Activation of Ntf4, a Tobacco Mitogen-Activated Protein Kinase, during Plant Defense Response and Its Involvement in Hypersensitive Response-Like Cell Death

Abstract

Mitogen-activated protein kinase (MAPK) cascades are important signaling modules in eukaryotic cells. They function downstream of sensors/receptors and regulate cellular responses to external and endogenous stimuli. Recent studies demonstrated that SIPK and WIPK, two tobacco (Nicotiana spp.) MAPKs, are involved in signaling plant defense responses to various pathogens. Ntf4, another tobacco MAPK that shares 93.6% and 72.3% identity with SIPK and WIPK, respectively, was reported to be developmentally regulated and function in pollen germination. We found that Ntf4 is also expressed in leaves and suspension-cultured cells. Genomic analysis excluded the possibility that Ntf4 and SIPK are orthologs from the two parental lines of the amphidiploid common tobacco. In vitro and in vivo phosphorylation and activation assays revealed that Ntf4 shares the same upstream MAPK kinase, NtMEK2, with SIPK and WIPK. Similar to SIPK and WIPK, Ntf4 is also stress responsive and can be activated by cryptogein, a proteinaceous elicitin from oomycetic pathogen Phytophthora cryptogea. Tobacco recognition of cryptogein induces rapid hypersensitive response (HR) cell death in tobacco. Transgenic Ntf4 plants with elevated levels of Ntf4 protein showed accelerated HR cell death when treated with cryptogein. In addition, conditional overexpression of Ntf4, which results in high cellular Ntf4 activity, is sufficient to induce HR-like cell death. Based on these results, we concluded that Ntf4 is multifunctional. In addition to its role in pollen germination, Ntf4 is also a component downstream of NtMEK2 in the MAPK cascade that regulates pathogen-induced HR cell death in tobacco.The active defense of plants against invading pathogens often includes rapid programmed cell death, known as the hypersensitive response (HR), the generation of reactive oxygen species, the activation of a complex array of defense genes, and the production of antimicrobial phytoalexins (Dangl and Jones, 2001;Dixon, 2001;Lam et al., 2001;Martin et al., 2003;Greenberg and Yao, 2004;Torres and Dangl, 2005). 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 pathogens (Durrant and Dong, 2004;Bostock, 2005). The induction of these defense responses is regulated by a complex signaling network initiated after the plant recognition of pathogens, which is mediated either by gene-for-gene interactions between plant resistance (R) genes and pathogen avirulence (Avr) genes, or by the binding of nonhost-specific elicitors, also known as pathogenassociated molecular patterns, to their receptors (Staskawicz et al., 1995;Baker et al., 1997;Dangl and Jones, 2001;Martin et al., 2003;Innes, 2004;Ausubel, 2005;Boller, 2005).Mitogen-activated protein kinase (MAPK) cascades are highly conserved signaling modules in eukaryotes. They function downstream of sensors/receptors and convert signals generated by the sensors/receptors to cellular responses (H...

Tomato MAPKs LeMPK1, LeMPK2, and LeMPK3 function in the systemin-mediated defense response against herbivorous insects

“…The authors also concluded that some of the published loss-of-function studies that targeted SIPK (41,42) most likely also silenced Ntf4, and that systematic loss-offunction studies of each of the two MAPK paralogs are lacking. They further suggested that the presence of two highly homologous MAPKs in certain solanaceous plants may be adaptive, either because the two MAPKs do have some different yetunknown functions or because of a gene-dosage effect (40). Our results are consistent with the latter scenario.…”

“…The tobacco genome also contains two highly homologous MAPKs of the A2 subgroup, SIPK and Ntf4. Recently, it was shown that their functions are largely redundant (40). The authors also concluded that some of the published loss-of-function studies that targeted SIPK (41,42) most likely also silenced Ntf4, and that systematic loss-offunction studies of each of the two MAPK paralogs are lacking.…”

A Comparison of Two Nicotiana attenuata Accessions Reveals Large Differences in Signaling Induced by Oral Secretions of the Specialist Herbivore Manduca sexta

“…Naf4 is a close homolog of SIPK, which might be derived from a recent gene duplication event (Ren et al, 2006;Wu et al, 2007a). Remarkably, qRT-PCR showed that even when UT and AZ plants were not induced (Fig.…”

“…Naf4 shows high sequence homology to SIPK and thus may have a similar function (Ren et al, 2006). MPK4 is known to be involved in pathogen resistance and woundinduced responses but has as yet no known function in herbivore resistance (Petersen et al, 2000;Gomi et al, 2005).…”

WRKY Transcription Factors Phosphorylated by MAPK Regulate a Plant Immune NADPH Oxidase in Nicotiana benthamiana

“…In tobacco (Nicotiana tabacum), wound-induced protein kinase (WIPK) and salicylic acid-induced protein kinase (SIPK) are pathogen-responsive MAPKs (Seo et al, 1995;Zhang and Klessig, 1997). MEK2, a tobacco MAPK kinase, functions upstream of SIPK and WIPK (Ren et al, 2006). The MEK2-SIPK/WIPK cascade (MEK2 cascade) is highly conserved in diverse plant species and participates in defense responses (Tanaka et al, 2009).…”