The Arabidopsis genome encodes similar to 20 different mitogen-activated pr
otein kinases (MAPKs) that are likely to be involved in growth, development
and responses to endogenous and environmental cues. Several plant MAPKs ar
e activated by a variety of stress stimuli, including pathogen infection, w
ounding, temperature, drought, salinity, osmolarity, UV irradiation, ozone
and reactive oxygen species. Recent gain-of-function studies show that two
tobacco MAPKs induce the expression of defense genes and cause cell death.
By contrast, loss-of-function studies of other MAPK pathways revealed negat
ive regulation of disease resistance. This'push-and-pull'regulation by diff
erent MAPK pathways might provide a more precise control of plant defense r
esponses.