Fig. 8: NHP treatment triggers eNAD(P) accumulation through RBOHF-generated ROS.

a eNAD(P) levels in the leaves treated with or without MV. Values are expressed relative to the eNAD(P) levels in the -MV samples, which were arbitrarily set to 1. Bars represent means ± SE (n = 3 independent AWF samples). The MV treatment induced significant eNAD(P) accumulation (two-tailed t test). The experiment was repeated with similar results. b, c SAR induction-triggered systemic accumulation of eNAD(P) in wild type (WT) and rbohF. Values are expressed relative to the eNAD(P) levels in the WT/-SAR samples, which were arbitrarily set to 1. Bars represent means ± SE (n = 3 independent AWF samples). The SAR induction-induced eNAD(P) accumulation was significantly inhibited in rbohF (one-way ANOVA with Tukey’s test). The experiment was performed three times with similar results. d, e NAD(P)⁺-induced systemic immunity in the indicated genotypes. The photo in (d) and samples in (e) were taken at 72 hpi. Bars in (e) represent means ± SE (n = 8 independent leaf disks). NAD(P)⁺ induced similar levels of systemic immunity in the WT, rbohD, and rbohF plants (two-tailed t test). The experiment was performed three times with similar results. f DAB (3, 3’-diaminobenzidine) staining of NHP-induced H₂O₂. Arabidopsis leaves were infiltrated with 0.5 mM NHP or water (-NHP) and collected 24 h later. g, h NHP-induced systemic accumulation of eNAD(P) in WT and rbohF. Values are expressed relative to the eNAD(P) levels in WT/-NHP samples, which were arbitrarily set to 1. Bars represent means ± SE (n = 3 independent AWF samples). The NHP-induced eNAD(P) accumulation was significantly inhibited in rbohF (one-way ANOVA with Tukey’s test). The experiment was conducted three times with similar results. i, j NHP-induced systemic immunity in the indicated genotypes. The photo in (j) and samples in (i) were taken at 72 hpi. Bars in (i) represent means ± SE (n = 10 independent leaf disks). The NHP-induced systemic immunity was significantly reduced in rbohD and rbohF (one-way ANOVA with Tukey’s test; p values are shown in the Source Data file). The experiment was performed three times with similar results. k A proposed working model for the function of eNAD(P) in SAR. Pathogen infections activate PTI, DTI, and ETI responses in the local leaf tissues, which are accompanied by generation of a blend of mobile signals among which are eNAD(P) and NHP. NHP likely contributes to eNAD(P) accumulation in the local leaf tissues. These mobile SAR signals are swiftly transported to systemic leaf tissues where the eNAD(P) from the local tissues binds to and activates its receptor complex LecRK-VI.2/BAK1, but this signal appears to be too weak to trigger downstream SAR signaling. Other mobile signals including NHP initiate a signaling amplification loop, triggering accumulation of ROS that induce de novo eNAD(P) accumulation in the systemic leaf tissues. The fresh eNAD(P) further activates the receptor complex to boost the force of the SAR signaling, triggering the downstream SA/NPR1-mediated SAR responses. A modest NHP-dependent but ROS- and eNAD(P)-independent pathway to SAR seems to exist. A question marks (?) indicates that the contribution of the pathway to SAR needs further investigation. The figure was created with BioRender.com.