The chromatin remodeler ERCC6 and the histone chaperone NAP1 are involved in apurinic/apyrimidinic endonuclease-mediated DNA repair.

During base excision repair (BER), the apurinic or apyrimidinic (AP) site serves as an intermediate product following base excision. In plants, APE-redox protein (ARP) represents the major AP site of cleavage activity. Despite the well-established understanding that the nucleosomal structure acts as a barrier to various DNA-templated processes, the regulatory mechanisms underlying BER at the chromatin level remain elusive, especially in plants. In this study, we identified plant chromatin remodeler Excision Repair Cross-Complementing protein group 6 (ERCC6) and histone chaperone Nucleosome Assembly Protein 1 (NAP1) as interacting proteins with ARP. The catalytic ATPase domain of ERCC6 facilitates its interaction with both ARP and NAP1. Additionally, ERCC6 and NAP1 synergistically contribute to nucleosome sliding and exposure of hindered endonuclease cleavage sites. Loss-of-function mutations in Arabidopsis (Arabidopsis thaliana) ERCC6 or NAP1 resulted in arp-dependent plant hypersensitivity to 5-fluorouracil, a toxic agent inducing BER, and the accumulation of AP sites. Furthermore, similar protein interactions are also found in yeast cells, suggesting a conserved recruitment mechanism employed by the AP endonuclease to overcome chromatin barriers during BER progression.

Arabidopsis γ-H2A.X-INTERACTING PROTEIN participates in DNA damage response and safeguards chromatin stability.

Upon the occurrence of DNA double strand breaks (DSB), the proximal histone variant H2A.X is phosphorylated as γ-H2A.X, a critical signal for consequent DSB signaling and repair pathways. Although γ-H2A.X-triggered DNA damage response (DDR) has been well-characterized in yeast and animals, the corresponding pathways in plant DDR are less well understood. Here, we show that an Arabidopsis protein γ-H2A.X-INTERACTING PROTEIN (XIP) can interact with γ-H2A.X. Its C-terminal dual-BRCT-like domain contributes to its specific interaction with γ-H2A.X. XIP-deficient seedlings display smaller meristems, inhibited growth, and higher sensitivity to DSB-inducing treatment. Loss-of-function in XIP causes transcriptome changes mimicking wild-type plants subject to replicative or genotoxic stresses. After genotoxic bleomycin treatment, more proteins with upregulated phosphorylation modifications, more DNA fragments and cell death were found in xip mutants. Moreover, XIP physically interacts with RAD51, the key recombinase in homologous recombination (HR), and somatic HR frequency is significantly reduced in xip mutants. Collectively, XIP participates in plant response to DSB and contributes to chromatin stability.

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.

Arabidopsis CHROMATIN REMODELING 19 acts as a transcriptional repressor and contributes to plant pathogen resistance.

Chromatin remodelers act in an ATP-dependent manner to modulate chromatin structure and thus genome function. Here, we report that the Arabidopsis (Arabidopsis thaliana) remodeler CHROMATIN REMODELING19 (CHR19) is enriched in gene body regions, and its depletion causes massive changes in nucleosome position and occupancy in the genome. Consistent with these changes, an in vitro assay verified that CHR19 can utilize ATP to slide nucleosomes. A variety of inducible genes, including several important genes in the salicylic acid (SA) and jasmonic acid (JA) pathways, were transcriptionally upregulated in the chr19 mutant under normal growth conditions, indicative of a role of CHR19 in transcriptional repression. In addition, the chr19 mutation triggered higher susceptibility to the JA pathway-defended necrotrophic fungal pathogen Botrytis cinerea, but did not affect the growth of the SA pathway-defended hemibiotrophic bacterial pathogen Pseudomonas syringae pv. tomato DC3000. Expression of CHR19 was tissue-specific and inhibited specifically by SA treatment. Such inhibition significantly decreased the local chromatin enrichment of CHR19 at the associated SA pathway genes, which resulted in their full activation upon SA treatment. Overall, our findings clarify CHR19 to be a novel regulator acting at the chromatin level to impact the transcription of genes underlying plant resistance to different pathogens.

