DNA Damage Triggers the Activation of Immune Response to Viral Pathogens via Salicylic Acid in Plants.

Plants are challenged by various pathogens throughout their lives, such as bacteria, viruses, fungi, and insects; consequently, they have evolved several defense mechanisms. In addition, plants have developed localized and systematic immune responses due to biotic and abiotic stress exposure. Animals are known to activate DNA damage responses (DDRs) and DNA damage sensor immune signals in response to stress, and the process is well studied in animal systems. However, the links between stress perception and immune response through DDRs remain largely unknown in plants. To determine whether DDRs induce plant resistance to pathogens, Arabidopsis plants were treated with bleomycin, a DNA damage-inducing agent, and the replication levels of viral pathogens and growth of bacterial pathogens were determined. We observed that DDR-mediated resistance was specifically activated against viral pathogens, including turnip crinkle virus (TCV). DDR increased the expression level of pathogenesis-related (PR) genes and the total salicylic acid (SA) content and promoted mitogen-activated protein kinase signaling cascades, including the WRKY signaling pathway in Arabidopsis. Transcriptome analysis further revealed that defense- and SA-related genes were upregulated by DDR. The atm-2atr-2 double mutants were susceptible to TCV, indicating that the main DDR signaling pathway sensors play an important role in plant immune responses. In conclusion, DDRs activated basal immune responses to viral pathogens.

Detection of Plant Pathogenic Viruses in Commercial Gochujang (Fermented Red Pepper Paste) from Korea.

The potential transmission of plant pathogenic viruses through processed foods could be a source of concern for global crop production; however, there is a lack of supporting evidence. The present study was conducted to investigate the presence of plant pathogenic viruses in five samples of gochujang (fermented red pepper paste) manufactured in Korea. Several viruses infecting pepper were detected by reverse transcriptionpolymerase chain reaction, among which the pepper mild mottle virus (PMMoV) was detected in all five samples, at concentrations ranging from 2.8 to 7.0 (log10 copies/ml). In addition, PMMoV was observed by transmission electron microscopy in all five samples. The samples exhibited viral pathogenicity to Nicotiana benthamiana plants, indicating that global trade of processed products could be a possible source of the transmission of plant viruses.

Mutation in DDM1 inhibits the homology directed repair of double strand breaks.

In all organisms, DNA damage must be repaired quickly and properly, as it can be lethal for cells. Because eukaryotic DNA is packaged into nucleosomes, the structural units of chromatin, chromatin modification is necessary during DNA damage repair and is achieved by histone modification and chromatin remodeling. Chromatin remodeling proteins therefore play important roles in the DNA damage response (DDR) by modifying the accessibility of DNA damage sites. Here, we show that mutation in a SWI2/SNF2 chromatin remodeling protein (DDM1) causes hypersensitivity in the DNA damage response via defects in single-strand annealing (SSA) repair of double-strand breaks (DSBs) as well as in the initial steps of homologous recombination (HR) repair. ddm1 mutants such as ddm1-1 and ddm1-2 exhibited increased root cell death and higher DSB frequency compared to the wild type after gamma irradiation. Although the DDM1 mutation did not affect the expression of most DDR genes, it did cause substantial decrease in the frequency of SSA as well as partial inhibition in the γ-H2AX and Rad51 induction, the initial steps of HR. Furthermore, global chromatin structure seemed to be affected by DDM1 mutations. These results suggest that DDM1 is involved in the homology directed repair such as SSA and HR, probably by modifying chromatin structure.

Ionizing radiation manifesting DNA damage response in plants: An overview of DNA damage signaling and repair mechanisms in plants.

Plants orchestrate various DNA damage responses (DDRs) to overcome the deleterious impacts of genotoxic agents on genetic materials. Ionizing radiation (IR) is widely used as a potent genotoxic agent in plant DDR research as well as plant breeding and quarantine services for commercial uses. This review aimed to highlight the recent advances in cellular and phenotypic DDRs, especially those induced by IR. Various physicochemical genotoxic agents damage DNA directly or indirectly by inhibiting DNA replication. Among them, IR-induced DDRs are considerably more complicated. Many aspects of such DDRs and their initial transcriptomes are closely related to oxidative stress response. Although many key components of DDR signaling have been characterized in plants, DDRs in plant cells are not understood in detail to allow comparison with those in yeast and mammalian cells. Recent studies have revealed plant DDR signaling pathways including the key regulator SOG1. The SOG1 and its upstream key components ATM and ATR could be functionally characterized by analyzing their knockout DDR phenotypes after exposure to IR. Considering the potent genotoxicity of IR and its various DDR phenotypes, IR-induced DDR studies should help to establish an integrated model for plant DDR signaling pathways by revealing the unknown key components of various DDRs in plants.

SOG1-dependent NAC103 modulates the DNA damage response as a transcriptional regulator in Arabidopsis.

The plant-specific transcription factor (TF) NAC103 was previously reported to modulate the unfolded protein response in Arabidopsis under endoplasmic reticulum (ER) stress. Alternatively, we report here that NAC103 is involved in downstream signaling of SOG1, a master regulator for expression of DNA damage response (DDR) genes induced by genotoxic stress. Arabidopsis NAC103 expression was strongly induced by genotoxic stress and nac103 mutants displayed substantial inhibition of DDR gene expression after gamma radiation or radiomimetic zeocin treatment. DDR phenotypes, such as true leaf inhibition, root cell death and root growth inhibition, were also suppressed significantly in the nac103 mutants, but to a lesser extent than in the sog1-1 mutant. By contrast, overexpression of NAC103 increased DDR gene expression without genotoxic stress and substantially rescued the phenotypic changes in the sog1-1 mutant after zeocin treatment. The putative promoters of some representative DDR genes, RAD51, PARP1, RPA1E, BRCA1 and At4g22960, were found to partly interact with NAC103. Together with the expected interaction of SOG1 with the promoter of NAC103, our study suggests that NAC103 is a putative SOG1-dependent transcriptional regulator of plant DDR genes, which are responsible for DDR phenotypes under genotoxic stress.

