Opinion TRENDS in Plant Science Vol.7 No.7 July 2002 301 18 Blazquez, M.A. and Weigel, D. (2000) Integration substrate change and catalytic simplification. of genomes. Proc. Natl. Acad. Sci. U. S. A. of floral inductive signals in Arabidopsis. Proc. Natl. Acad. Sci. U. S. A. 93, 9033–9038 99, 1405–1409 Nature 404, 889–892 27 Nowak, M.A. et al. (1997) Evolution of genetic 36 Shu, G. et al. (2000) LEAFY and the evolution of 19 Blazquez, M.A. et al. (1997) LEAFY expression redundancy. Nature 388, 167–171 rosette flowering in violet cress (Jonopsidium and flower initiation in Arabidopsis. Development 28 Lynch, M. et al. (2001) The probability of acaule, Brassicaceae). Am. J. Bot. 87, 634–641 124, 3835–3844 preservation of a newly arisen gene duplicate. 37 Ahearn, K.P. et al. (2001) NFL1, a Nicotiana 20 Parcy, F. et al. (1998) A genetic framework for Genetics 159, 1789–1804 tabacum LEAFY-like gene, controls meristem floral patterning. Nature 395, 561–566 29 Wagner, G.P. and Altenberg, L. (1996) Complex initiation and floral structure. Plant Cell Physiol. 21 Lohmann, J.U. et al. (2001) A molecular link adaptations and the evolution of evolvability. 42, 1130–1139 between stem cell regulation and floral patterning Evolution 50, 967–976 38 Gourlay, C.W. et al. (2000) Pea compound leaf in Arabidopsis. Cell 105, 793–803 30 Coen, E.S. and Meyerowitz, E.M. (1991) The war architecture is regulated by interactions among 22 Weigel, D. et al. (1992) LEAFY controls of the whorls: genetic interactions controlling the genes UNIFOLIATA, COCHLEATA, AFILA, floral meristem identity in Arabidopsis. flower development. Nature 353, 31–37 and TENDRIL-LESS. Plant Cell 12, 1279–1294 Cell 69, 843–859 31 Albert, V.A. et al. (1998) Ontogenetic systematics, 39 Kyozuka, J. et al. (1998) Down-regulation of RFL, 23 Himi, S. et al. (2001) Evolution of MADS-box gene molecular developmental genetics, and the the FLO/LFY homolog of rice, accompanied with induction by FLO/LFY genes. J. Mol. Evol. angiosperm petal. In Molecular Systematics of panicle branch initiation. Proc. Natl. Acad. Sci. 53, 387–393 Plants II: DNA Sequencing (Soltis, D.E. et al., U. S. A. 95, 1979–1982 24 Mouradov, A. et al. (1998) NEEDLY, a Pinus eds), pp. 349–374, Kluwer Academic Publishers 40 Gocal, G.F.W. et al. (2001) Evolution of floral radiata ortholog of FLORICAULA/LEAFY genes, 32 Endress, P.K. (1996) Structure and function of meristem identity genes. Analysis of Lolium expressed in both reproductive and vegetative female and bisexual organ complexes in Gnetales. temulentum genes related to APETALA1 and meristems. Proc. Natl. Acad. Sci. U. S. A. Int. J. Plant Sci. 157, S113–S125 LEAFY of Arabidopsis. Plant Physiol. 95, 6537–6542 33 Norstog, K.J. and Nicholls, T.J. (1997) The Biology 125, 1788–1801 25 Mellerowicz, E.J. et al. (1998) PRFLL – a Pinus of the Cycads, Cornell University Press 41 Soltis, D.E. et al. (2001) Missing links: the genetic radiata homologue of FLORICAULA and LEAFY 34 Shindo, S. et al. (2001) Characterization of a architecture of flowers and floral diversification. is expressed in buds containing vegetative shoot FLORICAULA/LEAFY homologue of Trends Plant Sci. 7, 22–31 and undifferentiated male cone primordia. Planta Gnetum parvifolium and its implications for the 42 Nixon, K.C. (1999) Winclada (BETA) ver. 0.9.9, 206, 619–629 evolution of reproductive organs in seed plants. Published by the author, Ithaca, New York 26 Helariutta, Y. et al. (1996) Duplication and Int. J. Plant. Sci. 162, 1199–1209 43 Krakauer, D.C. and Nowak, M.A. (1999) functional divergence in the chalcone synthase 35 Krakauer, D.C. and Plotkin, J.B. (2002) Evolutionary preservation of redundant duplicated gene family of Asteraceae: evolution with Redundancy, antiredundancy, and the robustness genes. Semin. Cell Dev. Biol. 10, 555–559 cascades mediate the intracellular transmission Mitogen-activated and amplification of extracellular stimuli, resulting in the induction of appropriate biochemical and physiological cellular responses [1–4]. MAPKs form protein kinase the terminal components of the prototypical sequential cascades, and are activated by MAPK kinases (MAPKKs or MEKs) via dual phosphorylation of cascades in plants: conserved threonine and tyrosine residues in the motif TxY located in the activation loop (T-loop) between kinase subdomains VII and VIII. MAPKKs a new nomenclature are themselves activated by MAPKK kinases (MAPKKKs or MEKKs) through phosphorylation of conserved serine and/or threonine residues in their MAPK Group (Kazuya Ichimura et al.) T-loop [1–4]. In plants, MAPK cascades are associated with various physiological, developmental and hormonal Mitogen-activated protein kinase (MAPK) cascades are universal signal responses. Molecular and biochemical studies