EVIDENCE THAT HETEROENCAPSIDATION BETWEEN 2 POTYVIRUSES IS INVOLVED IN APHID TRANSMISSION OF A NON-APHID-TRANSMISSIBLE ISOLATE FROM MIXED INFECTIONS

A non-aphid-transmissible isolate of zucchini yellow mosaic virus, ZYMV-NAT, which possesses a transmission-deficient capsid protein, was regularly transmitted by aphids from plants infected concomitantly by this virus and a transmissible isolate of papaya ringspot virus type W (PRSV-E2). This phenomenon was reproduced in in vitro acquisition experiments by mixing virions purified from plants co-infected by ZYMV-NAT and PRSV-E2, with PRSV-E2 helper component (HC) preparations. In contrast, ZYMV-NAT was not transmitted regardless of the HC used, when purified ZYMV-NAT virions were used either alone or mixed with purified PRSV-E2 virions. Immunosorbent electron microscopy experiments using the decoration technique were done with crude or purified extracts from singly or doubly infected plants. In extracts from plants infected by only one virus, particles were either fully decorated or not decorated when homologous or heterologous antisera were used, respectively. In extracts from plants infected by the two viruses, a third type of particle presenting irregular decorations was frequently observed. These particles that showed a phenotypic mixing were also revealed by the two-sites enzyme-linked immunosorbent assay technique: high absorbance values were observed at A405 nm with extracts from plants with a mixed infection, while no or weaker absorbance was obtained with extracts from single infections alone or mixed. It is concluded that ZYMV-NAT aphid transmission, in vivo from plants doubly infected by this virus and PRSV-E2 or in vitro from preparations containing purified virions from these doubly infected plants, occurred through heteroencapsidated particles and was mediated by PRSV-E2 HC.