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.