Effects of virus genotype and temperature on seed transmission of nepoviruses

Abstract

SUMMARYTransmission of different nepoviruses through chickweed (Stellaria media) seed was differently affected by ambient temperature during seed production. Raspberry ringspot and tomato black ring (Scottish isolate) viruses were similarly and frequently transmitted at14, 18 and 22 oC, whereas arabis mosaic virus was transmitted most frequently at 14 oC, and strawberry latent ringspot and tomato black ring (German isolate) viruses at 22 oC. When infected by seed‐borne nepoviruses, seedlings of S. media and other species were symptomless at 15–25 oC, and the viruses were therefore detected by inoculating sap to Chenopodium quinoa indicator plants. However, typical symptoms of arabis mosaic and tomato black ring viruses were induced by growing Nicotiana tabacum, N. clevelandii and C. quinoa seedlings infected with seed‐borne virus at 33–37 oC during the third and fourth weeks after sowing, preceded and followed by periods at 15–25 oC. The proportion of N. tabacum seedlings developing symptoms was the same as that of untreated seedlings yielding sap‐transmissible virus.Seed transmissibility of pseudo‐recombinant isolates of raspberry ringspot and tomato black ring viruses, containing RNA‐i from one virus strain and RNA‐2 from another strain, depended greatly on the transmissibility of the strain contributing RNA‐i. The source of RNA‐2 had an additional but smaller influence. The satellite RNA (RNA‐3) of tomato black ring virus was seed‐transmitted in S. media and its occurrence in cultures did not affect the frequency of transmission of the virus. Results of testing the infectivity of extracts of seed from infected mother plants suggested that failure of seed transmission reflected failure to become established in the seed, not subsequent inactivation.Whereas seed transmissibility of raspberry ringspot virus is primarily dependent on information carried in RNA‐i, transmissibility by nematode vectors, another property of major ecological importance, is determined by RNA‐2. In the field, selection pressures presumably can act independently on the two parts of the genome but evidence was also obtained of selection for mutual compatibility of RNA‐i and RNA‐2.