PARTIAL CHARACTERIZATION AND MOLECULAR-CLONING OF A CLOSTEROVIRUS FROM SWEET-POTATO INFECTED WITH THE SWEET-POTATO VIRUS-DISEASE COMPLEX FROM NIGERIA

Flexuous, filamentous, viruslike particles with a modal length of 950 nm were consistently observed in leaf dip preparations of Ipomoea setosa infected with the whitefly-transmitted component of the sweet potato virus disease (SPVD) from Nigeria. Ultrastructural examinations of infected tissue from I. setosa revealed membrane-enclosed vesicles containing fibrillar material in the phloem parenchyma, which are indicative of closterovirus infections. These structures were also present in thin sections prepared from SPVD-infected L setosa tissue. Analysis of dsRNA extracts revealed the presence of a large dsRNA (molecular weight about 6.1 X 10(6)) and several smaller dsRNA species in all sweet potato and I. setosa plants carrying the whitefly-transmitted component (WTA). Synthesis and cloning of cDNA using dsRNA isolated from WTA-infected sweet potato as template resulted in clones that specifically reacted with nucleic acid prepared from purified closterovirus as well as with RNA isolated from plants infected with the WTA. In Northern blot analysis a hybridization signal to an ssRNA with an estimated size of about 9 kb was obtained, which in concordance with the size of the largest dsRNA species found and the length of the closteroviruslike particles, was referred to as virion RNA. The data suggest that a closterovirus and sweet potato feathery mottle virus are the causal agents of sweet potato virus disease in Nigeria. Although definite proof of the whitefly transmissibility of the WTA is still lacking, evidence for the existence of other agents involved in the SPVD complex could not be substantiated.