Comparative Analysis of the Flax Immune Receptors L6 and L7 Suggests an Equilibrium-Based Switch Activation Model

Maud Bernoux, Hayden Burdett, Simon Williams, Xiaoxiao Zhang, Chunhong Chen, Kim Newell, Gregory Lawrence, Bostjan Kobe, Jeffrey Ellis, Peter Anderson, Peter Dodds, Peter Dodds

    Research output: Contribution to journalArticlepeer-review

    50 Citations (Scopus)

    Abstract

    NOD-like receptors (NLRs) are central components of the plant immune system. L6 is a Toll/interleukin-1 receptor (TIR) domain-containing NLR from flax (Linum usitatissimum) conferring immunity to the flax rust fungus. Comparison of L6 to the weaker allele L7 identified two polymorphic regions in the TIR and the nucleotide binding (NB) domains that regulate both effector ligand-dependent and -independent cell death signaling as well as nucleotide binding to the receptor. This suggests that a negative functional interaction between the TIR and NB domains holds L7 in an inactive/ADP-bound state more tightly than L6, hence decreasing its capacity to adopt the active/ATP-bound state and explaining its weaker activity in planta. L6 and L7 variants with a more stable ADP-bound state failed to bind to AvrL567 in yeast two-hybrid assays, while binding was detected to the signaling active variants. This contrasts with current models predicting that effectors bind to inactive receptors to trigger activation. Based on the correlation between nucleotide binding, effector interaction, and immune signaling properties of L6/L7 variants, we propose that NLRs exist in an equilibrium between ON and OFF states and that effector binding to the ON state stabilizes this conformation, thereby shifting the equilibrium toward the active form of the receptor to trigger defense signaling.

    Original languageEnglish
    Pages (from-to)146-159
    Number of pages14
    JournalPlant Cell
    Volume28
    Issue number1
    DOIs
    Publication statusPublished - Jan 2016

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