On the importance of virion stability and access to salivary glands for the circulative transmission of Tomato yellow leaf curl Sardinia virus by Bemisia tabaci.

The capsid protein (CP) of the begomovirus Tomato yellow leaf curl Sardinia virus (TYLCSV) is indispensable for systemic plant infection and vector transmission and a region between aa 129-152 is critical for virion assembly and insect transmissibility. Three non-transmissible (NT) mutants, one with the double Q129P and Q134H mutation (PNHD), a second with a further D152E change (PNHE) and another with a single N130D change (QDQD) were analyzed in their relationships with the whitefly vector Bemisia tabaci and the nonvector Trialeurodes vaporariorum, using the wild-type (wt) virus as reference. Retention kinetics experiments performed on whiteflies fed on plants infected by the mutants revealed that the QDQD mutant was hardly detectable in either whitefly species at any sampling time. It produced geminate virions that appeared nongeminate following purification. While PNHD was acquired and circulated in both whitefly species as the wt virus, PNHE circulated in B. tabaci only. Immunogold labelling experiments on whiteflies that acquired the different mutants showed the presence of both PNHD and PNHE CPs in salivary glands (SGs), as for the wt virus, while no labelling was found in tissues from whiteflies that acquired QDQD. Besides, the transmission of the wt virus was significantly inhibited after prior feeding on plants infected by the PNHE mutant. A model describing the circulation of TYLCSV virions in Bemisia will be presented where both virion stability and ability to cross the SG barrier are fundamental, but interactions with molecular components inside the SGs are also critical for transmission.