title: Identification of three MAPKKKs forming a linear signaling pathway leading to programmed cell death in Nicotiana benthamiana.
abstract: BACKGROUND
  The mitogen-activated protein kinase (MAPK) cascade is an evolutionarily ancient mechanism of signal transduction found in eukaryotic cells. In plants, MAPK cascades are associated with responses to various abiotic and biotic stresses such as plant pathogens. MAPK cascades function through sequential phosphorylation: MAPK kinase kinases (MAPKKKs) phosphorylate MAPK kinases (MAPKKs), and phosphorylated MAPKKs phosphorylate MAPKs. Of these three types of kinase, the MAPKKKs exhibit the most divergence in the plant genome. Their great diversity is assumed to allow MAPKKKs to regulate many specific signaling pathways in plants despite the relatively limited number of MAPKKs and MAPKs. Although some plant MAPKKKs, including the MAPKKKα of Nicotiana benthamiana (NbMAPKKKα), are known to play crucial roles in plant defense responses, the functional relationship among MAPKKK genes is poorly understood. Here, we performed a comparative functional analysis of MAPKKKs to investigate the signaling pathway leading to the defense response.
  
  
  RESULTS
  We cloned three novel MAPKKK genes from N. benthamiana: NbMAPKKKβ, NbMAPKKKγ, and NbMAPKKKε2. Transient overexpression of full-length NbMAPKKKβ or NbMAPKKKγ or their kinase domains in N. benthamiana leaves induced hypersensitive response (HR)-like cell death associated with hydrogen peroxide production. This activity was dependent on the kinase activity of the overexpressed MAPKKK. In addition, virus-induced silencing of NbMAPKKKβ or NbMAPKKKγ expression significantly suppressed the induction of programmed cell death (PCD) by viral infection. Furthermore, in epistasis analysis of the functional relationships among NbMAPKKKβ, NbMAPKKKγ, and NbMAPKKKα (previously shown to be involved in plant defense responses) conducted by combining transient overexpression analysis and virus-induced gene silencing, silencing of NbMAPKKKα suppressed cell death induced by the overexpression of the NbMAPKKKβ kinase domain or of NbMAPKKKγ, but silencing of NbMAPKKKβ failed to suppress cell death induced by the overexpression of NbMAPKKKα or NbMAPKKKγ. Silencing of NbMAPKKKγ suppressed cell death induced by the NbMAPKKKβ kinase domain but not that induced by NbMAPKKKα.
  
  
  CONCLUSIONS
  These results demonstrate that in addition to NbMAPKKKα, NbMAPKKKβ and NbMAPKKKγ also function as positive regulators of PCD. Furthermore, these three MAPKKKs form a linear signaling pathway leading to PCD; this pathway proceeds from NbMAPKKKβ to NbMAPKKKγ to NbMAPKKKα.
authors: Department of Agricultural and Environmental Biology, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-8657, Japan.
authors: Masayoshi
authors: M
authors: Hashimoto
authors: Ken
authors: K
authors: Komatsu
authors: Kensaku
authors: K
authors: Maejima
authors: Yukari
authors: Y
authors: Okano
authors: Takuya
authors: T
authors: Shiraishi
authors: Kazuya
authors: K
authors: Ishikawa
authors: Yusuke
authors: Y
authors: Takinami
authors: Yasuyuki
authors: Y
authors: Yamaji
authors: Shigetou
authors: S
authors: Namba
mesh_terms: D017209:Apoptosis / Q000235:genetics; D055162:Biocatalysis; D003001:Cloning, Molecular; D004843:Epistasis, Genetic; D015971:Gene Expression Regulation, Enzymologic; D018506:Gene Expression Regulation, Plant; D020868:Gene Silencing; D017343:Genes, Plant / Q000235:genetics; D020930:MAP Kinase Kinase Kinases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D009029:Mosaic Viruses / Q000502:physiology; D010802:Phylogeny; D018515:Plant Leaves / Q000166:cytology / Q000201:enzymology; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics; D017434:Protein Structure, Tertiary; D015398:Signal Transduction / Q000235:genetics; D014026:Nicotiana / Q000166:cytology* / Q000201:enzymology* / Q000235:genetics
chemical_list: D010940:Plant Proteins; D020930:MAP Kinase Kinase Kinases