SUPPRESSION OF AUXIN SIGNAL-TRANSDUCTION BY A MAPK CASCADE IN HIGHER-PLANTS

The plant hormone auxin activates many early response genes that are t hought to be responsible for diverse aspects of plant growth and devel opment(1). It has been proposed that auxin signal transduction is medi ated by a conserved signalling cascade consisting of three protein kin ases: the mitogen-activated protein kinase (MAPK), MAPK kinase (MAPKK) and MAPKK kinase (MAPKKK)(2), Here we show that a specific plant MAPK KK, NPK1 (ref. 3), activates a MAPK cascade that leads to the suppress ion of early auxin response gene transcription, A mutation in the kina se domain abolishes NPK1 activity, and the presence of the carboxy-ter minal domain diminishes the kinase activity, Moreover, the effects of NPK1 on the activation of a MAPK and the repression of early auxin res ponse gene transcription are specifically eliminated by a MAPK phospha tase(4). Transgenic tobacco plants overexpressing the NPK1 kinase doma in produced seeds defective in embryo and endosperm development. These results suggest that auxin sensitivity may be balanced by antagonisti c signalling pathways that use a distinct MAPK cascade in higher plant s.