The Potential of a Beneficial Satellite RNA of Cucumber Mosaic Cucumovirus to Acquire Deleterious Functions : Nature Versus Greenhouses
Part of the notoriety of cucumber mosaic cucumovirus (CMV) lies in its ability to support replication of a small (332–405 nucleotide) satellite RNA (satRNA) which can sometimes affect CMV replication, pathogenesis and symptom expression. Variants with attenuating effects (benign satRNAs) have been extensively studied as potential or actual biocontrol agents for the prevention of CMV-induced diseases. This approach has become the basis for experimental and practical field tests where satRNA-mediated protection has been achieved successfully by either preinoculation of seedlings (cross-protection or “vaccination”) or by transgenosis, and apparently no adverse effects have been detected.
The recurrent epidemics of CMV in Italy have offered the opportunity to carry out phylogenetic analysis of several wild-type variants of CMV satRNA in a “natural laboratory”. The population of CMV satRNA that we have studied since 1987 has had a propensity to maintain its original genetic heterogeneity from one year to the other. It seems that: i) molecules of the same lineage have limited possibilities to accumulate substitutions; ii) regions of the molecules with the possibility of accumulating substitutions are very likely the same in all the lineages; iii) the data are in agreement with the proposed maintenance of a functional molecular structure as a constraint on CMV satRNA evolution.
Although in principle we agree with the concept that benign CMV-SatRNAs should be used as biological control agent with extreme care, it should also be taken into account that: i) risks pointed out for a given satRNA variant may not be applicable to others; ii) on the basis of the long-term tests carried out in China, data obtained from nature do not seem to fit completely greenhouse experiments. In this view, a satRNA-mediated cross-protection strategy with a pseudorecombinant strain of CMV and several constructs to be used for transformation of tomato plants are under development in our laboratory. The results of some field and laboratory experiments will be briefly reported.
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