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Double-Virus Resistance of Transgenic Oriental Melon Conferred by Untranslatable Chimeric Construct Carrying Partial Coat Protein Genes of Two Viruses

Production of oriental melon (Cucumis melo var. makuwa) in Asia is often limited by two potyviruses, the watermelon infecting type of Papaya ringspot virus (PRSV W) and Zucchini yellow mosaic virus (ZYMV). In order to engineer transgenic resistance to these two viruses, an untranslatable chimeric DNA comprising partial coat protein (CP) sequences of ZYMV and PRSV W was constructed and used to transform the elite cultivar of oriental melon, Silver Light, by Agrobacterium. Greenhouse evaluation by mechanical challenges with ZYMV and PRSV W, alone or together, identified transgenic lines exhibiting different levels of resistance or complete immunity to ZYMV and PRSV W. Molecular analyses of transgenic lines revealed random insertion of transgene into the host genome, with insert numbers differing among transformants. There was no correlation between transgene insert numbers and the degree of resistance expressed by transgenic lines. The levels of accumulation of transgene transcript varied among transgenic lines. However, an inverse correlation was observed between the level of accumulation of transgene transcripts and the degree of virus resistance. Moreover, small interfering (si)RNA was readily detected from the immune and highly resistant lines, but not from the weakly resistant and susceptible lines. Altogether, our results indicated that RNA-mediated post-transcriptional gene silencing (PTGS) was the underlying mechanism of double-virus resistance of the transgenic melon lines. The segregation analysis of the R₁ progeny of the immune line ZW-1 indicated that the single inserted transgene is associated with the resistance phenotype and is inherited as a dominant trait. These transgenic melon lines with high degrees of resistance to ZYMV and PRSV W have great potential for the control of ZYMV and PRSV W in C. melo in Asia and elsewhere.