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

The capsid protein (CP) of the geminivirus Tomato yellow leaf curl Sardinia virus (TYLCSV) is indispensable for plant infection and vector transmission. A region between amino acids 129 and 152 is critical for virion assembly and insect transmissibility. Three nontransmissible (NT) mutants, one with a double Q129P Q134H mutation (PNHD), the second with a further D152E change (PNHE) and the third with a single N130D change (QDQD) were compared to the wild-type (wt, QNQD) virus in their relationships with the whitefly vector Bemisia tabaci and the nonvector Trialeurodes vaporariorum. Retention kinetics of NT mutants in whiteflies fed on infected plants were analyzed by quantitative dot blot hybridization. Virions of the QDQD mutant appeared nongeminate following purification and its DNA was hardly detectable in either whitefly species at any sampling time. The PNHD mutant was acquired and circulated for up to 10 days in both whitefly species, like the wt virus, while PNHE circulated in B. tabaci only. Both PNHD and PNHE CPs were immunogold labelled in B. tabaci salivary glands (SGs) like the wt virus, while no labelling was found in whitefly tissue with the QDQD mutant. Furthermore, a significant inhibition of transmission of the wt virus was observed after prior feeding of the insects on plants infected with the PNHE mutant, but not on plants infected with the other mutants. Virion stability and ability to cross the SG barrier are necessary for TYLCSV transmission, but interactions with molecular components inside SGs are critical for transmissibility.