Recognition, Activation, and Signaling Functions of the Arabidopsis NLR receptor RPP1
Microbial pathogens of plants secrete effector proteins into host cells to suppress basal immunity and manipulate host metabolism, but can be detected by a repertoire of innate immune receptors known as nucleotide-binding leucine rich repeat (NLR) proteins. Though NLRs were identified nearly 25 years ago, are ubiquitous across plant species, and are important components of pest resistance strategies in crops, the molecular mechanisms of NLR function are still unclear. This dissertation presents work on RPP1 (Resistance to Peronospora parasitica), an Arabidopsis NLR receptor that specifically recognizes the effector protein ATR1 from the naturally occurring oomycete pathogen Hyaloperonospora arabidopsidis. Genetic and molecular analysis of ATR1-RPP1 direct association demonstrates a variety of allele-specific recognition surfaces on the tandem WY-domain structure of ATR1. Mutations on these surfaces specifically activate a series of closely related RPP1 homologs, which show surprising structural variation in their recognition domain, a series of C-terminal leucine-rich repeats. Specificity is dictated by LRR binding affinity and full-length receptor sensitivity to activation, as demonstrated by chimeric NLR constructs and specific LRR mutations along a predicted ligand binding surface. Finally, recognized and unrecognized ATR1 alleles are used to probe inter-domain interactions and receptor oligomerization events upon effector binding. In addition, components of downstream signaling are studied using the autoactive, effector-independent TIR signaling domain of RPP1. Mechanistic details for how NLR receptors recognize pathogenic signatures may allow engineering of expanded NLR recognition specificity, yielding broad-spectrum, durable disease resistance in crop plants.