Development of an Effective Mild Chimeric Virus for the Concurrent Control of Two Important Aphid-Transmitted Viruses in Cucurbits

Papaya ringspot virus W-type (PRSV W) and cucumber mosaic virus (CMV) severely damage cucurbits worldwide. Recently, we showed that the attenuated mutant PRSV WAC protects cucurbits against severe PRSV W infection. Here, recombinants WAC-CP, WAC-CPn, and WAC-CPc, respectively carrying the whole, N-terminal half, and C-terminal half of the CMV CP reading frame, were constructed. They induced attenuated symptoms followed by recovery in horn melon (Cucumis metuliferus) plants, similar to WAC. In Chenopodium quinoa plants, all recombinants induced infection without lesions. In horn melon plants, all recombinants remained stable after seven transfers and displayed a zigzag accumulation pattern of the beneficial protective virus, similar to WAC. From three tests with 30 plants, WAC-CP provided 100% protection one month after the challenge with PRSV W-CI or CMV, showing no severe symptoms. The absence of the challenge virus was verified by local-lesion assay and RT-PCR. Additionally, 93.3% protection was observed against the mixed challenge of W-CI + CMV. Although WAC-CPn and WAC-CPc provided high degrees of protection (76.7% - 100%) against CMV or PRSV W-CI, they only delayed the development of severe symptoms after the mixed challenge. WAC-CP was further tested in muskmelon plants, where it conferred 90.0% or 93.3% protection against W-CI or CMV, respectively, and 76.7% against the mixed challenge. The protein of individual inserts was not detected, whereas small interfering RNA was detected, suggesting that the protection against CMV is mediated by RNA silencing. Thus, WAC-CP has potential for the concurrent control of PRSV W and CMV in cucurbits.