The Role of Epigenetic Processes in Controlling Flowering Time in Plants Exposed to Stress

Plants interact with their environment by modifying gene expression patterns. One mechanism for this interaction involves epigenetic modifications that affect a number of aspects of plant growth and development. Thus, the epigenome is highly dynamic in response to environmental cues and developmental changes. Flowering is controlled by a set of genes that are affected by environmental conditions through an alteration in their expression pattern. This ensures the production of flowers even when plants are growing under adverse conditions, and thereby enhances transgenerational seed production. In this review recent findings on the epigenetic changes associated with flowering in Arabidopsis thaliana grown under abiotic stress conditions such as cold, drought, and high salinity are discussed. These epigenetic modifications include DNA methylation, histone modifications, and the production of micro RNAs (miRNAs) that mediate epigenetic modifications. The roles played by the phytohormones abscisic acid (ABA) and auxin in chromatin remodelling are also discussed. It is shown that there is a crucial relationship between the epigenetic modifications associated with floral initiation and development and modifications associated with stress tolerance. This relationship is demonstrated by the common epigenetic pathways through which plants control both flowering and stress tolerance, and can be used to identify new epigenomic players.


Introduction
Plants can adapt their growth and developmental processes in response to environmental conditions. Under stress conditions such as drought, high salt, high temperature, and high light intensity, physiological processes are induced to reduce the cellular damage caused by stress and, at the same time, alter developmental timing to complete their life cycle in a timely manner. Plants that experience stress transition to reproductive development earlier than non-stress-treated plants, typically at the expense of decreased seed number that allows for some seed production to occur during periods of environmental stress. Stress environmental factors that induce flowering have been discussed thoroughly in a previous review . Most notably, salicylic acid, which usually induces defence genes, also induces early flowering under UV-C light stress, presumably by interacting with key floral transcription repressors such as FLOWERING LOCUS T (FT) and other components of the autonomous flowering pathway (Martinez et al., 2004;. However, evidence of physical interaction is not yet available. Similarly, high temperature stress induces flowering, a process mediated by FLOWERING LOCUS M (FLM) and FT, which integrates input from CONSTANS and other floral inductive pathways (Blazquez et al., 2003;Balasubramanian et al., 2006), while stress associated with low nitrate availability induces early flowering through a novel pathway (Castro Marín et al., 2010).
The effect of stress on flowering time can be ascribed, in part, to induced changes in the epigenome. Epigenetics refers to heritable, self-perpetuating changes in gene activities that are not caused by changes in nucleotide sequence and are associated with chemical modifications of chromatin (Bonasio et al., 2010). These modifications take place in the nucleosome at different levels through reversible biochemical reactions that include DNA methylation and histone tail modifications. DNA methylation occurs by covalently adding a methyl group to cytosine of the DNA backbone, while histone modifications occur when specific lysine or arginine residues within the amino acid terminal tail of histones are post-translationally modified either by acetylation (Grunstein, 1997), ADP-ribosylation (Tanigawa et al., 1984), glycosylation (Cervantes-Laurean et al., 1996), methylation (Zhang and Reinberg, 2001), phosphorylation (Lo et al., 2001), ubiquitination (Sridhar et al., 2007), or SUMOylation (David et al., 2002;Miller et al., 2010). Typically, DNA methylation leads to reduced gene expression, whereaas histone modifications are more complicated and can lead to various gene expression alterations depending on the modification (Richards and Elgin, 2002). More recently, it was discovered that small non-coding RNA plays an important role in regulating gene expression by specifying DNA methylation patterns (Matzke and Birchler, 2005). Each of these types of modifications have been involved in modulating flowering, and many of these are associated with response to abiotic and biotic stress.
