MUTATIONAL ANALYSIS OF POTATO YELLOW MOSAIC GEMINIVIRUS

Mutations have been inserted into the virion and complementary sense O RFs encoding proteins with M(r)s in excess of 9 kDa of both DNA A and DNA B of potato yellow mosaic geminivirus (PYMV). Wild-type and mutant monomeric clones were tested for their ability to replicate, produce PYMV-specific DNA, spread and cause symptoms in Nicotiana benthamiana plants following biolistic inoculation. Dimeric clones of the DNA A mu tants were also investigated by agroinoculation of leaf discs. In cont rast to N. benthamiana plants agro-inoculated with PYMV DNA A, in whic h the wild-type DNA A component was capable of limited independent rep lication and spread, both excised DNA A and B components were required for DNA replication and symptom development in plants inoculated by t he biolistic method. Mixtures of both genomic components were also inf ectious for potato plants following biolistic inoculation. Mutations i n ORFs AL1, AL2, BR1 and BL1 resulted in clones incapable of infecting N. benthamiana plants. However, the AL2 mutation, but not the AL1 mut ation, allowed viral DNA replication in leaf discs. Mutations to both the AR1 and AL3 ORFs produced clones which were infectious in plants b ut showed a considerable delay in the production of attenuated symptom s as compared to wild-type infections. Mutating the AL3 ORF dramatical ly reduced viral DNA replication in both whole plants and leaf discs. Mutations to the AL4 ORF produced clones which were as infectious for both N. benthamiana and potato plants as the wild-type clones. Our res ults are compared with those from mutagenesis studies on related bipar tite geminiviruses.