Activation of a helper NLR by plant and bacterial TIR immune signaling

Toll/interleukin-1 receptor (TIR) domains across different life kingdoms possess NADase activities and produce distinct small molecules including phosphoribosyl adenosine monophosphate/diphosphate (pRib-AMP/ADP) and two cyclic ADPR (cADPR) isomers 2'cADPR and 3'cADPR. Plant intracellular nucleotide-binding leucine-rich repeat (NLR) receptors with an N-terminal TIR domain sense pathogen effectors to initiate immune signaling and rely on downstream helper NLRs to execute immune function. Lipase-like proteins EDS1 and PAD4 transduce immune signals from sensor TIR-NLRs to a helper NLR called ADR1. We report the structure and function of Arabidopsis EDS1-PAD4-ADR1 (EPA) heterotrimer in complex with pRib-AMP/ADP activated by plant or bacterial TIR signaling. Bacterial TIRs that produce 2'cADPR, but not 3'cADPR, induce EPA complex formation and activate EPA signaling using pRib-AMP as the signaling molecule. 2'cADPR is hydrolyzed into pRib-AMP in vivo. 2'cADPR, but not 3'cADPR, induces EPA-dependent defense genes expression. Our findings shed light on the activation mechanisms of ADR1 by EDS1-PAD4 involving two structurally-related molecules with 2'cADPR likely being the storage form of the unstable signaling molecule pRib-AMP, as well as cross-talks between plant and bacterial TIR immune signaling. Overall design: To investigate the function of 2'cADPR and its dependence on EDS-PAD4-ADR1 pathway, we take a treatment of Col-0 Arabidopsis and PAD4 Knock out mutant with mock and 2'cADPR. Then comparative gene expression profiling analyses were conducted using RNA-seq data of tissues above.