Plants that are adapted to extreme environments have the capacity to cope with adverse environmental cues with minimum cellular damage. Eukaryotic cells respond to the environment by modifying their gene expression profiles, a process which usually involves specific chromatin modifications. The correlation between epigenetic changes in plants and stress tolerance has been previously discussed (Boyko and Kovalchuk, 2008;Chinnusamy and Zhu, 2009;Alvarez et al., 2010;. Chromatin modifications both influence and are influenced by other responses to abiotic and biotic stresses. DNA methylation, environmental stress, and the flowering process DNA methylation (5-methylcytosine) can account for >30% of the cytosine (CpG) residues in plants (Gruenbaum et al., 1981a(Gruenbaum et al., , 1981b and >60% in mammals (Gruenbaum et al., 1981b;Razin et al., 1984). In Arabidopsis, DNA methylation is often associated with gene repression (Zilberman et al., 2007), although some reports show a weak relationship between the hypermethylation status of the genes and their expression level. For example, Vaillant et al. (2006) showed that expression levels of the Arabidopsis 5S rRNA gene repeats can be increased in the MORPHEUS' MOLECULE 1 (MOM1) mutant lines despite the presence of the same amount of DNA methylation (Vaillant et al., 2006). In addition, recent evidence showed that short-term heat stress induced the expression of genes in heterochromatin that contain transcriptionally inert non-coding repeated DNA. This occurs despite the presence of epigenetic modifications that are typically associated with repression of gene expression such as a high level of DNA methylation and histone deacetylation, suggesting that the effect can vary depending on circumstance (Pecinka et al., 2010;Tittel-Elmer et al., 2010). DNA methylation is often a prerequisite for gene silencing via methylation directed by small RNAs. In particular, flowering time in Arabidopsis can be controlled by silencing of the homeodomain floral transcription factor FWA by de novo methylation of a specific region within the 5' end of the transcribed region. Once FWA is methylated, small interfering RNA (siRNA) is more efficient at directing further methylation to the locus, which subsequently enhances stable gene silencing (Chan et al., 2006).
The Arabidopsis genome encodes 12 methyl-CpG-binding domain (MBD) proteins, which function with chromatin remodelling proteins to inactive gene expression (Berg et al., 2003;Springer and Kaeppler, 2005). These proteins interact with other proteins to form chromatin modifier complexes. For example, AtMBD7 interacts with arginine methyltransferase (PRMT11) (Scebba et al., 2007), and the AtMBD5-AtMBD7 proteins co-localize in vivo and bind in vitro to the DECREASE IN DNA METHYLATION 1 (DDM1) protein (Zemach et al., 2008). Loss-of-function studies showed that plants with mutations in AtMBD9 display a pleiotropic phenotype that leads to a decrease in histone acetylation and an increase in DNA methylation at the FLC locus (Peng et al., 2006;Yaish et al., 2009). As a result, transcript levels for FLC decline, resulting in an early flowering phenotype. Prolonged exposure of some plant species to cold conditions or vernalization induces flowering, a process which is widely considered as a non-stress condition. AtMBD8 has been shown to control flowering in the Arabidopsis vernalization-responsive C24 ecotype. Mutation of AtMBD8 leads to a delay in flowering under both longand short-day photoperiods. While FLC expression is not affected in atmbd8-1, the expression of FT and SOC1, which are major flowering promoters, is down-regulated in the mutant (Stangeland et al., 2009). The mechanism by which the expression of these genes is decreased in the atmbd8-1 mutant has not yet been determined. Global gene expression analysis revealed that the C24 ecotype differentially expresses a set of biotic and abiotic stress-related genes during the vegetative stage compared with the Columbia ecotype. This finding may highlight a relationship between the flowering process and stress response, although the direct role of MBD proteins in the stress tolerance phenotype has not been determined.
The level of CpG methylation in Arabidopsis is controlled by the METHYTRANSFERASE1 (MET1), MET2, and MET3 genes (Henderson and Jacobsen, 2007), which are homologous to the (Dnmt1) mammalian DNA methyltransferase. In addition, the methylation level is also affected by the CHROMOMETHYLASE3 (CMT3) DNA METHYL-TRANSFERASE (Lindroth et al., 2001), which helps maintain DNA methylation at CpNpG and CpNpN sites. The met1-6 mutation leads to late flowering, and met1 and cmt3 mutants exhibit improper embryo development, cell division, seed viability, and abnormal auxin gradient (Xiao et al., 2006). Moreover, loss of DNA methylation reduces the ability of Arabidopsis plants to tolerate salt stress conditions. Loss-of-function met1-3 mutants are hypersensitive to salt stress due to a major loss in cytosine methylation in a putative small RNA target region that lowers the expression of the sodium transporter gene (AtHKT1), which is essential for salt tolerance (Baek et al., 2011). In rice, drought stress increases DNA methylation in a genotypic-specific fashion and only 70% of the total changes in DNA methylation are reset even after recovery in non-drought conditions . In addition, temperature stress modulates the flowering pattern by reducing the number of spikelets and overall fertility at anthesis in some rice genotypes (Jagadish et al., 2007).
