Plant Molecular Biology 43: 307–322, 2000.
M.A. Matzke and A.J.M. Matzke (Eds.), Plant Gene Silencing.
© 2000 Kluwer Academic Publishers. Printed in the Netherlands.
Plant DNA viruses and gene silencing
Simon N. Covey
and Nadia S. Al-Kaff
John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7UH, UK (
author for correspondence; e-mail
Key words: antipathogen response, Cruciferae, MRE, review, 35S promoter, viral vectors
Gene silencing is a multifaceted phenomenon leading to propagative down-regulation of gene expression. Gene
silencing, ﬁrst observed in plants containing transgenes, can operate both at the transcriptional and post-
transcriptional levels. Silencing effects can be triggered by nuclear transgenes and by cytoplasmic RNA viruses,
and it can be propagated between these elements and endogenous plant genes that share sequence homology.
Although some aspects of gene silencing are becoming better understood, little is yet known about the relationship
between nuclear and cytoplasmic events. Plant DNA viruses – both the ssDNA geminiviruses and the reverse-
transcribing pararetroviruses– have properties with the potential to initiate gene silencing in the nucleus and in the
cytoplasm. Characteristics include production of multiple copies of viral DNA genomes in the nucleus, illegitimate
integration of viral DNA into host chromosomes mimicking transgene transformation, and generation of abundant
viral RNAs in the cytoplasm. Evidence is emerging that geminiviruses and plant pararetrovirusescan interact with
the gene silencing system either from introduced DNA constructs or during viral pathogenesis. Some observations
suggest there are complex relationships between DNA viral activity, transcriptional and post-transcriptional gene
silencing mechanisms. DNA viruses also have properties consistent with an ability to counteract the plant silencing
response. In this article, features of plant DNA viruses are discussed in relation to gene silencing phenomena, and
the prospects for understanding the interaction between nuclear and cytoplasmic silencing processes.
Abbreviations: ACMV, African cassava mosaic virus; CaMV, cauliﬂower mosaic virus; CP, coat protein; CsVMV,
cassava vein mosaic virus; GFP, green ﬂuorescent protein; GUS, β-glucuronidase;PTGS, post-transcriptionalgene
silencing; PVX, potato virus X; TGMV, tomato golden mosaic virus; TGS, transcriptional gene silencing; TYDV,
tobacco yellow dwarf virus
In recent years, phenomena collectively described as
gene silencing have impacted many diverse areas of
plant molecular biology. Gene silencing is a cellular
mechanismthat targetsspeciﬁc nucleic acid sequences
for down-regulation or degradation and has the dis-
tinctive property of propagating inactivation to ho-
mologous nucleic acids within and between cells. Ini-
tially, gene silencing was associated with transgenes.
Transcriptional gene silencing (TGS) is often linked
with transgene multimerisation or re-arrangement.
Transgene-associated post-transcriptionalgene silenc-
ing (PTGS) targets the transgene RNA and RNA from
homologous endogenous genes for degradation in the
cytoplasm and is associated with continued transcrip-
tion of the transgene. PTGS can also be triggered by
cytoplasmic RNA viruses. A special feature of gene
silencing is the ability to propagate a silencing sig-
nal between the cytoplasm and nucleus, and between
cells, sometimes over long distances. The mobile sig-
nal might involve a small RNA molecule that has been
correlated with several types of gene silencing (Hamil-
ton and Baulcombe, 1999). As more is understood
about the diverse types of gene silencing – covered
in excellent articles and reviews (Matzke and Matzke,