Transcriptional silencing of geminiviral promoter-driven transgenes following homologous virus infection

Promoters isolated from the Tomato leaf curl virus (TLCV) drive both constitutive and tissue-specific expression in transgenic tobacco. Following systemic TLCV infection of plants stably expressing TLCV promoter:GUS transgenes, transgene expression driven by all six TLCV promoters was silenced. Silencing in the TLCV coat protein promoter:GUS plants (V2:GUSΔC) was characterized in more detail. Transgene silencing observed in leaf, stem, and preanthesis floral tissue occurred with the continued replication of TLCV in host tissues. Infection of the V2:GUSΔC plants with heterologous geminiviruses did not result in transgene silencing, indicating that silencing was specifically associated with TLCV infection. Nuclear run-on assays indicated that silencing was due to the abolition of transcription from the V2:GUSΔC transgene. Bisulfite sequencing showed that silencing was associated with cytosine hypermethylation of the TLCV-derived promoter sequences of the V2:GUSΔC transgene. Progeny derived from V2:GUSΔC plants silenced by TLCV infection were analyzed. Transgene expression was silenced in progeny seedlings but was partially reactivated in the majority of plants by 75 days postgermination. Progeny seedlings treated with the nonmethylatable cytosine analog 5-azacytidine or the histone deacetylase inhibitor sodium butyrate exhibited partial reactivation of expression. This is the first report of the hypermethylation of a virus-derived transgene associated with a DNA virus infection.Mark Seemanpillai, Ian Dry, John Randles and Ali Rezaia

Analysis of silencing escape of Tomato leaf curl virus: An evaluation of the role of DNA methylation

RNA silencing is a sequence-specific mechanism regulating gene expression and has been used successfully for antiviral defense against RNA viruses. Similar strategies to develop resistance against DNA containing Tomato leaf curl virus (TLCV) and some other geminiviruses have been unsuccessful. To analyze this silencing escape, we transformed tomato plants with a hairpin construct from the TLCV C2 open reading frame (ORF). The transgenic plants showed a strong RNA silencing response, and following TLCV inoculation, their infection was delayed. However, the viral infection was not prevented and TLCV DNA accumulated to the levels found in nontransgenic plants. To determine the fate of a transgene carrying homology to the virus, we used transgenic plants carrying the TLCV C4 gene, which induces a distinct phenotype. Upon TLCV infection, the phenotype was abolished and C4 transcript disappeared. Concurrently, TLCV-specific small interfering RNAs were produced. In situ hybridization showed abundant levels of TLCV DNA in phloem cells of TLCV-infected C4 trans-genic plants. However, the C4 transcripts were no longer detectable in nonvascular cells. Analysis of the transgene by methylation sequencing revealed a high level of de novo methylation of asymmetric cytosines in both the C4 ORF and its 35S promoter. A high level of methylation also was found at both symmetric and asymmetric cytosines of the complementary-sense strand of TLCV double-stranded DNA. Given the previous finding that methylated geminivi-ral DNA is not competent for replication, we provide a model whereby TLCV evades host defense through a population of de novo synthesized unmethylated DNA. Xue-Yu Bian, M. Saif Rasheed, Mark J. Seemanpillai, and M. Ali Rezaia

Fighting geminiviruses by RNAi and vice versa

Geminiviruses have recently emerged not only as the cause of devastating diseases of important crop plants but also as a tool to study fundamental aspects of RNA interference (RNAi) and virus-induced gene silencing. RNA silencing is an evolutionary conserved mechanism protecting cell from pathogenic RNA and DNA, which is increasingly viewed as an adaptive immune system of plants against viruses. Here we summarize recent developments in the field of geminivirology presented by several leading groups at the Meeting "Gemini2004" (a total of 85 participants from all over the world) with the main focus on the anti-viral strategies that exploit RNAi and related silencing phenomena

Genomic regions of tomato leaf curl virus DNA satellite required for replication and for satellite-mediated delivery of heterologous DNAs

