1999 Workshop Abstracts | Virus Evolution Home Page | Plant Biology Home Page

Producing a helper component to mediate vector transmission:
is this associated with a selective cost
during cauliflower mosaic virus infection cycle?

R. Froissart*", Y. Michalakis" and Stephane Blanc*
* Corresponding authors: Institut National de Recherche Agronomique 30380 St-Christol-lez-Ales FRANCE
" Centre National de Recherche Scientifique - Universite Paris VI, 75 005 Paris
e-mail : froissar@ensam.inra.fr

Some of the viruses transmitted by vectors in a non-circulative manner may interact with their vector directly via the coat protein (cucumo- and alphamoviruses), whereas others encode for a non-structural protein that binds to both the virus particle and the vector, thus bridging between the two (poty-, caulimo-, waika- and probably closteroviruses). This non-structural protein is named helper-component (HC). The most striking property of helper components is that they can assist the transmission of the virus encoding them but also that of other related viruses infecting the same plant or even another plant visited by the vector⁽¹⁾. This phenomenon of assistance between viruses during aphid-transmission has been named "hetero-assistance". In poty- and caulimoviruses, naturally-occurring strains that are self- (producing a functional HC) and non-self-transmissible (producing no HC or a non-functional HC) by vectors have been described.

The system we are studying is the cauliflower mosaic caulimovirus (CaMV) transmitted by aphids. In this system, the HC is encoded by the gene II⁽²⁾ which seems to be otherwise dispensable for the virus cycle in planta. Indeed, the naturally-occurring isolate CM4-184 exhibits a 421-bp deletion in the central region of gene II⁽³⁾. This isolate does not produce the corresponding HC and hence, though perfectly infectious in the host plant, it is never self-transmitted. Non-self-transmission of CM4-184 by aphids is however possible if the plant is co-infected with another isolate harboring a functional gene II or if the aphids have previously fed a plant infected with a HC-producing caulimovirus. Previous reports described a number of other strains which are non self-transmissible by vectors, due to point mutations or partial deletions in gene II ⁽⁴⁾, indicating that the appearance of "non-self-transmissibility" may be a rather frequent phenomenon in CaMV populations. A question then arises: is coding for a helper component associated with a selective cost?

To address this question, we engineered various gene II defective clones of CaMV and evaluated their infectivity and pathogenicity for the host plant. In the corresponding CaMV derivative mutants, gene II was altered as follows: "Del-S" has a 421-bp deletion identical to that exhibited by the isolate CM4-184 mentioned above; "Top-S-6, 100 & 120" have point mutations that introduce early stop-codon in CaMV HC at amino acid position 6, 100 & 120, respectively; "Tart-S" has a point mutation that changes the gene II start-codon from ATG to AAG. The genome of these clones was otherwise strictly identical to that of the wild type clone, Cabb-S.

Infections of plants were made with all clones. We have evaluated the infectivity and the timing of symptoms appearance for each mutant, relative to wild type Cabb-S. The poster presents preliminary results surprisingly suggesting that the fitness of CaMV variants may be decreased by alterations of the HC-coding gene. How this is answering the question we addressed in the title is discussed in light of the mechanisms controlling the genome expression.

References
(1) Pirone & Blanc, 1996. Ann. Rev. Phytopathol. 34:227-247.
(2) Blanc et al., 1993. Virology 192:643-650.
(3) Howarth et al., 1981. Virology 112:678-685.
(4) Al-Kaff & Covey 1994. J Gen. Virol. 75:3137-3145.

Abstract - Presented at the Virus Evolution Workshop
Ardmore, OK
October 21 - 24th, 1999

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e-mail: mroossinck@noble.org

Dr. Marilyn Roossinck
Plant Biology Division
The Noble Foundation
P.O. Box 2180
Ardmore, OK 73402

phone: 580 224-6630