Cell cycle regulation will be studied in the roots of legumes at various intervals after infection with Rhizobium. Early stages of Rhizobium infection are followed by either an activation or derepression of mitotic activity in cortical cells of the root and culminate in the formation of the root nodule, the site of nitrogen fixation in the plant/bacterial symbiosis. The proposed experiments will focus on changes in the expression, abundance and activity of the cell-cycle regulators, p34cdc2 and B-type cyclin in cortical cells of pea roots after infection. The working hypothesis is that p34cdc2 or cyclin is limiting in the quiescent plant cells. Experiments are also directed toward the study of a protein kinase that appears to respond to extracellular signals such as those that are involved in the early transduction events of infection by Rhizobium. Changes in the regulation of mitotic activity of otherwise terminally-differentiated cells in roots of the garden pea will be assessed after infection with the nitrogen-fixing bacterium, Rhizobium. During the earliest phases of root nodule formation, there is a significant increase in mitotic activity in what had been quiescent cells. The regulatory aspects of nodule formation are not known. Experiments in this project are directed toward discerning the factors that limit mitotic activity in differentiated cells under normal conditions, through a biochemical, molecular and immunological analysis of known activators of cell cycle progression during and after the infection process. This research problem is extremely important in plant biology, because insight into the signal transduction events that result in changes in the patterns or rates of cell division are central to almost all aspects of plant growth and development.
