KU80, a key factor for non-homologous end-joining, retards geminivirus multiplication

H3.1K27me1 loss confers Arabidopsis resistance to Geminivirus by sequestering DNA repair proteins onto host genome

The H3 methyltransferases ATXR5 and ATXR6 deposit H3.1K27me1 to heterochromatin to prevent genomic instability and transposon re-activation. Here, we report that atxr5 atxr6 mutants display robust resistance to Geminivirus. The viral resistance is correlated with activation of DNA repair pathways, but not with transposon re-activation or heterochromatin amplification. We identify RAD51 and RPA1A as partners of virus-encoded Rep protein. The two DNA repair proteins show increased binding to heterochromatic regions and defense-related genes in atxr5 atxr6 vs wild-type plants. Consequently, the proteins have reduced binding to viral DNA in the mutant, thus hampering viral amplification. Additionally, RAD51 recruitment to the host genome arise via BRCA1, HOP2, and CYCB1;1, and this recruitment is essential for viral resistance in atxr5 atxr6. Thus, Geminiviruses adapt to healthy plants by hijacking DNA repair pathways, whereas the unstable genome, triggered by reduced H3.1K27me1, could retain DNA repairing proteins to suppress viral amplification in atxr5 atxr6.

Differential expression of genes during recovery of Nicotiana tabacum from tomato leaf curl Gujarat virus infection

MAIN CONCLUSION

Nicotiana tabacum exhibits recovery response towards tomato leaf curl Gujarat virus. Transcriptome analysis revealed the differential expression of defense-related genes. Genes encoding for cysteine protease inhibitor, hormonal- and stress-related to DNA repair mechanism are found to be involved in the recovery process. Elucidating the role of host factors in response to viral infection is crucial in understanding the plant host-virus interaction. Begomovirus, a genus in the family Geminiviridae, is reported throughout the globe and is known to cause serious crop diseases. Tomato leaf curl Gujarat virus (ToLCGV) infection in Nicotiana tabacum resulted in initial symptom expression followed by a quick recovery in the systemic leaves. Transcriptome analysis using next-generation sequencing (NGS) revealed a large number of differentially expressed genes both in symptomatic as well as recovered leaves when compared to mock-inoculated plants. The virus infected N. tabacum results in alteration of various metabolic pathways, phytohormone signaling pathway, defense related protein, protease inhibitor, and DNA repair pathway. RT-qPCR results indicated that Germin-like protein subfamily T member 2 (NtGLPST), Cysteine protease inhibitor 1-like (NtCPI), Thaumatin-like protein (NtTLP), Kirola-like (NtKL), and Ethylene-responsive transcription factor ERF109-like (NtERTFL) were down-regulated in symptomatic leaves when compared to recovered leaves of ToLCGV-infected plants. In contrast, the Auxin-responsive protein SAUR71-like (NtARPSL) was found to be differentially down-regulated in recovered leaves when compared to symptomatic leaves and the mock-inoculated plants. Lastly, Histone 2X protein like (NtHH2L) gene was found to be down-regulated, whereas Uncharacterized (NtUNCD) was up-regulated in both symptomatic as well as recovered leaves compared to the mock-inoculated plants. Taken together, the present study suggests potential roles of the differentially expressed genes that might govern tobacco's susceptibility and/or recovery response towards ToLCGV infection.

Comparative analysis of the metabolome and transcriptome between green and albino zones of variegated leaves from Hydrangea macrophylla 'Maculata' infected by hydrangea ringspot virus

In nature, many different factors cause plants to present variegated leaves. The purpose of this study was to reveal the changes in the green and albino leaves of Hydrangea macrophylla 'Maculata'. It was found that in the albino zone, the leaves became thinner, the chloroplast structure disappeared, and a large number of leucoplasts replaced chloroplasts. In addition, the albino zone of the leaves contained almost no chlorophyll and showed no function related to transforming and utilizing light energy, and more intense oxidative stress was observed in the albino zone of the leaves than in the green zone. RNA-seq analysis showed that the chlorophyll synthesis pathway of the albino zone of leaves was blocked. Upregulated expression of the hydrangea ringspot virus (HdRSV) coat protein (CP) gene was detected in albino tissue by RT-qPCR. Finally, combined UPLC-MS/MS and RNA-seq analyses revealed metabolic changes involving multiple pathways in albino leaf tissue, centered on the TCA cycle. We hypothesize that HdRSV may alter energy metabolism in the albino zone of leaves, including increased lipid metabolism, reduced sugar metabolism, and increased synthesis of amino acids and the viral capsid protein from ribosomes.

Viral interactions with non-homologous end-joining: a game of hide-and-seek

There are extensive interactions between viruses and the host DNA damage response (DDR) machinery. The outcome of these interactions includes not only direct effects on viral nucleic acids and genome replication, but also the activation of host stress response signalling pathways that can have further, indirect effects on viral life cycles. The non-homologous end-joining (NHEJ) pathway is responsible for the rapid and imprecise repair of DNA double-stranded breaks in the nucleus that would otherwise be highly toxic. Whilst directly repairing DNA, components of the NHEJ machinery, in particular the DNA-dependent protein kinase (DNA-PK), can activate a raft of downstream signalling events that activate antiviral, cell cycle checkpoint and apoptosis pathways. This combination of possible outcomes results in NHEJ being pro- or antiviral depending on the infection. In this review we will describe the broad range of interactions between NHEJ components and viruses and their consequences for both host and pathogen.

