A geminivirus replication protein interacts with the retinoblastoma protein through a novel domain to determine symptoms and tissue specificity of infection in plants

Geminiviruses replicate in nuclei of mature plant cells after inducing the accumulation of host DNA replication machinery. Earlier studies showed that the viral replication factor, AL1, is sufficient for host induction and in teracts with the cell cycle regulator, retinoblastoma (pRb). Unlike other D NA virus proteins, AL1 does not contain the pRb binding consensus, LXCXE, a nd interacts with plant pRb homologues (pRBR) through a novel amino acid se quence. We mapped the pRBR binding domain of AL1 between amino acids 101 an d 180 and identified two mutants that are differentially impacted for AL1-p RBR interactions. Plants infected with the E-N140 mutant, which is wild-typ e for pRBR binding, developed wild-type symptoms and accumulated viral DNA and AL1 protein ill epidermal, mesophyll and vascular cells of mature leave s, Plants inoculated with the KEE146 mutant, which retains 16% pRBR binding activity, only developed chlorosis along the veins, and viral DNA, AL1 pro tein and the host DNA synthesis factor, proliferating cell nuclear antigen, were localized to vascular tissue, These results established the importanc e of AL1-pRBR interactions during geminivirus infection of plants.