Polycomb Repressive Complex 2 and KRYPTONITE regulate pathogen-induced programmed cell death in Arabidopsis

Hop stunt viroid infection induces heterochromatin reorganization

Viroids are pathogenic noncoding RNAs that completely rely on their host molecular machinery to accomplish their life cycle. Several interactions between viroids and their host molecular machinery have been identified, including interference with epigenetic mechanisms such as DNA methylation. Despite this, whether viroids influence changes in other epigenetic marks such as histone modifications remained unknown. Epigenetic regulation is particularly important during pathogenesis processes because it might be a key regulator of the dynamism of the defense response. Here we have analyzed the changes taking place in Cucumis sativus (cucumber) facultative and constitutive heterochromatin during hop stunt viroid (HSVd) infection using chromatin immunoprecipitation (ChIP) of the two main heterochromatic marks: H3K9me2 and H3K27me3. We find that HSVd infection is associated with changes in both H3K27me3 and H3K9me2, with a tendency to decrease the levels of repressive epigenetic marks through infection progression. These epigenetic changes are connected to the transcriptional regulation of their expected targets, genes, and transposable elements. Indeed, several genes related to the defense response are targets of both epigenetic marks. Our results highlight another host regulatory mechanism affected by viroid infection, providing further information about the complexity of the multiple layers of interactions between pathogens/viroids and hosts/plants.

Identification of epigenetically regulated genes involved in plant-virus interaction and their role in virus-triggered induced resistance

BACKGROUND

Plant responses to a wide range of stresses are known to be regulated by epigenetic mechanisms. Pathogen-related investigations, particularly against RNA viruses, are however scarce. It has been demonstrated that Arabidopsis thaliana plants defective in some members of the RNA-directed DNA methylation (RdDM) or histone modification pathways presented differential susceptibility to the turnip mosaic virus. In order to identify genes directly targeted by the RdDM-related RNA Polymerase V (POLV) complex and the histone demethylase protein JUMONJI14 (JMJ14) during infection, the transcriptomes of infected mutant and control plants were obtained and integrated with available chromatin occupancy data for various epigenetic proteins and marks.

RESULTS

A comprehensive list of virus-responsive gene candidates to be regulated by the two proteins was obtained. Twelve genes were selected for further characterization, confirming their dynamic regulation during the course of infection. Several epigenetic marks on their promoter sequences were found using in silico data, raising confidence that the identified genes are actually regulated by epigenetic mechanisms. The altered expression of six of these genes in mutants of the methyltransferase gene CURLY LEAF and the histone deacetylase gene HISTONE DEACETYLASE 19 suggests that some virus-responsive genes may be regulated by multiple coordinated epigenetic complexes. A temporally separated multiple plant virus infection experiment in which plants were transiently infected with one virus and then infected by a second one was designed to investigate the possible roles of the identified POLV- and JMJ14-regulated genes in wild-type (WT) plants. Plants that had previously been stimulated with viruses were found to be more resistant to subsequent virus challenge than control plants. Several POLV- and JMJ14-regulated genes were found to be regulated in virus induced resistance in WT plants, with some of them poisoned to be expressed in early infection stages.

CONCLUSIONS

A set of confident candidate genes directly regulated by the POLV and JMJ14 proteins during virus infection was identified, with indications that some of them may be regulated by multiple epigenetic modules. A subset of these genes may also play a role in the tolerance of WT plants to repeated, intermittent virus infections.

Expression of Genes Involved in Anthracnose Resistance in Chili (Capsicum baccatum) 'PBC80'-Derived Recombinant Inbred Lines

Chili anthracnose has long been a threat to chili production worldwide. Capsicum baccatum 'PBC80' has been identified as a source of resistance to anthracnose. Recently, a QTL for ripe fruit resistance from 'PBC80'-derived RILs was located on chromosome 4 (123 Mb) and contained over 80 defense-related genes. To identify the genes most related to anthracnose resistance, a fine map of the QTL region was developed using single-marker analysis. Nine genes were selected from the new QTL (1.12 Mb) to study their expression after being challenged with Colletotrichum scovillei 'MJ5' in two different RIL genotypes (Resistance/Resistance or R/R and Susceptible/Susceptible or S/S) at 0, 6 and 12 h. Of the nine genes, LYM2, CQW23_09597, CLF, NFXL1, and PR-14 were significantly up-regulated, compared to the control, in the R/R genotype. ERF was up-regulated in both chili genotypes. However, the expression was relatively and constantly low in the S/S genotype. Most up-regulated genes reached the highest peak (2.3-4.5 fold) at 6 h, except for ERF, which had the highest peak at 12 h (6.4 fold). The earliest and highest expressed gene was a pathogen receptor, LYM2.

Epigenetic regulation of plant immunity: from chromatin codes to plant disease resistance

Facing a deteriorating natural environment and an increasing serious food crisis, bioengineering-based breeding is increasing in importance. To defend against pathogen infection, plants have evolved multiple defense mechanisms, including pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI). A complex regulatory network acts downstream of these PTI and ETI pathways, including hormone signal transduction and transcriptional reprogramming. In recent years, increasing lines of evidence show that epigenetic factors act, as key regulators involved in the transcriptional reprogramming, to modulate plant immune responses. Here, we summarize current progress on the regulatory mechanism of DNA methylation and histone modifications in plant defense responses. In addition, we also discuss the application of epigenetic mechanism-based resistance strategies in plant disease breeding.

