Defining the Relationships Between Arabidopsis Innate Immunity Receptors and Phytopathogenic Immune Elicitors with Phylogenomics

The plant innate immune response begins with the binding of conserved microbial features called microbe-associated molecular patterns (MAMPs) to specific plant receptors.  While the importance of these events has long been understood, traditional approaches to identify the involved molecules remain problematic.  The recent profusion of genome sequences from bacterial phytopathogens has allowed for the development of bioinformatics approaches to MAMP identification.  Using a comparative genomics approach, we have identified new peptide MAMPs from the phytopathogenic bacterium Pseudomonas syringae and utilized a reverse genetic screen to identify Arabidopsis thaliana immune receptors required for their recognition. Using a novel high-throughput assay we have tested the activity of seven bacterial MAMPs on 187 immune receptor T-DNA insertion knockouts.  This primary screen has allowed us to identify clades of genes that play a role in increasing or decreasing MAMP responses globally, as well as lines unable to respond to a single MAMP, suggesting a role in MAMP binding.  We focused on the interaction between the xup25 MAMP from a bacterial xanthine/uracil permease and the Arabidopsis receptor xanthine/uracil permease sensing 1 (XPS1). We showed that xup25 treatment results in a classic immune response, including pathogenesis-related gene induction, callose deposition, and resistance to virulent bacteria, all in an XPS1-dependent manner, indicating that XPS1 is specifically required for xup25 recognition. The study demonstrates an efficient method to identify immune elicitors and the plant receptors responsible for their perception.  Further exploration of these molecules will increase our understanding of plant-pathogen interactions and the basis for host specificity.