Length of poly(A) tail affects transcript infectivity of three ZYMV symptom variants differing at only five amino acid positions

The incidence of plant virus diseases infecting important cucurbit vegetables in Korea has increased as new isolates have been introduced, associated with warming temperatures and vector movement caused by climate change. Transcript infectivity of full-length infectious clones of three new ZYMV isolates was dependent upon the length of the poly(A) tract; transcripts with 55 A residues were inefficiently infectious, whereas 60 A residues resulted in highly efficient infection and significantly reduced time to production of systemic symptoms. Sequences of isolates BR1 (MH042024), BR2 (MH042025), and BR3 (MH042026) showed 99% pair-wise identity and differed at only five amino acid positions in: HC-Pro (D134N in BR2), CI (F31 L in BR1), and 6 K2 (A24V in BR3), and two positions in NIb (T300S in BR2, H429Q in BR3). Cucurbita pepo plants inoculated with transcripts of clones with these amino acid differences showed symptoms that ranged from mild to severe. Phylogenetic analysis of these new ZYMV isolates with previously characterized isolates indicated that the new isolates had 87.8–97.5% identity to other ZYMV isolates and were most closely related to recent ZYMV isolates from Australia and Spain.