Barley stripe mosaic virus-encoded proteins triple-gene block 2 and γb localize to chloroplasts in virus-infected monocot and dicot plants, revealing hitherto-unknown roles in virus replication

Replication of Barley stripe mosaic virus (BSMV), genus Hordeivirus, is thought to be associated with vesicles in proplastids and chloroplasts, but the molecular details of the process and identity of virus proteins involved in establishing the virus replication complexes are unknown. In addition, BSMV encodes a triple-gene block of movement proteins (TGBs) that putatively share functional roles with their counterparts in other hordei-, pomo- and pecluviruses, but detailed information on the intracellular locations of the individual TGBs is lacking. Here, the subcellular localizations of BSMV-encoded proteins TGB2 and gamma b fused to green or red fluorescent proteins were examined in epidermal cells of Nicotiana benthamiana and barley (Hordeum vulgare 'Black Hulless'). The fusion proteins were expressed from a BSMV vector or under the control of the cauliflower mosaic virus 35S promoter. The subcellular localizations were studied by confocal laser-scanning microscopy (CLSM). CLSM studies showed that both proteins were recruited to chloroplasts in the presence of viral RNA and that virus RNA, coat protein and gamma b protein were detected in plastid preparations from infected leaves. Electron microscope images of thin sections of virus-infected leaves revealed abnormal chloroplasts with cytoplasmic inclusions containing virus-like particles. In addition, cellular localizations of BSMV TGB2 suggest subtle differences in function between the hordei-like TGB2 proteins. The results indicate that TGB2 and gamma b proteins play a previously unknown functional role at the site of virus replication.