using transduction modules in eukaryotes, including yeasts, animals and plants. specific antibodies to particular MAPKs have These protein phosphorylation cascades link extracellular stimuli to a wide revealed that MAPK activation correlates with range of cellular responses. In plants, MAPK cascades are involved in stimulatory treatments such as pathogen infection, responses to various biotic and abiotic stresses, hormones, cell division and wounding, low temperature, drought, hyper- and developmental processes. Completion of the Arabidopsis genome-sequencing hypo-osmolarity, high salinity, touch, and reactive project has revealed the existence of 20 MAPKs, 10 MAPK kinases and 60 MAPK oxygen species [5–8]. Genetic studies of the Arabidopsis kinase kinases. Here, we propose a simplified nomenclature for Arabidopsis mutants constitutive triple response 1 (ctr1) and MAPKs and MAPK kinases that might also serve as a basis for standard enhanced disease resistance 1 (edr1), which exhibit annotation of these gene families in all plants. altered responses to ethylene and pathogens, respectively, show that their wild-type alleles encode Published online: 13 June 2002 MAPKKKs related to the RAF protein kinase [9,10]. Reverse genetic analysis of the Arabidopsis mpk4 Mitogen-activated protein kinase (MAPK; see Glossary) knockout mutant revealed its importance in regulating cascades are universal modules of signal transduction systemic acquired resistance, including the ability to in eukaryotes. These protein phosphorylation accumulate salicylic acid [11]. A gain-of-function study http://plants.trends.com 1360-1385/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S1360-1385(02)02302-6 302 Opinion TRENDS in Plant Science Vol.7 No.7 July 2002 Glossary ANP MEK Ran Arabidopsis NPK1 homolog MAPK/ERK kinase; another name ras-related nuclear protein AtMRK1 for a MAPKK SAMK Arabidopsis thaliana MLK/Raf-related MEKK Stress-activated MAPK protein kinase 1 MAPK/ERK kinase kinase; another name for SIMK BWMK1 a MAPKKK Salt-stress-inducible MAPK Kazuya Ichimura Blast- and wound-induced MAP kinase 1 MHK SIMKK Kazuo Shinozaki* Bck1 Mak-homologous kinase SIMK kinase RIKEN Tsukuba Institute, Bypass of C kinase 1 MKK SIPK Japan. CD domain Another name for a MAPKK Salicylic-acid-induced protein kinase *e-mail: sinozaki@ Common docking domain MMK Ste11 rtc.riken.go.jp JNK Medicago MAPK Sterile 11 c-Jun amino(NH )-terminal kinase Guillaume Tena 2 MPK TDY LRR Jen Sheen Mitogen-activated protein kinase Putative activation motif in MAPK Leucine-rich repeat Harvard Medical School, NB-ARC gene TDY1 MA, USA. MAP3K Mitogen-activated protein kinase Nucleotide-binding adaptor shared by TIR domain Yves Henry kinase kinase APAF-1, certain R-gene products Toll- and interleukin-1-receptor domain Anthony Champion MAPK and CED-4 WIPK Martin Kreis Mitogen-activated protein kinase NPK1 Wound-induced protein kinase Institut de Biotechnologie MAPKK Nucleus- and phragmoplast-localized WRKY des Plantes, Orsay, Mitogen-activated protein kinase kinase protein kinase (renamed from Nicotiana Zinc-finger-type transcription factor France. MAPKKK protein kinase 1) WRKY domain Mitogen-activated protein kinase RAF 60-amino-acid motif that binds to the Shuqun Zhang University of Missouri – kinase kinase Transforming gene W-box domain Columbia, MO, USA. Heribert Hirt showed that ectopic expression of a constitutively RAF-like MAPKKKs have been investigated Cathal Wilson Erwin Heberle-Bors active mutant form of tobacco (Nicotiana tabacum) biochemically. Within the proposed plant MPK and Vienna Biocenter, Austria. NtMEK2 induced the activation of downstream MKK nomenclature, we respect the established rules Brian E. Ellis MAPK, the expression of defense genes and for plant gene naming and numbering, both for University of British hypersensitive-response-like cell death [12]. Recently, consistency and to avoid implying possible functions Columbia, Vancouver, an Arabidopsis MAPK cascade (MEKK1–MKK4-5–MPK6) for specific MPKs and MKKs. In keeping with the Canada. and WRKY transcription factors were shown to be TAIR suggestions for naming Arabidopsis genes Peter C. Morris involved in innate immunity and function downstream (http://www.arabidopsis.org/info/guidelines.html), Heriot–Watt University, of the flagellin receptor FLS2 [13]. we have also not incorporated an ‘At’ prefix into the Edinburgh, UK. In this article, we describe the complete sets formal gene name in this species. Although this is Roger W. Innes of Arabidopsis MAPKs and MAPKKs, and a appropriate within Arabidopsis, we also recognize Indiana University, IN, USA. proposed set of MAPKKKs, and analyze them the informational value of a taxonomic prefix for by phylogenetic methods, together with other situations