WELCOME ADDRESS BY CHAIRMAN ISHS WORKING GROUP FOR SMALL FRUIT VIRUS DISEASES

When I look back at the subjects presented and discussed at my first meeting with this group in Heidelberg in 1976, I amazed at the advances that have occurred in plant virology since then! To mention just a few - nucleic acid cloning and sequencing, cDNA probes of many kinds, riboprobes, monoclonal antibodies, PCR, RT-PCR, Agroinoculation, triparental mating, and transgenic plants that protect against viral infection!

With the great changes and advancements that we've seen, I am concerned about some present and future problems associated with these changes.

In 1991, G. de Zoeten wrote a Letter to the Editor of Phytopathology entitled 'Risk Assessment: Do We Let History Repeat Itself?'. He warned about possible problems that could arise as a result of viral-pathogen derived coat protein-mediated protection of transgenic crop plants. He stated that heterologous transcapsidation, phenotypic mixing and/or nucleic acid template switching could occur alone or in combination, as a result of a virus infecting the plant. He postulated that this could result in a 'bew disease', viral host ranges could be greatly altered and virus-vector relationships could be seriously altered.

In the past 3 or 4 years, a lot of transgenic cucurbits, sugar beets, oilseed rape plants, tomatoes and cotton plants have been released to the field for evaluation world-wide. In the past 1 or 2 years a few field trials have been done with transgenic cucurbits, tomatoes and potatoes and other crops for evaluation of CP-mediated protection against viral infection. There will be much more to come, including fruit crop plants.

In 1986 Bujarski & Kaesberg were the first to show complimentation between defective genes of brome mosaic virus (BMV), using normal RNA 1 & 2 plus a deletion mutant of RNA 3. Co-inoculation to barley resulted in fully infectious wild type BMV. In 1992, S. Gal et al. at the Freidrich-Meischer Institute (Virology 187: 525–533) showed by use of 2 constructs of CaMV (a pararetrovirus), both defective for the full compliment of genes, that in planta recombination of viral nucleic acid occured. One construct contained a portion of gene V and all of VI (inclusion body protein and systematic movement in some Nicotiana spp.) and was under the control of the 19s promoter. The other construct contained genes I through V, and was under the control of the 35s promoter. Brassica napus plants were Agroinoculated with the gene V/VI construct. These plants resulted in having gene VI incorporated into their genome. Non-transgenic plants inoculated with CaMV deficient for a portion of gene V and gene VI did not become infected. Transgenic plants inoculated with the same deficient CaMV became systemically infected. Viraln nucleic acids extracted from these plants were sequenced and it was found that in in planta RNA and DNA recombination events were involved in the production of fuctional virus. More recently, Geene & Allison at Michigan State University (Science 263: 1423–1425≈ in 1994, published an article demonstrating RNA recombination between the transgenic mRNA of tobacco plants and an inoculated mutant of cowpea chlorotic mottle virus (CCMV) deficient in the 3'-one third of its capsid gene. The portion of the CCMV RNA-3 complimetary to the missing 3'-part of the RNA contained 3 marker mutations, one of which was a silent mutation (Not I site) in the transgenic plants. In non-transgenic plants, inoculation of CCMV deletion mutants deficient in the 3'-one third of its capsid gene did not allow systemic infection. However, when inoculated to transgenic plants, systemic infection occured. Virions were purified from these systematically infected plants. Recovered viral RNA was sequenced. All 3marker mutations that were present only in the transgenic RNA were identified in the nucleotide sequence of the capsid gene. Only wild-type sequence was recovered from control transgenic plants inoculated with wild-type CCMV. These results indicated that the systemic infection resulted from recombination between the mRNA expressed by the plant and the challenging deletion inoculum. Of 125 transgenic plants tested, 4 recombinant viruses were verified from 3 different transgenic plant lines. Sequences derived from the recombinants revealed that each resulted from a distictly different aberrant homologous recombination event. Their work showed that precise recombination was not required to restore virus viability. In 1991, Mayo and Jolly (J. Gen. Virol 72; 25912595) demonstrated evidence that the 5'-end of the RNA of 2 strains of potato leafroll virus (PLRV) contained sequences from recombination with host plant chloroplast gene mRNA. The good news is that R. Martin (Proceedings from 2nd International Symposium on Biosafety Resultsof Field tests of Genetically Modified Plants and Microorganism-Goslar, Germany, May, 1992) tested potato plants transgenic for PLRV-CP for heterologous transcapsidation following acquisitions access to these plants were not able to transmit any of these 3 viruses.

At the last Symposium of our group held in Vienna, there were several papers concerning barbaceous plants transgenic for plumpox virus CP. At the present Symposium in Rome, there are three papers dealing with PPV-CP-mediated protection of transgenic plants, some of which are fruit trees. In addition there is one dealing with transgenic strawberry plants. Dr. R. Martinwill also be giving us a special presentation on altrnative to the use of CP genes in transgenic protection against virus infection.

We are nearing at this time the release of transgenic fruit plants for field testing. We are at very important juncture in the control of fruit virus diseases!

I worry that the aforementioned recent published findings on in planta recombination of transgenic viral nucleic acids with vitl nucleic acids of the challenging virus could resultsin infections whole virus, which could cause ne strains of virus with expanded host ranges and/or changed vector relations. If this were to happen or even be percieved to happen by regulatory agencies, the result could be very stringent regulations for the field testing and the use of transgenic fruit crops for protection against virus and virus-like diseases.

We should take a 'pro-active' stance and endeavour to do the necessary scientific tests to answer the critical questions that will arise. It is better to use scientific information than guesses and suppositin. Some guidlines to consider for future field testing of fruit crop plants transgenic for various parts of viral genomes were outlined at the 1992 Goslar, Germany Sympsium (Teso, B., Proceedings from 2nd International Symposium on Biosafety Results of Field Tests of Genetically Modified Palnts and Microorganisms, Gosslar, Germany, May, 1992).

Donald Ramsdell,
Chairman ISHS Working Group
Small Fruit Virus Diseases.