Puncta-localized TRAF domain protein TC1b contributes to the autoimmunity of snc1.

Immune receptors play important roles in the perception of pathogens and initiation of immune responses in both plants and animals. Intracellular nucleotide-binding domain leucine-rich repeat (NLR) type receptors constitute a major class of receptors in higher plants. In the Arabidopsis thaliana mutant snc1 (suppressor of npr1-1, constitutive 1), a gain-of-function mutation in the NLR gene SNC1 leads to SNC1 over-accumulation and constitutive activation of defense responses. From a CRISPR-Cas9-based reverse genetics screen in the snc1 autoimmune background, we identified that mutations in TRAF CANDIDATE 1b (TC1b), a gene encoding a protein with four TRAF (tumor necrosis factor receptor associated factor) domains, can suppress snc1 phenotypes. TC1b does not appear to be a general immune regulator as it is not required for defense mediated by other tested immune receptors. TC1b also does not physically associate with SNC1, affect SNC1 accumulation, or affect signalling of the downstream helper NLR ADR1-L2 (ACTIVATED DISEASE RESISTANCE PROTEIN 1-L2), suggesting that TC1b impacts snc1 autoimmunity in a unique way. TC1b can form oligomers and localizes to punctate structures of unknown function. The puncta localization of TC1b strictly requires its coiled-coil (CC) domain, whereas the functionality of TC1b requires the four TRAF domains in addition to the CC domain. Overall, we uncovered the TRAF domain protein TC1b as a novel positive contributor to plant immunity.