LAUSR

Papaya (Carica papaya L.) production is seriously limited by papaya ringspot virus (PRSV) worldwide, including Vietnam. Control of PRSV by cross protection is dependent on the availability of effective mild strains. Here, an infectious cDNA clone was constructed from PRSV isolate TG5 from South Vietnam. Site-directed mutagenesis with point mutations on the essential motifs of the helper component proteinase (HC-Pro) was performed, with or without deleting five amino acids (d5) in its N-terminal region. Mutants TG-d5, TG-d5I7, and TG-d5L206 containing d5, d5+F7I, and d5+F206L, respectively, induced mild mottling followed by symptomless recovery on papaya and infected Chenopodium quinoa without lesion formation. Each mutant accumulated in papaya at reduced levels with a zigzag pattern and was stable beyond six monthly passages. The cross-protection effectiveness of the three mutants in papaya against TG5 was investigated, each with 60 plants from three independent trials. The results showed that each mutant provided complete protection (100%) against TG5, one month after the challenge inoculation, as verified by no severe symptoms and no local lesions in C. quinoa. Further tests revealed that TG-d5I7 also confers high levels of protection against other severe PRSV isolates from South Vietnam, including isolates DN (97%) and ST2 (50%). However, TG-d5I7 is ineffective or less effective (0-33%) against seven other PRSV severe strains from different geographic origins, including the isolate HN from North Vietnam. Our results indicate that the protection by the three mutants is highly strain-specific and suitable for the control of PRSV in South Vietnam.