Structural Basis for Assembly and Function of a Heterodimeric Plant Immune Receptor

TLDR

Cytoplasmic plant immune receptors recognize pathogen effector proteins and initiate effector‑triggered immunity, with Arabidopsis RPS4 and RRS1 required to activate defense against three distinct pathogens. Crystal structures of the N‑terminal TIR domains of RPS4 and RRS1, individually and as a heterodimeric complex (2.05, 1.75, and 2.65 Å resolution), reveal a conserved TIR/TIR interaction interface that underlies their association. RPS4 and RRS1 physically associate, and TIR domain heterodimerization is essential for forming a functional effector‑recognition complex; RPS4 TIR activates effector‑independent defense that is inhibited by RRS1 TIR through the interface, illustrating distinct recognition and signaling roles.

Abstract

Cytoplasmic plant immune receptors recognize specific pathogen effector proteins and initiate effector-triggered immunity. In Arabidopsis, the immune receptors RPS4 and RRS1 are both required to activate defense to three different pathogens. We show that RPS4 and RRS1 physically associate. Crystal structures of the N-terminal Toll-interleukin-1 receptor/resistance (TIR) domains of RPS4 and RRS1, individually and as a heterodimeric complex (respectively at 2.05, 1.75, and 2.65 angstrom resolution), reveal a conserved TIR/TIR interaction interface. We show that TIR domain heterodimerization is required to form a functional RRS1/RPS4 effector recognition complex. The RPS4 TIR domain activates effector-independent defense, which is inhibited by the RRS1 TIR domain through the heterodimerization interface. Thus, RPS4 and RRS1 function as a receptor complex in which the two components play distinct roles in recognition and signaling.