Geminiviridae

Rob W Briddon, National Institute for Biotechnology and Genetic Engineering, Faisalabad, Pakistan
John Stanley, John Innes Centre, Norwich, UK

Published online: March 2009

DOI: 10.1002/9780470015902.a0000750.pub2

The Geminiviridae comprise a group of plant viruses with small, circular, single-stranded deoxyribonucleic acid (DNA) 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, single-stranded DNA betasatellites that are essential for disease progression. Their association with alphasatellites provides 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, such as using insecticides against their insect vectors and by breeding resistance, 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 diseases.

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 alphasatellites 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 organization of typical members of the four Geminiviridae genera. The position and orientation of virion-sense (V) and complementary-sense (C) genes of MSV (genus Mastrevirus), ACMV and AYVV (genus Begomovirus), BCTV (genus Curtovirus) and TPCTV (genus Topocuvirus) 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. MSV contains two noncoding regions referred to as the large and small intergenic regions (LIR and SIR, respectively) and two introns (boxes). The two genomic components of ACMV (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. Also shown are the small satellite (betasatellite) and satellite-like (alphasatellite) molecules (DNA- and DNA-1, respectively) associated with some monopartite begomoviruses such as AYVV. 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 C1 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 cryo-electron microscopy analysis (bar represents 10 nm; Zhang et al. (2001). Reproduced by permission of 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 which 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 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 BeYDV-infected Nicotiana benthamiana. (c) Vein yellowing in AYVV-infected Ageratum conyzoides. (d) Virus-induced leaflets (enations) developing on the abaxial surface of CLCuMV-infected cotton. (e) Chlorotic mosaic in AbMV-infected Abutilon sellovianum. (f) Upward leaf roll and vein-swelling in AYVV-infected N. benthamiana, symptoms typical of many monopartite begomoviruses and curtoviruses in this host.
Figure 5. Transmitting insects. (a) MSV vector Cicadulina mbila (Naudé), (b) BCTV vector Circulifer tenellus (Baker), (c) TPCTV vector Micrutalis malleifera (Fowler) and (d) begomovirus vector Bemisia tabaci (Genn).
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Related Sub-Topics:

Viruses Infecting Plants | DNA Viruses | Plant Disease
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Article Title: Geminiviridae
 
How to Cite close
Briddon, Rob W, and Stanley, John(Mar 2009) Geminiviridae. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000750.pub2]