TIR domain-associated nucleotides with functions in plant immunity and beyond

TIR (Toll/interlukin-1 receptor) domains are found in archaea, bacteria and eukaryotes, featured in proteins generally associated with immune functions. In plants, they are found in a large group of NLRs (nucleotide-binding leucine-rich repeat receptors), NLR-like proteins and TIR-only proteins. They are also present in effector proteins from phytopathogenic bacteria that are associated with suppression of host immunity. TIR domains from plants and bacteria are enzymes that cleave NAD+ (nicotinamide adenine dinucleotide, oxidized form) and other nucleotides. In dicot plants, TIR-derived signalling molecules activate downstream immune signalling proteins, the EDS1 (enhanced disease susceptibility 1) family proteins, and in turn helper NLRs. Recent work has brought major advances in understanding how TIR domains work, how they produce signalling molecules and how these products signal.