The Nicotiana benthamiana Mitogen-Activated Protein Kinase Cascade and WRKY Transcription Factor Participate in Nep1Mo-Triggered Plant Responses

Many bacterial, fungal, and oomycete species secrete necrosis and ethylene-inducing peptide 1 (Nep1)-like proteins (NLP) that trigger programmed cell death (PCD) and innate immune responses in dicotyledonous plants. However, how NLP induce such immune responses is not understood. Here, we show that silencing of the MAPKKKα–MEK2–WIPK mitogen-activated protein kinase (MAPK) cascade through virus-induced gene silencing compromises hydrogen peroxide accumulation and PCD induced by Nep1Mo from Magnaporthe oryzae. WIPK interacts with NbWRKY2, a transcription factor in Nicotiana benthamiana, in vitro and in vivo, suggesting an effector pathway that mediates Nep1Mo-induced cell death. Unexpectedly, salicylic acid–induced protein kinase (SIPK)- and NbWRKY2-silenced plants showed impaired Nep1Mo-induced stomatal closure, decreased Nep1Mo-promoted nitric oxide (NO) production in guard cells, and a reduction in Nep1Mo-induced resistance against Phytophthora nicotianae. Expression studies by real-time polymerase chain reaction suggested that the MEK2–WIPK–NbWRKY2 pathway regulated Nep1Mo-triggered NO accumulation could be partly dependent on nitrate reductase, which was implicated in NO synthesis. Taken together, these studies demonstrate that the MAPK cascade is involved in Nep1Mo-triggered plant responses and MAPK signaling associated with PCD exhibits shared and distinct components with that for stomatal closure.