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 1 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 to 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 RNAs were detected, suggesting that protection against CMV is mediated by RNA silencing. Thus, WAC-CP has potential for the concurrent control of PRSV W and CMV in cucurbits.