A geminivirus betasatellite damages the structural and functional integrity of chloroplasts leading to symptom formation and inhibition of photosynthesis.

Geminivirus infection often causes severe vein clearing symptoms in hosts. Recently a betasatellite has emerged as a key regulator of symptom induction. To understand the host-betasatellite interactions in the process of symptom development, a systematic study was carried out involving symptoms induced by a betasatellite associated with radish leaf curl disease (RaLCB) in Nicotiana benthamiana. It has been found that βC1 protein localized to chloroplasts of host cells, and RaLCB lacking βC1, which failed to produce symptoms, had no effect on chloroplast ultrastructure. Vein flecking induced by transiently expressed βC1 was associated with chloroplast ultrastructure. In addition, the betasatellite down-regulates expression of genes involved in chlorophyll biosynthesis as well as genes involved in chloroplast development and plastid translocation. Interestingly, the expression of key host genes involved in chlorophyll degradation remains unaffected. Betasatellite infection drastically reduced the numbers of active reaction centres and the plastoquinol pool size in leaves exhibiting vein clearing symptoms. Betasatellite-mediated impediments at different stages of chloroplast functionality affect the photosynthetic efficiency of N. benthamiana. To the best of the authors' knowledge, this is the first evidence of a chloroplast-targeting protein encoded by a DNA virus which induces vein clearing and structurally and functionally damages chloroplasts in plants.