Plant DNA Viruses as Gene Vectors

Summary

Caulimoviruses and geminiviruses are the only known plant DNA viruses. Both groups are candidates for carrying foreign DNA into plants, spreading it systemically and expressing high yields of the corresponding gene product. This has been achieved with hybrids of cauliflower mosaic virus (CaMV) and certain model genes. A major obstacle to the use of this technology is the high mutation rate of CaMV, probably caused by its mode of replication via reverse transcription, involving switches of nascent DNA strands on single-stranded RNA and DNA templates. If these occur at illegal positions, deletions and duplications are created. These are rarely observed with wild-type infections but deletions of foreign sequences are selected for if the total length of the hybrid genome is too large, when inserted sequences interfere with virus transcription and translation, when the secondary structure of replicative intermediates is changed, or if expressed payload protein is disadvantageous to virus or plant cell. Similar problems arise with geminiviruses and single-stranded RNA viruses with single-stranded genomic replicative intermediates in their life cycles. This problem of instability could be avoided by creating master copies of double-stranded DNA of the hybrid virus in the plant cells from which the single-stranded replicative intermediates are produced continuously. This could be achieved by agroinfection (transfer of virus genomes as double-stranded DNA multimers into the host cell with the help of agrobacteria). An interesting achievement in this field is agroinfection with the geminivirus, maize streak virus.