Activation of a mitogen-activated protein kinase pathway is involved in disease resistance in tobacco

Abstract

The active defense mechanisms of plants against invading pathogens often include rapid cell death, known as the hypersensitive response (HR), the activation of a complex array of defense genes, and the production of antimicrobial phytoalexins (1, 2). Several lines of evidence suggest HR cell death during plant disease resistance is a form of programmed cell death, as it requires active transcription and translation and is genetically defined (3–5). Although the details for the regulation and execution of HR remain poorly understood, production of reactive oxygen species, ion fluxes, protein phosphorylation/dephosphorylation, and gene activation have been implicated in the process (5). In addition to these local responses, the uninfected portions of the plant usually develop systemic acquired resistance, which is manifested as enhanced resistance to a subsequent challenge by the initial or even unrelated pathogens (2).

Plant-defense responses are initiated by the recognition of pathogens, which is mediated either by a gene-for-gene interaction between a plant resistance (R) gene and a pathogen avirulence (Avr) gene or by the binding of nonrace-specific elicitors such as elicitins to a putative receptor (1, 6–9). Signals generated by such interactions are transduced into cellular responses via several interlinked pathways. Recent studies suggest that plant mitogen-activated protein kinase (MAPK) cascade is one of the converging points after the perception of different pathogens and pathogen-derived elicitors (10–15).

MAPK cascades are major pathways by which extracellular stimuli are transduced into intracellular responses in yeast and mammalian cells (16, 17). The basic assembly of MAPK cascade is a three-kinase module conserved in all eukaryotes. MAPK, the last kinase in the three-kinase cascade, is activated by dual phosphorylation of Thr and Tyr residues in a TXY motif by MAPK kinase (MAPKK). MAPKK is, in turn, activated by MAPKK kinase. In mammals, three of the five subgroups of the MAPK family, the stress-activated protein kinase/Jun N-terminal kinase (SAPK/JNK), the p38 kinase, and extracellular signal-regulated protein kinase 5, are activated in response to various stress signals, including UV and ionizing radiation, hyperosmolarity, oxidative stress, and cytokines (16, 17). The outcome of SAPK/JNK and/or p38 activation depends on the magnitude and duration of their activation. A transient activation of these MAPKs induces various defense responses and allows the cells to adapt to an adverse environment. By contrast, a persistent activation leads to apoptosis (16, 17).

The induction of MAPK-like activities has been associated with HR cell death in tobacco cells treated with xylanase, a fungal elicitor from Trichoderma viride and Arabidopsis cells treated with harpin, a bacterial elicitor from Pseudomonas syringae pv. Syringae (18, 19). Through the use of kinase inhibitors, we linked the activation of salicylic acid (SA)-induced protein kinase (SIPK) and wounding-induced protein kinase (WIPK), two tobacco MAPKs, to HR cell death induced by fungal elicitins (20). SIPK was first identified as SA-induced protein kinase (21). It is also activated by various biotic and abiotic stresses, much like mammalian JNK/SAPK (12, 16). WIPK, which was first cloned as a gene whose transcript is induced by wounding (22), is activated by pathogens and pathogen-derived elicitors as well (10, 11, 20). Its ortholog in parsley, ERMK, is also implicated in plant-defense response (13). However, the exact roles of these MAPKs remain unknown. Here, we report the identification of NtMEK2, a tobacco MAPKK. as the upstream kinase for both SIPK and WIPK based on in vitro and in vivo evidence. Expression of NtMEK2^{, a constitutively active mutant of NtMEK2, activates endogenous SIPK and WIPK and leads to HR-like cell death and the activation of a subset of defense genes, demonstrating that the NtMEK2-SIPK/WIPK cascade controls multiple defense responses during plant disease resistance.}

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