNAs (miRNA) that guide sequence-specific posttranscriptional gene silencing play an important role in gene expression required for both developmental processes and responses to environmental conditions in plants. However, little is known about the transcriptional and posttranscriptional regulation of miRNA expression. Histone acetylation plays an important role in chromatin remodeling and is required for gene activation. By analyzing the accumulation of subset of miRNAs and the corresponding primary miRNAs in mutants of Arabidopsis, we show that histone acetyltransferase GCN5 (General control non-repressed protein5) has a general repressive effect on miRNA production, while it is required for the expression of a subset of (e.g. stress-inducible) MIRNA genes. The general negative function of GCN5 in miRNA production is likely achieved through an indirect repression of the miRNA machinery genes such as DICER LIKE1 (DCL1), SERRATE (SE), HYPONASTIC LEAVES1 (HYL1) and ARGONAUTE1 (AGO1). Chromatin immunoprecipitation assays revealed that GCN5 targets to a subset of MIRNA genes and is required for acetylation of histone H3 lysine 14 at these loci. Moreover, inhibition of histone deacetylation by trichostatin A treatment or in histone deacetylase gene mutants impaired the accumulation of certain miRNAs. These data together suggest that Arabidopsis GCN5 interferes with the miRNA pathway at both the transcriptional and posttranscriptional levels and histone acetylation/deacetylation is an epigenetic mechanism involved in the regulation of miRNA production.

MGOUN3: evidence for chromatin-mediated regulation of FLC expression.

The MGOUN3(MGO3)/BRUSHY1(BRU1)/TONSOKU(TSK) gene of Arabidopsis thaliana encodes a nuclear leucine-glycine-asparagine (LGN) domain protein that may be implicated in chromatin dynamics and genome maintenance. Mutants with defects in MGO3 display a fasciated stem and disorganized meristem structures. The transition to flowering was examined in mgo3 mutants and it was found that, under short days, the mutants flowered significantly earlier than the wild-type plants. Study of flowering-time associated gene expression showed that the floral transition inhibitor gene FLC was under-expressed in the mutant background. Ectopic expression of the flower-specific genes AGAMOUS (AG), PISTILLATA (PI), and SEPALLATA3 (SEP3) in mgo3 vegetative organs was also detected. Western blot and chromatin immunoprecipitation experiments suggested that histone H3 acetylation may be altered in the mgo3 background. Together, these data suggest that MGO3 is required for the correct transition to flowering and that this may be mediated by histone acetylation and associated changes in FLC expression.

Arabidopsis HAF2 gene encoding TATA-binding protein (TBP)-associated factor TAF1, is required to integrate light signals to regulate gene expression and growth.

Plant growth and development are sensitive to light. Light-responsive DNA-binding transcription factors have been functionally identified. However, how transcription initiation complex integrates light signals from enhancer-bound transcription factors remains unknown. In this work, we characterized mutations within the Arabidopsis HAF2 gene encoding TATA-binding protein-associated factor TAF1 (or TAF(II)250). The mutation of HAF2 induced decreases on chlorophyll accumulation, light-induced mRNA levels, and promoter activity. Genetic analysis indicated that HAF2 is involved in the pathways of both red/far-red and blue light signals. Double mutants between haf2-1 and hy5-1, a mutation of a light signaling positive DNA-binding transcription factor gene, had a synergistic effect on photomorphogenic traits and light-activated gene expression under different light wavelengths, suggesting that HAF2 is required for interaction with additional light-responsive DNA-binding transcription factors to fully respond to light induction. Chromatin immunoprecipitation assays showed that the mutation of HAF2 reduced acetylation of histone H3 in light-responsive promoters. In addition, transcriptome analysis showed that the mutation altered the expression of about 9% of genes in young leaves. These data indicate that TAF1 encoded by the Arabidopsis HAF2 gene functions as a coactivator capable of integrating light signals and acetylating histones to activate light-induced gene transcription.

MGOUN3, an Arabidopsis gene with TetratricoPeptide-Repeat-related motifs, regulates meristem cellular organization.

In order to understand the functioning of apical meristems in Arabidopsis more clearly, a new mutant, mgoun3 (mgo3), affected in the structural organization and the functional regulation of both shoot and root meristems has been isolated. mgo3 plants display perturbations in leaf morphogenesis, in the spatial and the temporal formation of primordia, and frequent fasciation of the inflorescence stem. Cellular analysis showed that both cellular organization and cell identity patterning are impaired in the mutant meristems. The MGO3 gene has been isolated by positional cloning. The protein deduced from the cDNA sequence contains TetratricoPeptide Repeats (TPR) and Leucine-Rich Repeats (LRR), two motifs that are thought to act in protein-protein interactions. This gene appears to be unique in the Arabidopsis genome. Although the MGO3 protein presents TPR as in the Arabidopsis proteins HOBBIT and SPINDLY, the MGO3 motifs are more similar to those present in LGN-related proteins, which are regulators for some of the asymmetric cell divisions in animal development. These features suggest a key role for MGO3 in meristematic cell divisions and would be of interest for the comparison between plant and animal development.

Arabidopsis histone acetyltransferase AtGCN5 regulates the floral meristem activity through the WUSCHEL/AGAMOUS pathway.

Histone acetyltransferases, which are able to acetylate histone and non-histone proteins, play important roles in gene regulation. Many histone acetyltransferases are related to yeast Gcn5, a component of two transcription regulatory complexes SAGA and ADA. In this work, by characterizing a mutation in the Arabidopsis GCN5 gene (AtGCN5) we studied the regulatory function of this gene in controlling floral meristem activity. We show that in addition to pleiotropic effects on plant development, this mutation also leads to the production of terminal flowers. The flowers show homeotic transformations of petals into stamens and sepals into filamentous structures and produce ectopic carpels. The phenotypes correlate to an expansion of the expression domains within floral meristems of the key regulatory genes WUSCHEL (WUS) and AGAMOUS (AG). These results suggest that AtGCN5 is required to regulate the floral meristem activity through the WUS/AG pathway. This study brings new elements on the elucidation of specific developmental pathways regulated by AtGCN5 and on the control mechanism of meristem regulatory gene expression.