Treating plants with the cytosine methyltransferase inhibitor 5-azacytidine (5-azaC) (Jones, 1985;Haaf, 1995) promotes flowering in the vernalization-requiring Arabidopsis ecotype C24 (Burn et al., 1993a;Dennis et al., 1998;Finnegan et al., 1998a) and in wheat (Brock and Davidson, 1994). This treatment was sufficient to substitute for the vernalization process. Reducing the amount of DNA methylation can also be achieved by genetically manipulating the enzymes that catalyse DNA methylation and demethylation in the cell. Alterations in DNA methylation levels show inconsistent effects on observed phenotypes. Reverse genetic studies have shown that mutations within the genes DE-CREASE IN DNA METHYLATION1 (DDM1) and DDM2, the DNA METHYLTRANSFERASE1 gene (MET1) (Vongs et al., 1993;Kakutani et al., 1996;Jackson et al., 2004), and the DNA demethylase gene, REPRESSOR OF SILENCING GENES1 (ROS1) (Agius et al., 2006), affect the global level of cytosine methylation and lead to some developmental abnormalities in Arabidopsis including changes in flowering time. The effect of DNA methylation level on Arabidopsis phenotype depends on the Arabidopsis ecotype studied. For example, reduction of DNA methylation in ddm1 and met1-1 mutant lines causes late flowering in Columbia and Landsberg erecta ecotypes (Kakutani et al., 1996;Kakutani, 1997;Kankel et al., 2003). However, low DNA methylation induced by vernalization or 5-azaC treatment promoted flowering in the vernalization-responsive Arabidopsis ecotype C24 (Burn et al., 1993b;Finnegan et al., 1998b). In other plant species such as the long-day plant Silene armeria, whose flowering state is photoperiodically stable, and the short-day plant Pharbitis nil, whose flowering state is photoperiodically unstable, DNA demethylation using 5azaC induced flowering under non-inductive photoperiod. However, floral induction of other species such as Xanthium strumarium and Lemna paucicostata could not be achieved using similar agents under the same conditions (Kondo et al., 2007).

Histone modifications associated with environmental stress and flowering
The role of histone acetylation and methylation in controlling eukaryotic gene expression was first described in 1964 (Allfrey et al., 1964). Trichostatin A (TSA) has been used to reduce histone deacetylation globally and, consequently, leads to an increase in acetylated histones. However, TSA has negative pleiotropic effects since increased acetylation has been shown to lead to impaired sister chromatid separation in human fibroblasts (Cimini et al., 2003) and also induces chromosomal abnormalities during tobacco cell division (Li et al., 2005). In addition, increasing histone acetylation activity in Arabidopsis by expressing antisense of the histone deacetylase gene AtHD1 results in pleiotropic phenotypes with various developmental defects including ectopic expression of silenced genes, suppression of apical dominance, heterochronic shift toward juvenility, floral structure abnormalities, and male and female sterility (Tian and Chen, 2001).
Similar to DNA methylation, histone modification is regulated by environmental conditions (Boyko and Kovalchuk, 2008;Kim et al., 2010). Recently, global gene expression analysis coupled with chromatin immunoprecipitation (ChIP) assays showed that histone H3 Lys4 methylation (H3K4) patterns respond dynamically to dehydration stress in Arabidopsis (van Dijk et al., 2010). One example of the interplay between environmental stressors and flowering is demonstrated in studies of the floral initiator SHK1 KINASE BINDING PROTEIN1 (SKB1) mutant line skb1. SKB1 binds to chromatin and increases the histone 4 Arg3 (H4R3) symmetric dimethylation (H4R3sme2) level, which in turn leads to the down-regulation of FLC expression as well as a number of stress-responsive genes. As a result, the phenotypes present in this mutant exhibit salt hypersensitivity, late flowering, and growth retardation (Zhang et al., 2011). In the same study, H4R3sme2 expression is reduced in wild-type Arabidopsis plants that are exposed to high salinity conditions, allowing for the release of SKB1 from chromatin and therefore enhancing the expression of FLC and other stress-induced genes.
