Download The virus world of fig mosaic disease

The virus world of fig mosaic disease
G.P. Martelli1, D. Boscia2, M.A. Castellano1, M. Conti2, A. De Stradis2,
M. Digiaro3, T. Elbeaino3, G. Gattoni1 and A. Minafra2
1Dipartimento
di Protezione delle Piante e Microbiologia Applicata, Università degli Studi, Via Amendola 165/A, 70126 Bari.
di Virologia Vegetale del CNR, Strada delle Cacce 73, 10135 Torino e Via Amendola 165/A, 70126 Bari.
3Istituto Agronomico Mediterraneo, Via Ceglie 9, 70010 Valenzano (BA).
2Istituto
Introduction
Mosaic disease (FMD) is a cosmopolitan disorder of fig (Ficus carica),
characterized by various patterns of discolouration (mosaic, chlorotic
mottling and blotching, vein banding, ringspots, line patterns) and
malformation (twisting, puckering, rosetting) of the leaves. Infected plants
have reduced vigour and may bear small mottled fruits.
HEALTHY
In 1971, an isometric virus [Sowbane mosaic virus (SoMV)] was reported
from Italy, then viruses with filamentous particles were found in Japan
(unidentified possibile carlavirus), Herzegovina (unnamed potyvirus) and
Spain (unassigned unidentified virus). A turning point was in 2006-2007
when a putative closterovirus [Fig leaf mottle-associated virus 1 (FLMaV1)-Fig.2], a putative ampelovirus [Fig leaf mottle-associated virus 2
(FLMaV-2)-Fig.3] and an unnamed partially characterized flexivirus
(Fig.4), were reported first from Italy, then from different European,
Mediterranean, North and South American countries ( Martelli, 2009).
From symptomatic leaves of fig seedlings exposed to viruliferous A. ficus,
which contained only DMBs, a battery of nine dsRNAs was extracted, the
largest of which was c. 7 kbp in size (T. Elbeaino, unpublished infornation).
FLMaV-1, FLMaV-2, the flexivirus and the c. 7 kDa RNA, which
proved to be of negative polarity, were partially sequenced, and primers
for virus-specific PCR detection were designed. The extensive use of these
primers coupled with the observation of thin-sectioned tissues, is providing
increasing evidence that DMBs consistently occur whenever symptoms are
shown, regardless of their type, whereas the flexivirus seems to be
associated with symptomless infections. Whether the two closteroviruses
are symptom inducers or not, remains to be established.
A
FMD has a viral aetiology but its causal agents are still under scrutiny.
FMD is perpetuated by vegetative propagation and is graft- but not seedtransmitted. However, fig mosaic virus (i.e. double-membrane bodies,
DMB) is transmitted by the eriophyid mite Aceria ficus and other viruses
(FLMaV-2 and an unnamed flexivirus) may also spread naturally.
B
Fig 1. Aspects of DMBs in the cytoplasm of a symptomatic fig leaf. A) A group of DMBs Bar =
100 nm. B) DMBs showing the membranous envelope (Bar = 50 nm. C) Convoluted, electrondense filamentous elements accompanying DMBs Bar = 100 nm.
Fig 2. Cythopathology of leaf tissues of fig infected by FLMaV-1. D) Closterovirus-like particles
in a dip from a symptomatic leaf. Bar = 100 nm. E) Phloem companion cell with a massive
aggregated of virus particles (V). Bar = 500 nm. F) Clusters of membranous vesicles (arrows) in
the cytoplasm of a phloem companion cell. Bar = 500 nm.
Enviromental scanning electron microscope of a member of the species Aceria
The contemporary presence of multiple viruses in symptomatic plants from
widely separated geographical areas confirms that FMD is a complex
disorder and suggests that its putative agents have travelled around with
infected propagative material, so as to have now a worldwide distribution.
Fig 3. Cythopathology of leaf tissues of fig infected by FLMaV-2. G) Closterovirus-like
particles in a dip from a symptomatic leaf. Bar = 100 nm. H) Heavily damaged phloem tissues
showing necrotic and crushed cells with thickened cell walls (Cw). Aggregates of virus particles
(V) fill the lumen of a cell with evident signs of plasmolysis and containing rows of small
vesicles lining the plasmalemma. Bar = 150 nm. I) Close-up of the small vesicles (arrows). Bar =
100 nm.
Molecular analysis and Electron microscopy
Up to 1971, enveloped round to ovoid bodies 90-200 nm in diameter
[double-membrane bodies (DMB)-Fig.1A], were the only anomalous
intracellular structures consistently associated with FMD (G.P. Martelli et
al., 1993). DMB are the likely particles of what can tentatively be referred
to as Fig mosaic virus (FMV). DMBs have an envelope that seems to derive
from the endoplasmic reticulum and consists of a lipoprotein unit
membrane about 12 nm thick (Fig.1B), containing carbohydrates. They
also contain proteinaceous material and fine fibrils and often gather around
masses of convoluted, electron-dense filamentous elements (Fig.1C) which
contain carbohydrates and are partially digested by pronase. By contrast,
DMBs are insensitive to tetracycline.
Fig 4. Cythopathology of leaf tissues of fig infected by the flexivirus. L)Virus-like particles in a
dip from a symptomatic leaf. Bar = 100 nm. Bundles of filamentous particles in the cytoplasm
of a symptomless leaf (M, bar = 100 nm) and in a symptomatic leaf, which contains aslo DMBs
(N, bar = 50 nm).Cw=cell wall, Ch=chloroplast, M=mitochondrion, N=nucleus, V=virus.
REFERENCES
Martelli G.P., 2009. Fig mosaic. In: Hadidi A., Barba. M., Candresse T., Jelkmann W. (eds.). Virus and Virus-like Diseases of Pome and
Stone Fruits. APS Press, St. Paul, MN, USA (in press)
Martelli G.P., Castellano M.A., Lafortezza R., 1993. An Ultrastructural study of fig mosaic. Phytopathologia mediterranea 32: 33-43.