Geminiviridae

Abstract

The Geminiviridae comprise a group of plant viruses with small, circular, single‐stranded deoxyribonucleic acid (ssDNA) genomes encapsidated in particles that have a twinned quasi‐icosahedral (geminate) shape, from which the family derives its name. They are among the smallest of viruses known and are transmitted exclusively by specific insect vectors. Some are associated with highly diverse circular, ssDNA satellites that play a part in disease progression and provide evidence of the interaction of geminiviruses with other families of plant‐infecting viruses. Collectively, geminiviruses are pathogens of cereals, vegetable and fibre crops, posing a serious threat to agriculture worldwide. Efforts to control these viruses by conventional means have either proven unsuccessful or have not been durable. For this reason, researchers are increasingly looking to engineered resistance as a means to control these important viruses. This has recently led to the first crop with engineered resistance to a geminivirus reaching the field.

Key Concepts

  • Geminiviruses are important insect‐transmitted pathogens of cereal, vegetable and fibre crops in tropical and temperate regions throughout the world, including developed and developing countries.
  • Geminiviruses often cause striking symptoms in plants, indeed the earliest description of virus symptoms in plants is believed to relate to those caused by a geminivirus and such symptoms occasionally are considered to be horticulturally desirable.
  • All geminiviruses have single‐stranded DNA genomes and a twinned particle morphology that is unique to this family of plant viruses.
  • Some geminiviruses are associated with single‐stranded DNA satellites, referred to as alpha‐ and betasatellites, the latter playing an essential role in maintenance of the disease.
  • Because their small DNA genomes encode only a limited number of genes, geminiviruses manipulate the metabolism of the infected cell to produce conditions suitable for their proliferation.
  • Geminiviruses are proving invaluable in the investigation of fundamental biological processes such as DNA replication, cell‐cycle regulation, gene expression and pathogen resistance in plants.
  • Geminiviruses are highly recombinogenic, and the derivation of a large number of geminivirus and satellite DNA sequences is providing important insights into their diversity, adaptability and evolution.
  • Conventional control strategies, such as the use of chemicals to eliminate their insect vectors and breeding natural host plant resistance, have proven ineffective in reducing crop losses to geminivirus infection, leading to the development of promising alternative (genetically engineered) strategies.
  • Sequences derived from geminiviruses are being used to develop systems for high‐level expression of valuable proteins in plants.

Keywords: geminivirus; plant DNA virus; DNA satellite; gene expression and function; gene vectors; disease and control

Figure 1. Genome organisation of typical members of the seven genera of the family . The position and orientation of virion‐sense (V) and complementary‐sense (C) genes of viruses typical of the genera s, , , , , and are shown in relation to the initiation site of virion‐sense DNA replication (arbitrarily designated as nucleotide one), located within the conserved nonanucleotide TAATATTAC at the apex of a conserved hairpin structure. The hairpin structure lies within non‐coding, intergenic region (IR). Mastreviruses and becurtoviruses contain two non‐coding regions referred to as the large and small intergenic regions (LIR and SIR, respectively). The eragrovirus ECSV also has two intergenic regions, although in this case the LIR is smaller than the SIR. The mastreviruses contain two introns (boxes). The two genomic components of bipartite begomoviruses (DNAs A and B) contain an almost identical sequence of approximately 200 nucleotides referred to as the common region (CR) located almost entirely within the intergenic region (the non‐coding sequences). Also shown are the small satellite (betasatellite) and satellite‐like (alphasatellite) molecules associated with some begomoviruses. These contain a region of sequence rich in adenine (A‐rich), and the betasatellites contain a region of highly conserved sequence known as the satellite conserved region (SCR). The gene products (where their function is known) are indicated as coat protein (CP), replication‐associated protein (REP), transcriptional activator protein (TRAP), replication enhancer protein (REN), movement protein (MP) and nuclear shuttle protein (NSP). The only product encoded by betasatellites is known as βC1, whereas the alphasatellites encode REP.
Figure 2. Electron micrographs of (a) purified MSV particles showing their characteristic twinned (geminate) morphology (bar represents 100 nm), (b) three‐dimensional image reconstruction of an MSV particle based on a cryoelectron microscopy analysis (bar represents 10 nm; reproduced with permission from Zhang . © Elsevier) and (c) ACMV circular ssDNA showing the predominant monomeric DNA of approximately 2.8 kbp and an example of half‐size defective DNA (bar represents 100 nm).
Figure 3. Geminivirus infection cycle. Following insect transmission, virus particles are uncoated and the unencapsidated ssDNA is converted to a dsDNA intermediate that adopts a minichromosome structure for gene expression. The dsDNA serves as a template to produce ssDNA by a rolling circle mechanism involving REP, REN and host factors. Viral DNA associates with NSP, MP and possibly CP, V2 and βC1 during cell‐to‐cell movement and long‐distance systemic spread throughout the plant.
Figure 4. Geminivirus disease symptoms. (a) Mild and severe streak symptoms in MSV‐infected maize. (b) Downward leaf curl in CpCDV‐infected . (c) Vein yellowing in AYVV‐infected . (d) Virus‐induced leaflets (enations) developing on the abaxial surface of CLCuMV‐infected cotton. (e) Chlorotic mosaic in AbMV‐infected . (f) Upward leaf roll and vein‐swelling in AYVV‐infected , symptoms typical of many monopartite begomoviruses and curtoviruses in this host.
Figure 5. Transmitting insects. (a) MSV vector (Naudé), (b) BCTV vector (Baker), (c) TPCTV vector (Fowler) and (d) begomovirus vector (Genn).
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Briddon, Rob W(Mar 2015) Geminiviridae. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000750.pub3]