Virome Analysis Deciphers the First Virus Occurrence in Melothria scabra, Revealing Two Potyviruses, Including a Highly Divergent Zucchini Yellow Mmosaic Virus Isolate

Melothria scabra, an annual vine plant belonging to the family Cucurbitaceae, is usually found as a weed in agricultural ecosystems, making it a potential reservoir for crop viruses. Nonetheless, no plant virus has been documented to infect M. scabra to date. In the present study, M. scabra leaves with plant virus disease symptoms were sampled and subjected to sequencing through metatranscriptome and small RNA methods. High-throughput data analysis revealed the presence of two potyvirus species, zucchini tigre mosaic virus (ZTMV) and zucchini yellow mosaic virus (ZYMV), which were subsequently confirmed through reverse transcription PCR (RT-PCR) detection. The complete genome sequences of ZTMV and ZYMV in M. scabra, designated as ZTMV-ms (PQ720520) and ZYMV-ms (PQ720521), were determined by a combination of RT-PCR, rapid amplification of cDNA ends and Sanger sequencing. The full-genome length of ZTMV-ms and ZYMV-ms is 10,331 nt and 9,602 nt, respectively, excluding the 3' poly(A) tail. Notably, ZYMV-ms showed 80.15% similarity to its best BLASTn hit (AJ515911.1, ZYMV-WM), approaching the threshold for defining new Potyvirus species, thus classifying ZYMV-ms as a highly divergent ZYMV isolate. Both ZTMV-ms and ZYMV-ms show typical virus-derived small interfering RNA (vsiRNAs) characteristics of plant viruses, with 21- and 22-nt vsiRNAs, the latter being the most abundant, a feature rare among plant viruses. These findings provide new insights into the diversity of plant host antiviral RNAi response, as well as the evolution and host expansion of ZTMV and ZYMV, with implications for virus prevention and control.