Effects of point mutations in the readthrough domain of the beet western yellows virus minor capsid protein on virus accumulation in planta and on transmission by aphids

Point mutations were introduced into or near Eve conserved sequence motifs of the readthrough domain of the beet western yellows, virus minor capsid p rotein P74. The mutant virus was tested for its ability to accumulate effic iently in agroinfected plants and to be transmitted by its aphid vector, My zus persicae. The stability of the mutants in the agroinfected and aphid-in fected plants was followed by sequence analysis of the progeny virus. Only the mutation Y201D was found to strongly inhibit virus accumulation in plan ts following agroinfection, but high accumulation levels were restored by r eversion or pseudoreversion at this site. Four of the five mutants were poo rly aphid transmissible, but in three cases successful transmission was res tored by pseudoreversion or second-site mutations. The same second-site mut ations in the nonconserved motif PVT(32-34) were shown to compensate for tw o distinct primary mutations (R24A and E59/D60A), one on each side of the P VT sequence. In the latter case, a second-site mutation in the PVT motif re stored the ability of the virus to move from the hemocoel through the acces sory salivary gland following microinjection of mutant virus into the aphid hemocoel but did not permit virus movement across the epithelium separatin g the intestine from the hemocoel. Successful movement of the mutant virus across both barriers was accompanied by conversion of A59 to E or T, indica ting that distinct features of the readthrough domain in this region operat e at different stages of the transmission process.