THE RELATIONSHIP BETWEEN APHID-TRANSMISSIBILITY OF POTATO LEAFROLL VIRUS AND SURFACE EPITOPES OF THE VIRAL CAPSID

A panel of nine monoclonal antibodies (MAbs) was used to investigate t he relationship between aphid-transmissibility and surface epitopes of the capsid of potato leafroll virus (PLRV). In immunoblocking experim ents, mixtures of purified PLRV and MAbs were fed to I-day-old Myzus p ersicae nymphs, and their transmission efficiency was then studied. Th e MAbs were applied in triple-antibody sandwich enzyme-linked immunoso rbent assay (TAS ELISA) to characterize antigenically two phenotypic v ariants of the Wageningen isolate of PLRV (PLRV-Wag) that differ in tr ansmissibility and a highly aphid-transmissible isolate of PLRV. It wa s previously established that these MAbs reacted with epitopes exposed on the surface of the virion. However, the MAbs can be separated into two groups on the basis of their reactivity with intact and disrupted particles of PLRV. One group of six MAbs identified epitopes located on individual subunits of the viral capsid, since they reacted equally well with either conformational state of the virus. Four of these MAb s-WAU-A5, -A6, -A7, and -A13-affected the transmission of PLRV in immu noblocking experiments by significantly increasing the latency period of the virus in the aphid. The epitopes to which they are directed wer e previously found to be closely related tppologically. WAU-A13 reacte d significantly more strongly in TAS ELISA with the readily transmitte d phenotypic variant of PLRV-Wag isolated from upper leaves of Physali s floridana than with the poorly transmissible variant from lower leav es. The other MAbs did not differentiate between these variants. Only WAU-A13 detected a highly aphid-transmissible isolate of PLRV (hat-PLR V) obtained after a selective transmission pressure was exerted on PLR V-Wag. Thus the epitope delineated by this MAb or topologically relate d capsid domains might be associated with the aphid transmission of PL RV. Western blot analysis of purified PLRV revealed the presence of tw o proteins with molecular masses of 23 and 56 kDa. Since WAU-A13 tagge d both capsid-associated proteins, this MAb is specific for the major coat protein species of 23 kDa. The second group of MAbs-WAU-A12, A24, and -B9-reacted exclusively with intact particles of PLRV, which sugg ests that they are directed to epitopes that depend on the quarternary protein structure. These MAbs did not interfere in PLRV transmission in the immunoblocking experiments, did not detect hat-PLRV, and were h ardly reactive to the readily transmissible variant of PLRV-Wag compar ed to the poorly transmissible one. The epitopes delineated by these M Abs therefore could not be associated with aphid-transmissibility,