Identification and characterization of genes frequently responsive to Xanthomonas oryzae pv. oryzae and Magnaporthe oryzae infections in rice

Background

Disease resistance is an important factor that impacts rice production. However, the mechanisms underlying rice disease resistance remain to be elucidated.

Results

Here, we show that a robust set of genes has been defined in rice response to the infections of Xanthomonas oryzae pv. oryzae (Xoo) and Magnaporthe oryzae (Mor). We conducted a comprehensive analysis of the available microarray data from a variety of rice samples with inoculation of Xoo and Mor. A set of 12,932 genes was identified to be regulated by Xoo and another set of 2709 Mor-regulated genes was determined. GO enrichment analysis of the regulated genes by Xoo or Mor suggested mitochondrion may be an arena for the up-regulated genes and chloroplast be another for the down-regulated genes by Xoo or Mor. Cytokinin-related processes were most frequently repressed by Xoo, while processes relevant to jasmonic acid and abscisic acid were most frequently activated by Xoo and Mor. Among genes responsive to Xoo and Mor, defense responses and diverse signaling pathways were the most frequently enriched resistance mechanisms. InterPro annotation showed the zinc finger domain family, WRKY proteins, and Myb domain proteins were the most significant transcription factors regulated by Xoo and Mor. KEGG analysis demonstrated pathways including ‘phenylpropanoid biosynthesis’, ‘biosynthesis of antibiotics’, ‘phenylalanine metabolism’, and ‘biosynthesis of secondary metabolites’ were most frequently triggered by Xoo and Mor, whereas ‘circadian rhythm-plant’ was the most frequent pathway repressed by Xoo and Mor.

Conclusions

The genes identified here represent a robust set of genes responsive to the infections of Xoo and Mor, which provides an overview of transcriptional reprogramming during rice defense against Xoo and Mor infections. Our study would be helpful in understanding the mechanisms of rice disease resistance.

Barcoding of Plant Viruses with Circular Single-Stranded DNA Based on Rolling Circle Amplification

The experience with a diagnostic technology based on rolling circle amplification (RCA), restriction fragment length polymorphism (RFLP) analyses, and direct or deep sequencing (Circomics) over the past 15 years is surveyed for the plant infecting geminiviruses, nanoviruses and associated satellite DNAs, which have had increasing impact on agricultural and horticultural losses due to global transportation and recombination-aided diversification. Current state methods for quarantine measures are described to identify individual DNA components with great accuracy and to recognize the crucial role of the molecular viral population structure as an important factor for sustainable plant protection.

Differential methylation of the circular DNA in geminiviral minichromosomes

Geminiviral minichromosomes were purified to explore epigenetic modifications. The levels of methylation in their covalently closed circular DNA were examined with the help of methylation-dependent restriction (MdR). DNA with 12 superhelical turns was preferentially modified, indicating minichromosomes with 12 nucleosomes leaving an open gap. MdR digestion yielded a specific product of genomic length, which was cloned and Sanger-sequenced, or amplified following ligation-mediated rolling circle amplification and deep-sequenced (circomics). The conventional approach revealed a single cleavage product indicating specific methylations at the borders of the common region. The circomics approach identified considerably more MdR sites in a preferential distance to each other of ~200 nts, which is the DNA length in a nucleosome. They accumulated in regions of nucleosome-free gaps, but scattered also along the genomic components. These results may hint at a function in specific gene regulation, as well as in virus resistance.

The recombination mediator RAD51D promotes geminiviral infection

To study a possible role for homologous recombination in geminivirus replication, we challenged Arabidopsis recombination gene knockouts by Euphorbia yellow mosaic virus infection. Our results show that the RAD51 paralog RAD51D, rather than RAD51 itself, promotes viral replication at early stages of infection. Blot hybridization analyses of replicative intermediates using one- and two-dimensional gels and deep sequencing point to an unexpected facet of recombination-dependent replication, the repair by single-strand annealing (SSA) during complementary strand replication. A significant decrease of both intramolecular, yielding defective DNAs and intermolecular recombinant molecules between the two geminiviral DNA components (A, B) were observed in the absence of RAD51D. By contrast, DNA A and B reacted differentially with the generation of inversions. A model to implicate single-strand annealing recombination in geminiviral recombination-dependent replication is proposed.

The contribution of translesion synthesis polymerases on geminiviral replication

Geminiviruses multiply primarily in the plant phloem, but never in meristems. Their Rep protein can activate DNA synthesis in differentiated cells. However, when their single-stranded DNA is injected into the phloem by insects, no Rep is present for inducing initial complementary strand replication. Considering a contribution of translesion synthesis (TLS) polymerases in plants, four of them (Polη, Polζ, Polκ, Rev1) are highly and constitutively expressed in differentiated tissues like the phloem. Two geminiviruses (Euphorbia yellow mosaic virus, Cleome leaf crumple virus), inoculated either biolistically or by whiteflies, replicated in Arabidopsis thaliana mutant lines of these genes to the same extent as in wild type plants. Comparative deep sequencing of geminiviral DNAs, however, showed a high exchange rate (10(-4)-10(-3)) similar to the phylogenetic variation described before and a significant difference in nucleotide substation rates if Polη and Polζ were absent, with a differential response to the viral DNA components.