The 6K2 protein and the VPg of potato virus A are determinants of systemicinfection in Nicandra physaloides

Infection with the isolate PVA-M of potato virus A (PVA; genus Potyvirus) i s restricted to the inoculated leaves of Nicandra physaloides (Solanaceae), whereas the isolate PVA-B11 infects plants systemically by 10 days post in oculation. Resistance to systemic infection was shown to develop during pla nt growth. A recombinant virus (B11-M) in which a 1,208-nucleotide sequence of the full-length cDNA done of PVA-B11 was replaced with the correspondin g sequence from PVA-M displayed a phenotype similar to that of PVA-M. The r eplaced sequence contained four amino acid differences between the two isol ates: one in the 6K2 protein and three in the viral genome-linked protein ( VPg). Site-directed mutagenesis of the cDNA clones and inoculation of the m utants to N. physaloides indicated that the amino acid substitutions of Met 5Val in the 6K2 protein or Leu185Ser in the VPg permitted vascular movement and systemic infection. However, resistance was only partially overcome by these changes, since systemic infection proceeded at a slower rate than wi th PVA-B11, The amino acid substitution Val116Met in the VPg alone was suff icient to overcome resistance and recover the phenotype of the isolate PVA- B11. These data indicated that both the 6K2 protein and the VPg were avirul ence determinants of PVA-M in N. physaloides and suggested a possibly coord inated function of them in the vascular movement of PVA.