Copyright © 2007 by the Society for General Microbiology.Tomato leaf curl virus (TLCV) satellite DNA (sat-DNA) is a 682 nt, circular, single-stranded molecule that lacks an open reading frame (ORF) or an apparent promoter. It contains binding motifs for the TLCV replication-associated protein, but these are dispensable for replication. To identify the regions of the sat-DNA critical for replication, the entire sequence was scanned by deletion/replacement mutagenesis. Transient assays using Nicotiana benthamiana revealed that sequences within nt 296–35 (through nt 682) are essential for replication. Sequence deletions and replacements between nt 35 and 296 were tolerated but with a significant loss of infectivity, indicating that genome size strongly influences replication efficiency. Within the permissible region, inserts of 100–700 nt were retained in transient assays although with a slight reduction in replication. In addition, sat-DNA constructs containing short non-viral DNAs replicated and spread in tobacco plants, indicating their potential as gene-delivery vectors.Dongmei Li, S. A. Akbar Behjatnia, Ian B. Dry, John W. Randles, Omid Eini and M. Ali Rezaia

Engineering resistance to geminiviruses--review and perspectives.

peer reviewedaudience: researcher, professionalFollowing the conceptual development of virus resistance strategies ranging from coat protein-mediated interference of virus propagation to RNA-mediated virus gene silencing, much progress has been achieved to protect plants against RNA and DNA virus infections. Geminiviruses are a major threat to world agriculture, and breeding resistant crops against these DNA viruses is one of the major challenges faced by plant virologists and biotechnologists. In this article, we review the most recent transgene-based approaches that have been developed to achieve durable geminivirus resistance. Although most of the strategies have been tested in model plant systems, they are ready to be adopted for the protection of crop plants. Furthermore, a better understanding of geminivirus gene and protein functions, as well as the native immune system which protects plants against viruses, will allow us to develop novel tools to expand our current capacity to stabilize crop production in geminivirus epidemic zones

Small RNA-based antimicrobial immunity

Protection against microbial infection in eukaryotes is provided by diverse cellular and molecular mechanisms. Here, we present a comparative view of the antiviral activity of virus-derived small interfering RNAs in fungi, plants, invertebrates and mammals, detailing the mechanisms for their production, amplification and activity. We also highlight the recent discovery of viral PIWI-interacting RNAs in animals and a new role for mobile host and pathogen small RNAs in plant defence against eukaryotic pathogens. In turn, viruses that infect plants, insects and mammals, as well as eukaryotic pathogens of plants, have evolved specific virulence proteins that suppress RNA interference (RNAi). Together, these advances suggest that an antimicrobial function of the RNAi pathway is conserved across eukaryotic kingdoms

Plasmodesmata: Channels for Viruses on the Move

The symplastic communication network established by plasmodesmata (PD) and connected phloem provides an essential pathway for spatiotemporal intercellular signaling in plant development but is also exploited by viruses for moving their genomes between cells in order to infect plants systemically. Virus movement depends on virus-encoded movement proteins (MPs) that target PD and therefore represent important keys to the cellular mechanisms underlying the intercellular trafficking of viruses and other macromolecules. Viruses and their MPs have evolved different mechanisms for intracellular transport and interaction with PD. Some viruses move from cell to cell by interacting with cellular mechanisms that control the size exclusion limit of PD whereas other viruses alter the PD architecture through assembly of specialized transport structures within the channel. Some viruses move between cells in the form of assembled virus particles whereas other viruses may interact with nucleic acid transport mechanisms to move their genomes in a non-encapsidated form. Moreover, whereas several viruses rely on the secretory pathway to target PD, other viruses interact with the cortical endoplasmic reticulum and associated cytoskeleton to spread infection. This chapter provides an introduction into viruses and their role in studying the diverse cellular mechanisms involved in intercellular PD-mediated macromolecular trafficking