2006 Annual Report 4d.Progress report. This reports serves to document research conducted under a specific cooperative agreement between ARS and the University of Wisconsin-Madison. Additional details of research can be found in the report for the parent CRIS 3655-21000-042-00D, Potato Genetics, Cytogenetics, Disease Resistance and Pre-Breeding Utilizing Wild and Cultivated Species. This cooperative agreement is focused on studying the function of a major potato gene, RB, which confers resistance to the late blight pathogen Phytophthora infestans. The overall goal of this project is to characterize the resistance response mediated by RB and identify other genes that might be involved. We have successfully made stable transgenic plants that contain RB protein fused to the green fluorescent protein (GFP) and separate lines with RB fused to a hemaglutenin (HA) tag. Testing for the disease resistance function of RB in these lines is underway. These transgenic plants will allow us to monitor the amount of RB protein present after inoculation with P. infestans and also to determine where the RB protein is located within the cell using microscopy. We have also begun work using potato as a model for studying the accumulation of defense related proteins after inoculation with P. infestans. At various time points after inoculation with P. infestans, leaf tissue was harvested and total protein was extracted. These proteins were then separated using a two dimensional gel. We are currently in the process of analyzing individual protein spots to determine what proteins are more or less abundant after exposure to the pathogen in both resistant and susceptible plants.