The histone deacetylase HDA6 of Arabidopsis is involved in modulating seed germination and salt stress as well as the abscisic acid (ABA) response. HDA6 RNA interfering lines are hypersensitive to salt and ABA . Along with phytochrome B, HDA6 regulates the global chromatin organization in some Arabidopsis genotypes that are typically grown in different geographical latitudes (Tessadori et al., 2009). Quantitative trait locus (QTL) mapping based on relative heterochromatin fraction (RHF) analysis and microscopic examination showed that HDA6 controls chromatin remodelling capacity, which also depends on light intensity, a factor that usually regulates flowering time, thereby providing evidence of direct involvement of HDA6 in environmental adaptation. Similarly, hda19-1 mutants are hypersensitive to salt and ABA, and the expression of ABA-responsive genes, ABI1, ABI2, KAT1, KAT2, and RD29B, is reduced in this mutant .
The Arabidopsis histone deacetylase 2 gene (AtHD2C) is highly expressed in ovules, embryos, shoot apical meristems, and primary leaf tissues (Sridha and Wu, 2006). The same study showed that AtHD2C is repressed by ABA, and transgenic lines overexpressing AtHD2C showed an ABA insensitivity phenotype where the expression of several ABA-related genes is affected. In addition, transgenic lines exhibited a reduction in the transpiration rate and enhanced tolerance to salt and drought stresses.
In Brassica napus, the putative transcription factor harbouring a kinase-inducible domain bnKCP1 interacts with HDA19, through which it controls expression of the gene. The bnKCP1 gene, which is induced by cold and highly expressed in flowers, may have a transcriptional regulatory role in cold stress (Gao et al., 2003). Interestingly, overexpression of HDA19 resulted in increased expression of jasmonic acid (JA)-and ethylene-regulated pathogenesis-related genes such as the ETHYLENE RE-SPONSE FACTOR1, basic chitinase, and b-1,3-glucanase. Moreover, these overexpression lines are more resistant to the pathogen Alternaria brassicicola but also displayed late flowering and a reduction in seed fertility (Zhou et al., 2005). Therefore, HDA19 interconnects the hormone response to pathogen pathways and floral induction through a common epigenetic mechanism.
The Arabidopsis acetyltransferase GENERAL CON-TROL NON-REPRESSED PROTEIN5 (AtGCN5) (Stockinger et al., 2001) is a major histone acetyltransferase in Arabidopsis. Mutation within the coding region of this gene causes pleiotropic effects on plant development and also leads to impaired floral production where petals are transformed into stamens and sepals into filamentous like-structures (Bertrand et al., 2003). It was also found that gcn5 mutants have altered expression of a large number of genes, including those involved in floral initiation and development as well as those associated with stress tolerance. The transcriptional co-activators ADA2a and ADA2b are components of AtGCN5-containing complexes in Arabidopsis (Stockinger et al., 2001). Mutation of ADA2a leads to delayed flowering and fruit setting and to the production of shorter inflorescences. Recently it was shown that ADA2b positively regulates salt-induced genes by maintaining the required acetylation level of histones H4 and H3, with the ada2b-1 mutant being hypersensitive to salt and ABA (Hark et al., 2009;Kaldis et al., 2010). Interestingly, the gcn5-1 mutant line also displays an ABA hypersensitivity phenotype (Hark et al., 2009), indicating that AtGCN5 and ADA2b integration are important for proper ABA response in Arabidopsis. SGF29A-1 is another component of the AtGCN5 complex that helps control floral initiation. Compared with wildtype plants, the sgf29a-1 mutant displays late flowering, and smaller and fewer rosette leaves. Unlike the ada2b-1 mutant, sgf29a-1 displays enhanced salt tolerance compared with the wild type (Kaldis et al., 2010).
In addition to its conventional role in controlling gene expression by acetylating and deacetylating specific histones, a recent study showed that AtGCN5 is also involved in the production of microRNAs (miRNAs), including those induced by environmental stress (W Kim et al., 2009). These results demonstrate a diverse role for the AtGCN5 complex in controlling the expression of stressand flowering-related genes by tightly controlling the histone acetylation levels of their loci. Hence AtGCN5 represents a central point in the relationship between histone modification and miRNA production, which is discussed below.

