The retinoblastoma protein (pRb) and the E2F transcription factors are key regulators of cell division and differentiation in animals. Related proteins have been identified in plants, but little is known about their functions. 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 under-phosphorylated pRb (the form associated with cell cycle arrest and differentiation) and disrupt pRb/E2F complexes, leading to altered host gene expression and cell cycle reentry. Recent experiments suggested that geminiviruses use a similar mechanism to alter cell cycle controls in plants. The overall goals of this research are to analyze host gene expression during geminivirus infection and to use these important plant viruses as tools to study how pRb and E2F regulate plant gene expression. The first aim is to develop an Arabidopsis system for studying geminivirus-mediated host induction and pRb interactions. This system will combine geminiviruses with Arabidopsis genetic and database tools to study pRb and E2F function in plants. The second aim is characterize the global changes in Arabidopsis gene expression associated with geminivirus infection using gene profiling technology. The third aim is to generate transgenic plants that express wild type and mutant viral proteins that differ only in their pRb binding capacities. Comparison of the expression profiles of these lines will provide insight into the identities of plant genes controlled by pRb and E2F. Together, these experiments are likely to identify genes whose products are associated with the cell division cycle, endoreduplication, differentiation, and disease in plants.
The studies represent a unique opportunity to study the dedifferentiation process and the return to the cell division cycle in intact mature tissues of a higher eukaryote. They have the potential to uncover plant-specific components of these two important processes and may have a profound impact on agriculture by providing insight into strategies for altering plant growth.