A CUCUMBER NECROSIS VIRUS VARIANT DEFICIENT IN FUNGAL TRANSMISSIBILITY CONTAINS AN ALTERED COAT PROTEIN SHELL DOMAIN

Little is currently known regarding the specific interactions that gov ern transmission of plant viruses by their vectors. A cucumber necrosi s virus (CNV) variant (LL5) deficient in fungal transmissibility has b een isolated from mechanically passaged CNV and characterized. Althoug h LL5 accumulates to wild-type (WT) levels, is capable of rapid system ic infection, and produces stable, highly infectious particles, it is only inefficiently transmitted by Olpidium bornovanus zoospores. The L L5 coat protein (CP) gene was amplified by RT-PCR and cloned in place of the WT CNV CP gene in an infectious CNV cDNA clone. Particles deriv ed from this construct also failed to be efficiently transmitted. The LL5 CP gene was sequenced and found to contain two amino acid substitu tions relative to WT CNV CP. One substitution (Phe to Cys) occurred in the arm region and another (Glu to Lye) in the shell domain. These am ino acid changes were separately introduced into the WT CNV genome thr ough in vitro mutagenesis and it was found that the Glu to Lys change in the LL5 CP shell domain is largely responsible for the loss of tran smissibility. in vitro binding assays were developed to determine if t he defect in transmissibility was due to a defect in binding zoospores . LL5 particles were found to bind less efficiently than WT CNV. Furth ermore, the nontransmissible tomato bushy stunt virus did not detectab ly bind zoospores. These binding studies suggest that the specificity of CNV transmission by O. bornovanus occurs through specific recogniti on of a putative zoospore receptor. (C) 1997 Academic Press.