Cold treatment affects flowering via epigenetic modifications
As was noted earlier, exposure to cold conditions for a sufficient time induces flowering in some plant species through the vernalization process and is a comparatively well studied example of how cold induces epigenetic changes that in turn affect flowering. In addition to vernalization, which is crucial to induce flowering in some plant species by modulating the expression of certain genes, cold stress also modulates the expression of some genes, including those involving chromatin modulation. For example, global gene expression analysis of cold-stressed Arabidopsis showed up-regulation of some epigenetic modifiers such as NRPD1, which is a DNA-binding bromodomain-containing protein, AtGCN5-related GNAT family 5 (acetyltransferase 5), and histone deacetylase (Lee et al., 2005). In general, low temperature often has been shown to be associated with DNA demethylation in Arabidopsis and other plant species such as maize (Steward et al., 2002), Antirrhinum majus (Hashida et al., 2003), and wheat (Sherman and Talbert, 2002). Vernalization down-regulates the expression of FLC, a MADS box transcriptional repressor that maintains the vegetative stage in Arabidopsis apices. Thus, epigenetic changes at the FLC locus accelerate flowering (DH Kim et al., 2009). Trimethylation of Lys27 of H3 histones (H3K27me3) is crucial for the regulation of some genes that are involved in plant development, including those that control flowering time in Arabidopsis. Interestingly, the same genes are also affected by vernalization (Shindo et al., 2006;Finnegan and Dennis, 2007;Greb et al., 2007). Likewise, H3K27me3 decreases the expression of the floral regulators AGL19, FT, and AGAMOUS (Schonrock et al., 2006;Schubert et al., 2006;Saleh et al., 2007;Jiang et al., 2008).
Exposure to cold induces expression of the Arabidopsis VERNALIZATION INSENSITIVE 3 gene (VIN3), a chromatin remodelling plant homeodomain (PHD) finger protein that increases acetylation levels. This protein is required to repress FLC and enhance flowering. Mutant lines for VIN3 do not respond to vernalization and therefore remain in a vegetative state longer because FLC expression is not reduced by cold treatment (Sung and Amasino, 2004). Increased H3K27me3 levels at FLC after vernalization are due to a reaction mediated by the Polycomb-group Repressive Complex 2 (PRC2) (De Lucia et al., 2008). This complex binds to chromatin of the VIN3 locus during vernalization (Schonrock et al., 2006). In contrast, a decrease in H3K27me3 modifications within histones of the coldresponsive gene COR15A and the GALACTINOL SYN-THASE gene ATGOLS3 (Taji et al., 2002) leads to increased gene expression in Arabidopsis (Kwon et al., 2009). Likewise, the plant trithorax factor (ATX1) (Alvarez-Venegas et al., 2003) tri-methylates Lys4 residues of histone H3 (H3K4me3), thereby regulating floral organ development and modulating expression of transcription factor WRKY70 during dehydration stress Ndamukong et al., 2010). Mutation of ATX1 causes major defects in the floral architecture (Alvarez-Venegas et al., 2003).

Small RNA production is associated with environmental stress tolerance and flowering
Flowering in Arabidopsis is induced through several pathways including autonomous, gibberellic acid, photoperiod, and vernalization. Global gene expression analysis of Arabidopsis harbouring defective genes in the photoperiod signal pathway and the pathway integrator genes suggests a critical involvement of miRNAs in mediating the effects of floral induction (Schmid et al., 2003).
miRNAs that control gene expression at a posttranscriptional level are encoded by 20-24 nucleotides that are not translated. These small RNA molecules are also able to direct DNA methylation to a particular locus by an RNA-directed DNA methylation (RdDM) process (Matzke et al., 2001(Matzke et al., , 2007Pikaard, 2006). miRNAs have been shown to control the expression of some genes when plants are exposed to biotic (Madlung and Comai, 2004;Ruiz-Ferrer and Voinnet, 2009;Covarrubias and Reyes, 2010) and abiotic stress (Madlung and Comai, 2004;Hirayama and Shinozaki, 2010;Urano et al., 2010). This is often accompanied by a reprogramming of genes associated with floral initiation and development.
