ABSTRACT
Advances in the techniques of molecular biology have made significant contributions to our understanding of host pathogen interactions in the past decade. We now know the structure of many resistance (R) genes and the proteins they encode. In some cases, the structure of the corresponding avirulence gene product in the pathogen has been determined as well. We also know the identity of some of the downstream genes that are induced as a result of pathogen infection, such as the pathogenesis-related (PR) genes. In some cases, particularly in Arabidopsis, some of the genes which encode proteins that function at intermediate steps in the signal transduction pathway for response to pathogens have been identified and cloned. Some examples are: NDR1 (necessary for defence response) (Century et al., 1997); EDSl (enhanced disease symptoms) (Falk et al., 1999); and NPRl/NIMl (non-inducible PRl/non-inucible immunity) (Cao et al., 1991; Ryals et al., 1997). These genes are required for a functional resistance response to some, though not all, pathogens. While we now know of some of the genes that participate in defence response pathways, how they all interact to mount an effective resistance response is still very much a mystery. This chapter discusses some new techniques and tools that may be helpful in bridging the gap between R gene structure and functional resistance and in studying other plant defence mechanisms as well. This includes new methods that can be used to enhance traditional genetics-based approaches; techniques that will enhance our ability to identify and clone genes acting at all stages in a signal transduction pathway, from the initial regulatory steps through intermediate signalling steps to the final response genes at the end of the pathway. Strategies that are being used to facilitate genetic approaches are discussed in the first section.
