The nature of the associations between Olpidium brassicae and lettuce big-vein and tobacco necrosis viruses

Abstract

Tobacco necrosis virus (TNV) was associated intimately with zoospores of Olpidium brassicae (Wor.) Dang. Transmission of TNV was not prevented when zoospores were washed by consecutive centrifugations that removed TNV demonstrable by mechanical assay. Transmission was prevented when zoospores from roots infected with TNV and Olpidium were discharged into TNV antiserum or when the antiserum was mixed with zoospores either 1 minute before or after TNV was added in vitro. The TNV antiserum reduced, but did not completely prevent, transmission when added to TNV-zoospore mixtures that had been prepared for more than 15 minutes. Olpidium survived in dry roots for 39 months, withstood acid treatment, and transmitted lettuce big-vein virus (BVV). Transmission of TNV, in contrast, was not obtained from roots that were air-dried for 8 days and was prevented by acid treatment of resting spores from freshly harvested roots. Thus, BVV seems to be internally borne in resting spores whereas TNV is not. It is proposed that TNV is acquired by zoospores after the separate entities are independently released from roots, rather than being carried internally by these zoospores, as BVV seems to be. This acquisition probably consists of adsorption of TNV to the zoosporic plasma membrane, from which it is released after the fungal protoplasm infects the host. Although it is possible that TNV might be taken into the fungal protoplasm during flagellar retraction, or during prolonged exposure of zoospores to TNV, there is no clear evidence of this. The few transmissions obtained when zoospores and TNV were mixed at least 15 minutes before antiserum was added may have been caused by in vitro encystment of zoospores and consequent protection of the virus.