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CELL BIOLOGY AND SIGNAL TRANSDUCTION

Arabidopsis Mitogen-Activated Protein Kinase Kinases MKK1 and MKK2 Have Overlapping Functions in Defense Signaling Mediated by MEKK1, MPK4, and MKS1^1,[W]

Department of Biology, University of Copenhagen, 2200 Copenhagen N, Denmark (J.-L.Q., B.K.F., K.P., J. Mundy); Heriot-Watt University, School of Life Sciences, Riccarton, Edinburgh EH14 4AS, United Kingdom (L.Z., J. MacKinlay, P.C.M.); Institute of Molecular Plant Sciences, University of Edinburgh, King's Buildings, Edinburgh EH9 3JH, United Kingdom (B.-W.Y., G.J.L.); and Center for Biological Sequence Analysis, Department of Systems Biology, Technical University of Denmark, Kemitorvet, DK–2800 Kgs. Lyngby, Denmark (H.B.N.)

The Arabidopsis (Arabidopsis thaliana) MKK1 and MKK2 mitogen-activated protein kinase kinases have been implicated in biotic and abiotic stress responses as part of a signaling cascade including MEKK1 and MPK4. Here, the double loss-of-function mutant (mkk1/2) of MKK1 and MKK2 is shown to have marked phenotypes in development and disease resistance similar to those of the single mekk1 and mpk4 mutants. Because mkk1 or mkk2 single mutants appear wild type, basal levels of MPK4 activity are not impaired in them, and MKK1 and MKK2 are in part functionally redundant in unchallenged plants. These findings are confirmed and extended by biochemical and molecular analyses implicating the kinases in jasmonate- and salicylate-dependent defense responses, mediated in part via the MPK4 substrate MKS1. In addition, transcriptome analyses delineate overlapping and specific effects of the kinases on global gene expression patterns demonstrating both redundant and unique functions for MKK1 and MKK2.

² These authors contributed equally to the article.

³ Present address: Carlsberg Laboratory, Gamle Carlsberg Vej 10, 2500 Valby, Denmark.

⁴ Present address: NWCRF Institute, School of Biological Sciences, University of Wales, Bangor, Gwynedd LL57 2UW, UK.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Peter C. Morris (p.c.morris{at}hw.ac.uk).

^[W] The online version of this article contains Web-only data.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.120006

^* Corresponding author; e-mail p.c.morris{at}hw.ac.uk.

Received March 28, 2008; accepted June 27, 2008; published July 3, 2008.