TRENDS in Plant Science Vol.7 No.7 July 2002 http://plants.trends.com 1360-1385/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S1360-1385(02)02302-6 301 Opinion Mitogen-activated protein kinase (MAPK; see Glossary) cascades are universal modules of signal transduction in eukaryotes. These protein phosphorylation cascades mediate the intracellular transmission and amplification of extracellular stimuli, resulting in the induction of appropriate biochemical and physiological cellular responses [1–4]. MAPKs form the terminal components of the prototypical sequential cascades, and are activated by MAPK kinases (MAPKKs or MEKs) via dual phosphorylation of conserved threonine and tyrosine residues in the motif TxY located in the activation loop (T-loop) between kinase subdomains VII and VIII. MAPKKs are themselves activated by MAPKK kinases (MAPKKKs or MEKKs) through phosphorylation of conserved serine and/or threonine residues in their T-loop [1–4]. In plants, MAPK cascades are associated with various physiological, developmental and hormonal responses. Molecular and biochemical studies using specific antibodies to particular MAPKs have revealed that MAPK activation correlates with stimulatory treatments such as pathogen infection, wounding, low temperature, drought, hyper- and hypo-osmolarity, high salinity, touch, and reactive oxygen species [5–8]. Genetic studies of the Arabidopsis mutants constitutive triple response 1 (ctr1) and enhanced disease resistance 1 (edr1), which exhibit altered responses to ethylene and pathogens, respectively, show that their wild-type alleles encode MAPKKKs related to the RAF protein kinase [9,10]. Reverse genetic analysis of the Arabidopsis mpk4 knockout mutant revealed its importance in regulating systemic acquired resistance, including the ability to accumulate salicylic acid [11]. A gain-of-function study M itogen-activated protein kinase cascades in plants: a new nomenclature MAPK Group (Kazuya Ichimura et al.) Mitogen-activated protein kinase (MAPK) cascades are universal signal transduction modules in eukaryotes, including yeasts, animals and plants. These protein phosphorylation cascades link extracellular stimuli to a wide range of cellular responses. In plants, MAPK cascades are involved in responses to various biotic and abiotic stresses, hormones, cell division and developmental processes. Completion of the Arabidopsis genome-sequencing project has revealed the existence of 20 MAPKs, 10 MAPK kinases and 60 MAPK kinase kinases. Here, we propose a simplified nomenclature for Arabidopsis MAPKs and MAPK kinases that might also serve as a basis for standard annotation of these gene families in all plants. Published online: 13 June 2002 18 Blazquez, M.A. and Weigel, D. (2000) Integration of floral inductive signals in Arabidopsis. 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