Plant Virus Satellites

Abstract

Plant virus satellites are molecular parasites of plant viruses that rely on a helper virus for replication but do not supply any essential function to the helper virus. They can affect the accumulation of the helper virus and alter the host symptoms induced by the helper virus. They include satellite viruses, satellite ribonucleic acids (RNAs) and satellite deoxyribonucleic acids (DNAs). Size, genome structure and replication strategies vary greatly among plant virus satellites. Some plant virus satellites have been used as biocontrol reagents or as vectors to express exogenous genes. They have also been used as reporters for studying the evolutionary mechanisms of the helper virus.

Key Concept:

  • Plant virus satellites depend on a helper virus for replication, but they may use different host factors.

  • Plant virus satellites do not provide any essential function for the helper virus.

  • Plant virus satellites have varied effects on their helper viruses and hosts, including reduction in virus titer and exacerbation or attenuation of symptoms.

  • Some plant satellite RNAs use ribozymes to resolve multimers generated during replication.

  • Satellite DNAs are the symptom determinants in an increasing number of crop diseases caused by geminiviruses.

Keywords: subviral agents; RNA structure; ribozymes; plant necrosis; programmed cell death; betasatellites

Figure 1.

Rolling circle replication of sats. (a) Replication of circular satRNAs. Plus strand (blue line) circular satRNA is used as a template for the synthesis of linear minus strand multimers (orange line; steps 1–3). There are two alternate models for the following steps. In model a, the multimeric minus strand satRNA is used as a linear template for the synthesis of plus strand and the resulting plus strand satRNA multimer self‐cleaves into monomeric plus strand, which self‐ligates and becomes the circular satRNA (step 4a–7a). In model b, the multimeric minus strand satRNA self‐cleaves into a monomer, followed by self‐ligation to become circular minus strand satRNA, which is used as a template for the synthesis of multimeric plus strand satRNA (steps 4b–8b). Through self‐cleavage and ligation, circular plus strand satRNA is produced (step 9b–10b). (b) Repliction of circular satDNAs. Circular satDNA (green line) is converted into a covalently closed dsDNA replicative form (RF). The plus strand of the RF dsDNA is nicked by the Rep protein encoded by the helper virus at the nanosequence site. Rep also covalently binds the 5′ end of plus strand (steps 1–3). The synthesis of new plus strand satDNA is initiated from the corresponding 3′ end of the minus strand (pink line). The nascent plus strand displaces the parental plus strand, which self‐ligates and becomes circular plus strand satDNA (steps 4–5a). The nascent plus strand and the minus strand form RF dsDNA for further replication (steps 4–5b).

Figure 2.

Ribozyme structures shown as secondary (above) and three dimensional structures. Nucleotides involved in catalysis are shown in yellow boxes; cleavage occurs between the nucleotides boxed in red. (a) Hammerhead ribozyme, consisting of three helices (Stems I, II and III). (b) Hairpin or paperclip ribozyme, consisting of the four helices (Stems A, B, C and D). Reprinted from Serganov and Patel (2007), with permission from Macmillan Publishers Ltd, copyright 2007.

Figure 3.

CMV satRNAs alter CMV symptoms in infected tomato plants. (a) Nucleotide sequence of the necrogenic region of CMV satRNAs. Mutations (highlighted in green) convert the necrogenic D4 satRNA to an attenuating strain (Dm satRNA). (b) Symptoms induced by CMV/Dm satRNA, CMV and CMV/D satRNA in tomato plants.

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Vogt P (1999) Satellites and Defective RNAs. New York: Springer. 179pp.

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Xu, Ping, and Roossinck, Marilyn J(Apr 2011) Plant Virus Satellites. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000771.pub2]