During plant-microbe interaction, plants have evolved specialized receptors, called nucleotide-binding leucine-rich repeat receptors (NLRs), to recognize pathogen effector proteins and induce defence responses. However, the molecular mechanism of NLR-mediated immunity is not fully understood. One of the major classes of NLRs contains the N-terminal Toll/interleukin-1 receptor (TIR) signalling domain, hereafter TNLs. Two TNLs, from Arabidopsis thaliana and Nicotiana benthamiana, form tetrameric protein complexes termed resistosomes. Given the diversity of TNLs in plants, it is important to establish if tetrameric assembly is a common feature. TNL resistosomes exhibit nicotinamide adenine dinucleotide (NAD+) hydrolase activity through the TIR domains, producing signalling molecules that are required to trigger cell death, including 2’cADPR. It remains unclear whether distinct signalling products are derived from NADase activity by different TNLs. We expressed and purified both the full-length and the TIR domain of two closely related flax TNLs, L6 and M, that are known to activate immune responses to rust fungi. We found that full-length TNLs form different oligomeric states in the absence of their cognate effectors, while the effectors induce tetrameric resistosome formation. We determined the M protein structure with and without its effector by cryogenic-electron microscopy. We also demonstrated that full-length L6 and M proteins cleave NAD+, producing 2’cADPR, while their TIR domains alone display weak enzymatic activity. Additionally, each TNL produces distinct metabolites that may contribute to signalling selectivity. Overall, our findings suggest that TNLs preferentially form tetrameric complexes and provide structural basis of TNL activation.