P-147 - Rice nuclear NLR Xo1’s detection of TALEs depends on their repeat backbone

The rice nuclear NLR Xo1 mediates resistance to Xanthomonas oryzae pathovars by detecting transcription activator-like effectors (TALEs) regardless of their DNA binding specificity. However, TALEs from other Xanthomonas species, which have minor repeat backbone sequence differences from X .oryzae TALEs, escape detection. Furthermore, some X. oryzae strains have truncated TALEs (truncTALEs, iTALEs) that inhibit Xo1 immunity. To understand how Xo1 detects TALEs and is inhibited by truncTALEs, X. oryzae transformants expressing TALE or truncTALE variants were tested in virulence assays for their ability to trigger or inhibit Xo1. Variants included those with substitutions in the repeat backbone mimicking Xo1-escaping TALEs, TALE-truncTALE chimeras, and domain deletions. Mutations in Xo1 at the predicted interaction interface, modeled using AlphaFold 3, were also tested in a protoplast cell death assay. The virulence assays demonstrated that the polymorphic backbone residues and overall chimeric effector protein size determine whether a variant triggers or inhibits Xo1. The protoplast results were consistent with the AlphaFold prediction that residues in the near-identical repeats of the Xo1 LRR are important for TALE recognition. This work broadens understanding of NLR effector recognition mechanisms, how effector variants can counter this immunity, and how Xo1 variants may be engineered to be insensitive to truncTALE suppression or to detect TALEs from other Xanthomonads.