IntroductionBegomoviruses are mainly transmitted by whiteflies. However, a few begomoviruses can be transmitted mechanically. Mechanical transmissibility affects begomoviral distribution in the field.Materials and methodsIn this study, two mechanically transmissible begomoviruses, tomato leaf curl New Delhi virus-oriental melon isolate (ToLCNDV-OM) and tomato yellow leaf curl Thailand virus (TYLCTHV), and two nonmechanically transmissible begomoviruses, ToLCNDV-cucumber isolate (ToLCNDV-CB) and tomato leaf curl Taiwan virus (ToLCTV), were used to study the effects of virus-virus interactions on mechanical transmissibility.ResultsNicotiana benthamianaand host plants were coinoculated through mechanical transmission with inoculants derived from plants that were mix-infected or inoculants derived from individually infected plants, and the inoculants were mixed immediately before inoculation. Our results showed that ToLCNDV-CB was mechanically transmitted with ToLCNDV-OM toN. benthamiana, cucumber, and oriental melon, whereas ToLCTV was mechanically transmitted with TYLCTHV toN. benthamianaand tomato. For crossing host range inoculation, ToLCNDV-CB was mechanically transmitted with TYLCTHV toN. benthamianaand its nonhost tomato, while ToLCTV with ToLCNDV-OM was transmitted toN. benthamianaand its nonhost oriental melon. For sequential inoculation, ToLCNDV-CB and ToLCTV were mechanically transmitted toN. benthamianaplants that were either preinfected with ToLCNDV-OM or TYLCTHV. The results of fluorescence resonance energy transfer analyses showed that the nuclear shuttle protein of ToLCNDV-CB (CBNSP) and the coat protein of ToLCTV (TWCP) localized alone to the nucleus. When coexpressed with movement proteins of ToLCNDV-OM or TYLCTHV, CBNSP and TWCP relocalized to both the nucleus and the cellular periphery and interacted with movement proteins.DiscussionOur findings indicated that virus-virus interactions in mixed infection circumstances could complement the mechanical transmissibility of nonmechanically transmissible begomoviruses and alter their host range. These findings provide new insight into complex virus-virus interactions and will help us to understand the begomoviral distribution and to reevaluate disease management strategies in the field.