Virion stability is important for the circulative transmission of Tomato yellow leaf curl Sardinia virus by Bemisia tabaci, but virion access to salivary glands does not guarantee transmissibility.

The capsid protein (CP) of the monopartite begomovirus Tomato yellow leaf curl Sardinia virus (TYLCSV), family Geminiviridae, is indispensable for plant infection and vector transmission. A region between amino acid 129 and 152 is critical for virion assembly and insect transmissibility. Two previously described mutants, one with a double Q129P/Q134H mutation (PNHD) and another with a further D152E change (PNHE), were found non-transmissible (NT). Another NT mutant with a single N130D change (QDQD) was retrieved from a new mutational analysis. In this study, these three NT mutants and the wild-type (wt) virus were compared in their relationships with the whitefly vector Bemisia tabaci and the non-vector Trialeurodes vaporariorum. Retention kinetics of NT mutants were analyzed by quantitative dot-blot hybridization in whiteflies fed on infected plants. QDQD, whose virions appeared non-geminate following purification, was hardly detectable in either whitefly species at any sampling time. PNHD was acquired and circulated in both whitefly species for up to 10 days, like the wt virus, while PNHE circulated in B. tabaci only. Using immunogold-labeling, both PNHD and PNHE CPs were detected in B. tabaci salivary glands (SGs) like the wt virus, while no labeling was found in any whitefly tissue with the QDQD mutant. Significant inhibition of transmission of the wt virus was observed after pre-feeding insects on the PNHE mutant, but not on the others. Virion stability and ability to cross the SG barrier are necessary for TYLCSV transmission, but interactions with molecular components inside the SGs are also critical for transmissibility.