Model of IBI1 as a regulator of BABA-induced resistance (A) and as a “depleted-self sensor” in basal resistance (B). (A) Role of IBI1in BABA-induced resistance (BABA-IR). Binding of R-BABA to ER-localized IBI1 protein (yellow circles) deprives the protein of canonical aspartyl-tRNA synthetase (AspRS) activity, which “primes” the protein's non-canonical defense activity against pathogen attack (orange circles). Detection of pathogen-associated molecular patterns (PAMPs) during the early stages of pathogen infection boosts IBI1 gene transcription and triggers translocation of IBI1 to the cytosol, where it activates defense activity through interaction with immune-regulatory proteins (red circles). (B) Role of IBI1 in basal resistance. Successful pathogen infection leads to suppression of PAMP recognition through the action of pathogen-derived effectors (blue asterisks). Ongoing parasitization and amino acid uptake by the pathogen lowers cellular L-asp levels, depriving IBI1 from its canonical AspRS activity. This, together with effector-weakened PAMP perception, boosts IBI1 transcription and elicits translocation of the protein from the ER to the cytosol, where it activates broad-spectrum defenses (red circles). Hence, IBI1 acts as a “depleted-self” sensor to counteract effector-triggered suppression of “nonself recognition.”