Transcriptome-based biological dosimetry of gamma radiation in Arabidopsis using DNA damage response genes.

Plants are used as representative reference biota for the biological assessment of environmental risks such as ionizing radiation due to their immobility. This study proposed a faster, more economical, and more effective method than conventional cytogenetic methods for the biological dosimetry of ionizing radiation in plants (phytodosimetry). We compared various dose-response curves for the radiation-induced DNA damage response (DDR) in Arabidopsis thaliana after relatively "low-dose" gamma irradiation (3, 6, 12, 24, and 48 Gy) below tens of Gy using comet (or single-cell gel electrophoresis), gamma-H2AX, and transcriptomic assays of seven DDR genes (AGO2, BRCA1, GRG, PARP1, RAD17, RAD51, and RPA1E) using quantitative real time PCR. The DDR signal from the comet assay was saturated at 6 Gy, while the gamma-H2AX signal increased up to 48 Gy, following a linear-quadratic dose-response model. The transcriptional changes in the seven DDR genes were fitted to linear or supra-linear quadratic equations with significant dose-dependency. The dose-dependent transcriptional changes were maintained similarly until 24 h after irradiation. The integrated transcriptional dose-response model of AGO2, BRCA1, GRG, and PARP1 was very similar to that of gamma-H2AX, while the transcriptional changes in the BRCA1, GRG, and PARP1 DDR genes revealed significant dependency on the dose-rate, ecotype, and radiation dose. These results suggest that the transcriptome-based dose-response model fitted to a quadratic equation could be used practically for phytodosimetry instead of conventional cytogenetic models, such as the comet and gamma-H2AX assays. The effects of dose-rate and ecotype on the transcriptional changes of DDR genes should also be considered to improve the transcriptome-based phytodosimetry model.

Chromosomal Aberrations in Human Peripheral Blood Lymphocytes after Exposure to Ionizing Radiation.

Biological dosimetry using chromosome aberration analyses in human peripheral blood lymphocytes is suitable and useful tool for estimating the dose when a nuclear or radiological emergency is investigated. Blood samples from five healthy donors were obtained to establish dose-response calibration curves for chromosomal aberrations after exposure to ionizing radiation. In this work, dicentric assay and CBMN assay were compared considering the sensitivity and accuracy of dose estimation. In a total of 21,688 analyzed metaphase spreads, 10,969 dicentric chromosomes, 563 centric rings and 11,364 acentric chromosomes were found. The number of metaphase cells decreased with increasing radiation dose. The centric rings were not found in the non-irradiated control. There was no relationship between radiation dose and acentric ring induction. The frequency of total MN increased in a dose-dependent manner. In comparison with the control value, MN increased about 9, 32, 75, 87, and 52 fold higher after treatment with 1, 2, 3, 4, and 5 Gy, respectively. The results revealed that the mean frequency of chromosomal aberrations, both in dicentric and in micronuclei analyses increased with increasing radiation dose.

Genotoxicity in earthworm after combined treatment of ionising radiation and mercury.

This study was performed to investigate the acute genotoxic effects of mercury and radiation on earthworms (Eisenia fetida). The levels of DNA damage and the repair kinetics in the coelomocytes of E. fetida treated with mercuric chloride (HgCl2) and ionising radiation (gamma rays) were analysed by means of the comet assay. For detection of DNA damage and repair, E. fetida was exposed to HgCl2 (0-160 mg kg(-1)) and irradiated with gamma rays (0-50 Gy) in vivo. The increase in DNA damage depended on the concentration of mercury or dose of radiation. The results showed that the more the oxidative stress induced by mercury and radiation the longer the repair time that was required. When a combination of HgCl2 and gamma rays was applied, the cell damage was much higher than those treated with HgCl2 or radiation alone, which indicated that the genotoxic effects were increased after the combined treatment of mercury and radiation.

Effect of N-acetyl-l-cysteine on Saccharomyces cerevisiae irradiated with gamma-rays.

Ionizing radiation (IR) induces DNA strand breaks (DSBs), base damage, inhibition of protein activity, apoptosis by reactive oxygen species (ROS). Detoxification or removal of generated ROS can reduce oxidative damage. Antioxidant enzymes such as superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase are immediately triggered for ROS scavenging. N-acetyl-l-cysteine (NAC) having a thiol, a precursor for reduced glutathione (GSH), is known as one of the antioxidants. In this study, the effect of NAC as an antioxidant and a radioprotector was investigated on survival rate, transcriptional level of antioxidant enzymes gene, and protein level including SOD activity and intracellular GSH in yeast Saccharomyces cerevisiae W303-1A strain mutated YBP1 gene irradiated with gamma-rays. NAC did not protect the gamma-ray-induced cell death. The gene expression of antioxidant enzymes including SOD1, SOD2, GPX1, and GPX2 was induced by gamma-rays. In contrast, the pretreatment of NAC reduced the expression of these genes. NAC reduced SOD activity and intracellular GSH level in yeast. These data suggest that NAC is able to reduce radiation-induced ROS levels in vivo but does not protect yeast cells against radiation-induced death.