Environmental cues regulate the expression of miRNAs in plants. For example, stress-inducible miRNAs and their predicted targets were identified in Arabidopsis and found to be conserved among other plant species (Sunkar and Zhu, 2004). In rice, global expression analysis revealed a crucial role for miRNAs in controlling gene expression when plants are exposed to stress conditions such as cold, drought, high salt, and ABA treatment (Shen et al., 2010). A study of lossof-function mutations in the miRNA biogenesis machinery DICER-LIKE 1-4 genes (DCL1, DCL2, DCL3, and DCL4) revealed a predominant epigenetic role for miRNAs in controlling gene expression in Arabidopsis (Laubinger et al., 2010). In addition to conventional mechanisms of gene repression via binding of miRNA to the target genes, the same study showed that DCL1 is involved in a process that leads to the repression of a subset of transposons by enhancing DNA methylation. Because they control the production of miRNA, DCL2 and DCL3 proteins are believed to direct the transgenerational memory of stress in plants (Boyko and Kovalchuk, 2010).
The relationship between miRNA biogenesis machinery proteins, stress response, and flowering is clear in some Arabidopsis mutant lines. For example, ABH1 and CBP20 (Papp et al., 2004) encode cap-binding factors that are necessary for RNA maturation. The abh1 mutant displays ABA hypersensitivity and the cbp20 mutant line shows enhanced drought tolerance as well as ABA hypersensitivity (Hugouvieux et al., 2001;Kwak et al., 2005). The abh1 mutant also displays an early flowering phenotype due to the production of an alternative form of mRNAs for the key flowering time genes CONSTANS, FLC, and FLM compared with the wild type (Kuhn et al., 2007). The STRESS RESPONSE SUPPRESSOR1 and 2 (STRS1 and 2) genes code for DEAD-box RNA helicases that are suppressed when plants are exposed to salt and osmotic stress conditions. Mutant lines for these genes display higher tolerance than the wild type (Kant et al., 2007). Once again, the relationship between stress conditions and flowering is clear in this example as the strs mutants showed slightly early flowering, perhaps suggesting a common epigenetic pathway in controlling both mechanisms.
In addition to their role in environmental stress responses (Sunkar and Zhu, 2004), miRNAs are also involved in controlling flowering in Arabidopsis (Aukerman and Sakai, 2003;Chen, 2004). Interestingly, recent evidence showed that H3K27me3 at FLC is mediated by a long intronic noncoding RNA (COLDAIR). The association of COLDAIR triggers PRC2 targeting to FLC, a situation which leads to FLC repression during vernalization (Heo and Sung, 2010). Expression of FLC is partially controlled by miRNAs since mutations within miRNA biogenesis genes DCL1 and DCL3 lead to delayed flowering due to excessively high expression of FLC in these mutant backgrounds (Schmitz et al., 2007). Late flowering phenotypes were also observed in HYPONASTIC LEAVES 1 (HYL1) mutant lines (Lu and Fedoroff, 2000). This gene encodes a double-stranded RNA (dsRNA)-binding protein that also plays a role in miRNA-mediated gene regulation (Han et al., 2004). Recent studies also show that, in addition to hyl1, mutants within the miRNA biogenesis factors SERRATE (SE), DCL1, HUA-ENHANCER 1 (HEN1), and HASTY of Arabidopsis display a salt and ABA hypersensitivity phenotype (Lu and Fedoroff, 2000;Han et al., 2004;Zhang et al., 2008;Rasia et al., 2010).
miRNAs are important regulators of ABA and salt tolerance genes; sets of these molecules were identified recently and found to have roles in stress tolerance in different plant species such as Arabidopsis (Liu et al., 2008), rice (Zhao et al., 2007), and maize (Ding et al., 2009). For example, miR159, which controls the expression of MYB101 and MYB33 transcription factors by mediating their cleavage and is also involved in floral development (Reyes and Chua, 2007), and miR160, which controls floral morphology by modulating the expression of an AUXIN RESPONSE FACTOR 10 (ARF10) (Liu et al., 2007), are also potential ABA regulatory miRNA molecules and are induced by ABA.
Plants interact with their environment and accordingly modify their flowering programmes. Recent research shows that plants use common and parallel epigenetic modification pathways in order to modify the expression of genes that are involved in stress tolerance and flowering processes. These modifications are associated with changes in DNA methylation, histone modifications such as acetylation and methylation, and also the production of specific miRNA molecules. Together these changes underlie intricate mechanisms that ensure plant survival and optimize reproductive success under a variety of stress conditions. Information derived from epigenomic profiles of plants exposed to abiotic stresses is highly important for the production of genetically fertile crop species that can tolerate a warmer globe.