TIR-catalyzed nucleotide signaling molecules in plant defense

Toll and interleukin-1 receptor (TIR) domain is a conserved immune module in prokaryotes and eukaryotes. Signaling regulated by TIR-only proteins or TIR domain-containing intracellular immune receptors is critical for plant immunity. Recent studies demonstrated that TIR domains function as enzymes encoding a variety of activities, which manifest different mechanisms for regulation of plant immunity. These enzymatic activities catalyze metabolism of NAD+, ATP and other nucleic acids, generating structurally diversified nucleo-tide metabolites. Signaling roles have been revealed for some TIR enzymatic products that can act as second messengers to induce plant immunity. Herein, we summarize our current knowledge about catalytic production of these nucleotide me-tabolites and their roles in plant immune signaling. We also highlight outstanding questions that are likely to be the focus of future investigations about TIR-produced signaling molecules.

Automated summary

The Toll and TIR domains are conserved immune modules in both prokaryotes and eukaryotes. Signaling regulated by TIR-only proteins or TIR domain-containing intracellular immune receptors is critical for plant immunity. Recent studies have shown that TIR domains function as enzymes encoding a range of activities, which play different roles in the regulation of plant immunity. These enzymatic activities catalyze the metabolism of NAD+, ATP, and other nucleic acids, generating structurally diverse nucleotide metabolites. Some of these TIR enzymatic products have been identified as signaling molecules that act as second messengers to induce plant immunity. This article summarizes the current knowledge on the catalytic production of these nucleotide metabolites and their roles in plant immune signaling, and also highlights the outstanding questions that will likely be the focus of future investigations.