Activities of Chromatin Remodeling Factors and Histone Chaperones and Their Effects in Root Apical Meristem Development.

Eukaryotic genes are packaged into dynamic but stable chromatin structures to deal with transcriptional reprogramming and inheritance during development. Chromatin remodeling factors and histone chaperones are epigenetic factors that target nucleosomes and/or histones to establish and maintain proper chromatin structures during critical physiological processes such as DNA replication and transcriptional modulation. Root apical meristems are vital for plant root development. Regarding the well-characterized transcription factors involved in stem cell proliferation and differentiation, there is increasing evidence of the functional implications of epigenetic regulation in root apical meristem development. In this review, we focus on the activities of chromatin remodeling factors and histone chaperones in the root apical meristems of the model plant species Arabidopsis and rice.

AtINO80 and AtARP5 physically interact and play common as well as distinct roles in regulating plant growth and development.

The proper modulation of chromatin structure is dependent on the activities of chromatin-remodeling factors and their interplays. Here, we show that the Arabidopsis chromatin-remodeler AtINO80 interacts with the actin-related protein AtARP5 and can form a larger protein complex. Genetic analysis demonstrated that AtARP5 acts in concert with AtINO80 during plant cellular proliferation and replication stress response. At the same time, AtARP5 is not required for AtINO80-mediated control of flowering time and related transcriptional regulation, and their chromatin distribution patterns on regions of flowering-repressor genes FLC/MAF4/MAF5 are also different. An in vitro DNase I digestion assay revealed that the AtINO80N-terminus can weakly bind DNA, an interaction that is significantly inhibited by H2A.Z/H2B addition. AtARP6, a specific subunit of SWR1-C that mediates the H2A.Z exchange, was found to have a previously unexpected inhibitory role in the local chromatin enrichment of AtINO80. Further genetic analyses revealed the functional interplay between AtINO80 and AtARP6 and their critical roles in embryogenesis and post-embryonic organ development, as well as the synergy of AtARP5 and AtARP6 in maintaining genomic stability. Our findings provide insights into the common and distinct roles of AtINO80 and AtARP5 in diverse aspects of plant development.

Functional Coordination of the Chromatin-Remodeling Factor AtINO80 and the Histone Chaperones NRP1/2 in Inflorescence Meristem and Root Apical Meristem.

Chromatin structure requires proper modulation in face of transcriptional reprogramming in the context of organism growth and development. Chromatin-remodeling factors and histone chaperones are considered to intrinsically possess abilities to remodel chromatin structure in single or in combination. Our previous study revealed the functional synergy between the Arabidopsis chromatin-remodeling factor INOSITOL AUXOTROPHY 80 (AtINO80) and the histone chaperone NAP1-RELATED PROTEIN 1 (NRP1) and NRP2 in somatic homologous recombination, one crucial pathway involved in repairing DNA double strand breaks. Here, we report genetic interplay between AtINO80 and NRP1/2 in regulating inflorescence meristem (IM) and root apical meristem (RAM) activities. The triple mutant atino80-5 m56-1 depleting of both AtINO80 (atino80-5) and NRP1/2 (m56-1) showed abnormal positioning pattern of floral primordia and enlargement of IM size. Higher mRNA levels of several genes involved in auxin pathway (e.g., PIN1, FIL) were found in the inflorescences of the triple mutant but barely in those of the single mutant atino80-5 or the double mutant m56-1. In particular, the depletion of AtINO80 and NRP1/2 decreased histone H3 levels within the chromatin regions of PIN1, which encodes an important auxin efflux carrier. Moreover, the triple mutant displayed a severe short-root phenotype with higher sensitivity to auxin transport inhibitor NPA. Unusual high level of cell death was also found in triple mutant root tips, accompanied by double-strand break damages revealed by γ-H2A.X loci and cortex cell enlargement. Collectively, our study provides novel insight into the functional coordination of the two epigenetic factors AtINO80 and NRP1/2 in apical meristems during plant growth and development.