Geminiviruses are a large family of plant viruses with circular, single-stranded DNA genomes that replicate through double-stranded intermediates. Because of their limited coding capacity, geminiviruses supply only the factors required to initiate viral replication and depend on host DNA polymerases to amplify their genomes. Many geminiviruses replicate in differentiated plant cells that no longer contain detectable levels of host DNA polymerases and associated factors.
Mature cells infected with Tomato golden mosaic virus (TGMV) express proliferating cell nuclear antigen (PCNA), the processivity factor for DNA polymerase delta. Because PCNA is only expressed in meristematic cells of healthy plants, geminiviruses reprogram host transcriptional controls to induce quiescent plant cells to reenter the cell cycle and regain the capacity to support high levels of DNA replication.
The retinoblastoma protein (pRb or pRBR) and the E2F transcription factors are key regulators of cell division and differentiation in eukaryotes. In animals, much of our knowledge of pRb and E2F is from studies of DNA tumor viruses and their tumor antigen proteins. These viral proteins interact with underphosphorylated pRb (the form associated with cell cycle arrest and differentiation) and drive quiescent cells into the cell division cycle. Geminiviruses induce transcription of the PCNA gene by relieving E2F-mediated repression. Geminiviruses encode two proteins, AL1 and AL3, that interact with plant pRBR through novel sequences. Mutations in the pRBR binding motif of AL1 alter tissue specificity of infection, establishing the importance of AL1/pRBR interactions in the infection process.

One goal of our research program is to characterize global changes in host gene expression associated with geminivirus infection. For these experiments, we are using Affymetrix ATH1 microarrays to examine steady state mRNA profiles in Arabidopsis plants infected with Cabbage leaf curl virus (CaLCuV).

Mammalian DNA tumor viruses also interface with signal transduction pathways and cell cycle regulatory machinery through interactions with protein kinases. We showed that AL1 interacts with a protein kinase designated as GRIK (Geminivirus Rep Interacting Kinase). Arabidopsis encodes two GRIK proteins, both of which are expressed in meristems and young leaves of healthy plants and accumulate in geminivirus-infected  cells of mature leaves. The GRIKs show homology to yeast SNF1-activating kinases and can complement the corresponding yeast mutants, suggesting that they are upsteam activators of plant SNF1-related kinases (SnRKs).

We collaborate with Niki Robertson (NCSU) and Rino Cella (University of Pavia) on geminivirus mechanisms.

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