NAD acts as an integral regulator of multiple defense layers

Pyridine nucleotides, such as nicotinamide adenine dinucleotide (NAD), are crucial redox carriers and have emerged as important signaling molecules in stress responses. Previously, we have demonstrated in Arabidopsis thaliana (Arabidopsis) that the inducible NAD-overproducing nadC lines are more resistant to an avirulent strain of Pseudomonas syringae pv. tomato (Pst-AvrRpm1), which was associated with salicylic acid-dependent defense. Here, we have further characterized the NAD-dependent immune response in Arabidopsis. Quinolinate-induced stimulation of intracellular NAD in transgenic nadC plants enhanced resistance against a diverse range of (a)virulent pathogens, including Pst-AvrRpt2, Dickeya dadantii and Botrytis cinerea. Characterization of the redox status demonstrated that elevated NAD levels induce reactive oxygen species (ROS) production and expression of redox marker genes of the cytosol and mitochondrion. Using pharmacological and reverse genetics approaches, we show that NAD-induced ROS production functions independently of NADPH oxidase activity and light metabolism but depends on mitochondrial respiration which was increased at higher NAD. We further demonstrate that NAD primes pathogen-induced callose deposition and cell death. Mass spectrometry analysis reveals that NAD simultaneously induces different defense hormones and that the NAD-induced metabolic profiles are similar to that of defense-expressing plants after treatment with pathogen-associated molecular patterns (PAMPs). We thus conclude that NAD triggers metabolic profiles rather similar to that of PAMPs and discuss how signaling crosstalk between defense hormones, ROS and NAD explains the observed resistance to pathogens.