Geminiviruses and the plant cell cycle.

Geminiviruses are plant DNA viruses with a small genome that infect a large variety of plant species. Viral proteins regulate viral DNA replication and transcription. Also they appear to have an impact on cellular gene expression. Cellular proteins directly involved in DNA replication, such as PCNA, have long been known to accumulate in cells expressing Rep tomato golden mosaic geminivirus. This effect can be a direct effect of the viral protein and/or be mediated by interference with the G1/S transition control, namely the pathway controlled by the retinoblastoma-related (RBR) protein, analogous to the human retinoblastoma (RB) tumour suppressor protein. Different geminiviruses seem to have evolved two mechanisms to interact with plant RBR proteins. One is dependent on a LxCxE amino acid motif present in proteins, such as RepA, encoded by members of the Mastrevirus genus, and another seems to be mediated by the viral Rep protein, which lacks the LxCxE motif, encoded by members of the Begomovirus, and perhaps the Curtovirus genus. These and other aspects of the relationships between geminivirus replication and cell cycle control pathways will be discussed.