Analysis of the RPP13/ATR13 interaction complex between downy mildew and Arabidopsis

Plants and their diseases are locked in a constant battle for supremacy. To prevent these diseases from destroying crops we use pesticides or genetics, as plants have evolved natural mechsnisms of disease resistance. How these resistance mechanisms evolve and function is important to understand as it will allow us to more effectively use these natural processes in protecting crop plants. The downy mildew disease occurs in the wild on the model plant species Arabidopsis, so the two organisms are co-evolving. The plant has developed genes that recognise the presence of the pathogen. We have cloned one of these genes (RPP13) and shown it to be highly variable in structure between plants suggesting that it is in a war with the disease to detect a protein in the pathogen that is rapidly evolving to avoid detection. We have also isolated the pathogen gene (ATR13) that RPP13 detects and indeed it is amazingly variable. However, it is not that simple as we have also shown that RPP13 can detect other pathogen proteins and that ATR13 can be detected by other plant resistance proteins. This suggests then, that resistance proteins and the pathogen proteins that they detect are working in complexes that are under massive pressure to alter their structure to detect a pathogen or avoid detection by the plant. Having identified more components of the RPP13/ATR13 system, we have a unique opportunity to analyse the relationship between gene structure and function in a natural system that is evolving rapidly. This study will identify these novel components of these protein complexes and allow us to more fully understand how this war between host and pathogen is fought in the wild. These new insghts will allow us to develop strategies for deployment of naturally occuring resistance genes in crops to maximise their effectiveness in preventing disease and maintain food yeilds.

The RPP13 gene is the most variable gene so far cloned from Arabidopsis and it detects the presence of the downy mildew (Hyaloperonospora parasitica) protein ATR13 which we have shown is also under massive diversifying selection. Previously we predicted that ATR13 would be detected by other resistance (R) proteins as the regions of the protein under diversifying selection extend beyond those that we have defined as being responsible for interacting with RPP13. This system is becoming a paradigm for R gene evolutionary analyses but has been limited by lack of functional data. We have recently added three new components to this study. Firstly, ATR13 is only recognised by a single clade of RPP13 genes, demonstrating tight linkage between R gene structure and recognition of ATR13. Secondly, we have confirmed that indeed at least one unlinked R gene can detect ATR13 alleles and have carried out preliminary mapping. Finally, we have demonstrated that RPP13 alleles in other clades recognise other pathogen proteins than ATR13. Therefore, having added functional data to the evolutionary analyses we are now in a unique position to extend this data set by cloning two new components (the new R and ATR genes we have defined) that are driving evolutionary change in this complex. By analysing allelic variation of these new components we will understand how they influence the evolution of the plant resistance and pathogen ATR genes and enable us to move beyond just evolutionary analyses to defining those amino acids that are key to the recognition responses that we see. This study will lead to an understanding of the underlying processes that are developing new resistance and pathogenicity determinants in the Arabidopsis/downy mildew pathosystem.