Some new results on citrus psorosis virus and Indian citrus ringspot virus, two important but so far unclassified viruses

摘要

Recent work is reviewed on citrus psorosis virus (CPsV) and Indian citrus ringspot virus (ICRSV), two important viruses of citrus that, until now, have not been classified and have not been amenable to routine diagnostic testing by laboratory methods.Results are reported that clarify the taxonomic positions and names of these two very different viruses, and open the way to testing for them by PCR and ELISA.The work has involved collaboration between laboratories in Argentina, India and Italy. CPsV is a multicomponent virus with probably three ssRNAs ranging in size from 10 to 1.5 kb; there is one coat protein of 48-50 kDa. The particles are fine filaments about 3 nm in diameter, circularized to form irregular rings which in some conditions collapseto form apparently linear structures; in this they resemble the particles of tenuiviruses and the nucleocapsids of tospoviruses; however the size of the coat protein of the latter viruses is around 31 kDa and 29 kDa respectively. Recently an antiserum to CPsV was produced in Turin that, after absorption with healthy plant antigens, was suitable for use in DAS-ELISA; this test has shown that different isolates from Argentina, Italy, Spain and the USA are related, and that at least in southern Italy, thevirus is widespread, even in nursery stocks. More recently our group in Turin and that in Bari, Italy, have each produced monoclonals that promise to be even more useful in diagnostics and also in differentiating different strains. Part of the genome ofCPsV has been sequenced, both in Argentina and the USA, and primers constructed on which to base RT-PCR tests. ELISA and PCR diagnostics are still largely confined to laboratory and greenhouse material due to low and variable virus concentrations in foeld trees and to uncertain degrees of genetic variation between field strains. However, the road to field diagnostics is now open. ICRSV (isolate Kinnow 1) is a monopartite ssRNA virus with flexuous filamentous particles that show strong cross-banding, morphologically resembling capilloviruses; however it is not serologically related to capilloviruses or any flexuous filamentous virus tested. Recently it has been mechanically transmitted to Phaseolus vulgaris test plants, from which it is easily purified. Antisera suitable for DAS ELISA have been made. Sequencing of the genome is preparing the way for a PCR test, and shows that toward the 3' end there are limited homologies with some potexviruses, carlaviruses and foveaviruses; however, the CP is too large for a potexvirus, the morphology is not that of a potexvirus or carlavirus, and homology of sequences available is not particularly close to the foveaviruses. Thus the virus does not, on present evidence, fit into any recognized taxonomic group. Recently a second virus, serologically distantly related, (Kinnow 2) has been isolated and is under study. The results of both published and unpublished work are discussed in relation to taxonomy and diagnostics of these two viruses.