Abstract
Nicotinamide adenine dinucleotide (NAD(+)) has emerged as a key component in prokaryotic and eukaryotic immune systems. The recent discovery that Toll/interleukin-1 receptor (TIR) proteins function as NAD(+) hydrolases (NADase) links NAD(+)-derived small molecules with immune signaling. We investigated pathogen manipulation of host NAD(+) metabolism as a virulence strategy. Using the pangenome of the model bacterial pathogen Pseudomonas syringae, we conducted a structure-based similarity search from 35,000 orthogroups for type III effectors (T3Es) with potential NADase activity. Thirteen T3Es, including five newly identified candidates, were identified that possess domain(s) characteristic of seven NAD(+)-hydrolyzing enzyme families. Most Pseudomonas syringae strains that depend on the type III secretion system to cause disease, encode at least one NAD(+)-manipulating T3E, and many have several. We experimentally confirmed the type III-dependent secretion of a novel T3E, named HopBY, which shows structural similarity to both TIR and adenosine diphosphate ribose (ADPR) cyclase. Homologs of HopBY were predicted to be type VI effectors in diverse bacterial species, indicating potential recruitment of this activity by microbial proteins secreted during various interspecies interactions. HopBY efficiently hydrolyzes NAD(+) and specifically produces 2'cADPR, which can also be produced by TIR immune receptors of plants and by other bacteria. Intriguingly, this effector promoted bacterial virulence, indicating that 2'cADPR may not be the signaling molecule that directly initiates immunity. This study highlights a host-pathogen battleground centered around NAD(+) metabolism and provides insight into the NAD(+)-derived molecules involved in plant immunity.