MAPK cascades in plant defense signaling.

The Arabidopsis genome encodes approximately 20 different mitogen-activated protein kinases (MAPKs) that are likely to be involved in growth, development and responses to endogenous and environmental cues. Several plant MAPKs are activated by a variety of stress stimuli, including pathogen infection, wounding, 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 negative regulation of disease resistance. This 'push-and-pull' regulation by different MAPK pathways might provide a more precise control of plant defense responses.