Summary: Pathogen infection in plants triggers large‐scale transcriptional changes, both locally and systemically. Emerging evidence suggests that theArabidopsisMediator complex plays a crucial role in these transcriptional changes. Mediator is highly conserved in eukaryotes, and its core comprises more than 20 subunits organized into three modules named head, middle and tail. The head and middle modules interact with general transcription factors andRNApolymerase II, whereas the tail module associates with activators, and signals through the head and middle modules to the basal transcription machinery. In Arabidopsis, three tail module subunits,MED14,MED15 andMED16, have been identified. BothMED15 andMED16 have been implicated in plant immunity, but the role ofMED14 has not been established. Here, we report the characterization of an Arabidopsis T–DNAinsertion mutant of theMED14gene. Similarly to themed15and/ormed16mutations, themed14mutation significantly suppresses salicylic acid‐induced defense responses, alters transcriptional changes induced by the avirulent bacterial pathogenPseudomonas syringaepv.tomato(Pst)DC3000/avrRpt2, and renders plants susceptible to bothPstDC3000/avrRpt2andPstDC3000. Themed14mutation also completely compromises biological induction of systemic acquired resistance (SAR), indicating that the tail module as a whole is essential forSAR. Interestingly, unlike themed16mutation, which differentially affects expression of severalSARpositive and negative regulators,med14inhibits induction of a large group of defense genes, including bothSARpositive and negative regulators, suggesting that individual subunits of the Mediator tail module employ distinct mechanisms to regulate plant immune responses.

An evolutionarily conserved gene regulates resistance to a devastating necrotrophic fungal crop plant pathogen by controlling major antifungal defense pathways.

Nicotinamide adenine dinucleotide (NAD⁺) participates in intracellular and extracellular signaling events unrelated to metabolism. In animals, purinergic receptors are required for extracellular NAD⁺ (eNAD⁺) to evoke biological responses, indicating that eNAD⁺ may be sensed by cell-surface receptors. However, the identity of eNAD⁺-binding receptors still remains elusive. Here, we identify a lectin receptor kinase (LecRK), LecRK-I.8, as a potential eNAD⁺ receptor in Arabidopsis. The extracellular lectin domain of LecRK-I.8 binds NAD⁺ with a dissociation constant of 436.5 ± 104.8 nM, although much higher concentrations are needed to trigger in vivo responses. Mutations in LecRK-I.8 inhibit NAD⁺-induced immune responses, whereas overexpression of LecRK-I.8 enhances the Arabidopsis response to NAD⁺. Furthermore, LecRK-I.8 is required for basal resistance against bacterial pathogens, substantiating a role for eNAD⁺ in plant immunity. Our results demonstrate that lectin receptors can potentially function as eNAD⁺-binding receptors and provide direct evidence for eNAD⁺ being an endogenous signaling molecule in plants.

The evolutionarily conserved Elongator complex functions in diverse biological processes including salicylic acid‐mediated immune response. However, how Elongator functions in jasmonic acid (JA)/ethylene (ET)‐mediated defense is unknown. Here, we show that Elongator is required for full induction of the JA/ET defense pathway marker gene PLANT DEFENSIN1.2 (PDF1.2) and for resistance to the necrotrophic fungal pathogens Botrytis cinerea and Alternaria brassicicola. A loss‐of‐function mutation in the Arabidopsis Elongator subunit 2 (ELP2) alters B. cinerea‐induced transcriptome reprogramming. Interestingly, in elp2, expression of WRKY33, OCTADECANOID‐RESPONSIVE ARABIDOPSIS AP2/ERF59 (ORA59), and PDF1.2 is inhibited, whereas transcription of MYC2 and its target genes is enhanced. However, overexpression of WRKY33 or ORA59 and mutation of MYC2 fail to restore PDF1.2 expression and B. cinerea resistance in elp2, suggesting that ELP2 is required for induction of not only WRKY33 and ORA59 but also PDF1.2. Moreover, elp2 is as susceptible as coronatine‐insensitive1 (coi1) and ethylene‐insensitive2 (ein2) to B. cinerea, indicating that ELP2 is an important player in B. cinerea resistance. Further analysis of the lesion sizes on the double mutants elp2 coi1 and elp2 ein2 and the corresponding single mutants revealed that the function of ELP2 overlaps with COI1 and is additive to EIN2 for B. cinerea resistance. Finally, basal histone acetylation levels in the coding regions of WRKY33, ORA59, and PDF1.2 are reduced in elp2 and a functional ELP2‐GFP fusion protein binds to the chromatin of these genes, suggesting that constitutive ELP2‐mediated histone acetylation may be required for full activation of the WRKY33/ORA59/PDF1.2 transcriptional cascade.