PHYTO MEDIZIN Mitteilungen der Deutschen ... - Die DPG

control strategy may be the use of cross-protection, the phenomenon in which
mild or attenuated strains of viruses protect plants against infection with
severe strains of the same virus. It is thought that in most cases posttranscriptional
gene silencing (PTGS) is the mechanism behind crossprotection.
We have used an engineered mutant of Fny-CMV lacking the 2b
gene, a suppressor of gene-silencing (Fny-CMV∆2b) in cross-protection
assays. This mutant successfully cross-protected against the parental wildtype
strain Fny-CMV in tobacco and N. benthamiana. Using RT-PCR to
distinguish between both strains, no Fny-CMV could be detected in systemic
leaves in most cases of cross-protected plants. However, in one experiment
the presence of Fny-CMV could be detected in systemic leaves arguing
against the hypothesis that Fny-CMV∆2b induces gene-silencing and hence
the degradation of viral RNA from the parental strain. Currently, work is
underway to look for the presence of small interfering RNAs, markers for the
induction of PTGS, in cross-protected tobacco plants. m species entering the
EU. This legislation also applies to previously imported untested true-seed
accessions present in EU gene-collections. In spring 2003 for one accession
of Solanum kurtzianum virus symptoms appeared upon mechanical inoculation
of test plants, indicating an infection by Alfalfa mosaic virus (AMV).
The presence of this virus was confirmed by DAS-ELISA using AMVspecific
antibodies. Testing of the remaining twenty-three plants of this accession
revealed no further infections, thus demonstrating that the rate of
seed transmission was low. The infected plant was destroyed, although AMV
is not listed as a quarantine organism in the EU. However, since true potato
seed is used in the initial stages of potato production, there is a high risk that
viruses may become widely spread. Finally, this finding emphasizes the importance
of test plants as a tool for post-entry quarantine testing, since this
method enables the detection of viruses apart from those under specific testing.
Alfalfa mosaic virus detected in true seed of Solanum kurtzianum during
post-entry quarantine testing
AW Werkman, JThJ Verhoeven & JW Roenhorst; Plantenziektenkundige Dienst, P.O.
Box 9102, 6700 HC Wageningen
Each year about sixty Solanum seed accessions from the Dutch-German
gene-bank are tested for the presence of quarantine viruses as part of the
annual seed reproduction programme. This testing is obliged by the legislation
of the European Union (EU) for all Solanum species entering the EU.
This legislation also applies to previously imported untested true-seed acces-
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