Suppression of auxin signal transduction by a MAPK cascade in higher plants

The plant hormone auxin activates many early response genes that are thought to be responsible for diverse aspects of plant growth and development1. It has been proposed that auxin signal

transduction is mediated by a conserved signalling cascade consisting of three protein kinases: the mitogen-activated protein kinase (MAPK), MAPK kinase (MAPKK) and MAPKK kinase (MAPKKK)2.

Here we show that a specific plant MAPKKK, NPK1 (ref. 3), activates a MAPK cascade that leads to the suppression of early auxin response gene transcription. A mutation in the kinase domain

abolishes NPK1 activity, and the presence of the carboxy-terminal domain diminishes the kinase activity. Moreover, the effects of NPK1 on the activation of a MAPK and the repression of early

auxin response gene transcription are specifically eliminated by a MAPK phosphatase4. Transgenic tobacco plants overexpressing the NPK1 kinase domain produced seeds defective in embryo and

endosperm development. These results suggest that auxin sensitivity may be balanced by antagonistic signalling pathways that use a distinct MAPK cascade in higher plants.

We thank T. Ulmasov and T. J. Guilfoyle for the GH3–GUS construct; J.-C. Jang for the tobacco cDNA; M. Minet for the Arabidopsis cDNA library; H. Sun and N. K. Tonks for the MKP1 cDNA; and

T. Jones, A. Khokhlatchev, G. Lazar, J. Nardone and J. Nishio for comments on the manuscript. J.S. is supported by Hoecht A.G., USDA and NSF.

Department of Genetics, and Department of Molecular Biology, Harvard Medical School, Massachusetts General Hospital, Boston, 02114, Massachusetts, USA

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