Databases and prospects of dynamic gene regulation in eukaryotes: A mini review

In eukaryotes, dynamic regulation enables DNA polymerases to catalyze a variety of RNA products in spatial and temporal patterns. Dynamic gene expression is regulated by transcription factors (TFs) and epigenetics (DNA methylation and histone modification). The applications of biochemical technology and high-throughput sequencing enhance the understanding of mechanisms of these regulations and affected genomic regions. To provide a searchable platform for retrieving such metadata, numerous databases have been developed based on the integration of genome-wide maps (e.g., ChIP-seq, whole-genome bisulfite sequencing, RNA-seq, ATAC-seq, DNase-seq, and MNase-seq data) and functionally genomic annotation. In this mini review, we summarize the main functions of TF-related databases and outline the prevalent approaches used in inferring epigenetic regulations, their associated genes, and functions. We review the literature on crosstalk between TF and epigenetic regulation and the properties of non-coding RNA regulation, which are challenging topics that promise to pave the way for advances in database development.

Histone modification and chromatin remodeling in plant response to pathogens

As sessile organisms, plants are constantly exposed to changing environments frequently under diverse stresses. Invasion by pathogens, including virus, bacterial and fungal infections, can severely impede plant growth and development, causing important yield loss and thus challenging food/feed security worldwide. During evolution, plants have adapted complex systems, including coordinated global gene expression networks, to defend against pathogen attacks. In recent years, growing evidences indicate that pathogen infections can trigger local and global epigenetic changes that reprogram the transcription of plant defense genes, which in turn helps plants to fight against pathogens. Here, we summarize up plant defense pathways and epigenetic mechanisms and we review in depth current knowledge's about histone modifications and chromatin-remodeling factors found in the epigenetic regulation of plant response to biotic stresses. It is anticipated that epigenetic mechanisms may be explorable in the design of tools to generate stress-resistant plant varieties.

Transcriptional regulation of plant innate immunity

Transcriptional reprogramming is an integral part of plant immunity. Tight regulation of the immune transcriptome is essential for a proper response of plants to different types of pathogens. Consequently, transcriptional regulators are proven targets of pathogens to enhance their virulence. The plant immune transcriptome is regulated by many different, interconnected mechanisms that can determine the rate at which genes are transcribed. These include intracellular calcium signaling, modulation of the redox state, post-translational modifications of transcriptional regulators, histone modifications, DNA methylation, modulation of RNA polymerases, alternative transcription inititation, the Mediator complex and regulation by non-coding RNAs. In addition, on their journey from transcription to translation, mRNAs are further modulated through mechanisms such as nuclear RNA retention, storage of mRNA in stress granules and P-bodies, and post-transcriptional gene silencing. In this review, we highlight the latest insights into these mechanisms. Furthermore, we discuss some emerging technologies that promise to greatly enhance our understanding of the regulation of the plant immune transcriptome in the future.

Zebularine induces enzymatic DNA-protein crosslinks in 45S rDNA heterochromatin of Arabidopsis nuclei

Loss of genome stability leads to reduced fitness, fertility and a high mutation rate. Therefore, the genome is guarded by the pathways monitoring its integrity and neutralizing DNA lesions. To analyze the mechanism of DNA damage induction by cytidine analog zebularine, we performed a forward-directed suppressor genetic screen in the background of Arabidopsis thaliana zebularine-hypersensitive structural maintenance of chromosomes 6b (smc6b) mutant. We show that smc6b hypersensitivity was suppressed by the mutations in EQUILIBRATIVE NUCLEOSIDE TRANSPORTER 3 (ENT3), DNA METHYLTRANSFERASE 1 (MET1) and DECREASE IN DNA METHYLATION 1 (DDM1). Superior resistance of ent3 plants to zebularine indicated that ENT3 is likely necessary for the import of the drug to the cells. Identification of MET1 and DDM1 suggested that zebularine induces DNA damage by interference with the maintenance of CG DNA methylation. The same holds for structurally similar compounds 5-azacytidine and 2-deoxy-5-azacytidine. Based on our genetic and biochemical data, we propose that zebularine induces enzymatic DNA–protein crosslinks (DPCs) of MET1 and zebularine-containing DNA in Arabidopsis, which was confirmed by native chromatin immunoprecipitation experiments. Moreover, zebularine-induced DPCs accumulate preferentially in 45S rDNA chromocenters in a DDM1-dependent manner. These findings open a new avenue for studying genome stability and DPC repair in plants.

Dynamics of H3K27me3 Modification on Plant Adaptation to Environmental Cues

Given their sessile nature, plants have evolved sophisticated regulatory networks to confer developmental plasticity for adaptation to fluctuating environments. Epigenetic codes, like tri-methylation of histone H3 on Lys27 (H3K27me3), are evidenced to account for this evolutionary benefit. Polycomb repressive complex 2 (PRC2) and PRC1 implement and maintain the H3K27me3-mediated gene repression in most eukaryotic cells. Plants take advantage of this epigenetic machinery to reprogram gene expression in development and environmental adaption. Recent studies have uncovered a number of new players involved in the establishment, erasure, and regulation of H3K27me3 mark in plants, particularly highlighting new roles in plants’ responses to environmental cues. Here, we review current knowledge on PRC2-H3K27me3 dynamics occurring during plant growth and development, including its writers, erasers, and readers, as well as targeting mechanisms, and summarize the emerging roles of H3K27me3 mark in plant adaptation to environmental stresses.