Plant mitogen-activated protein kinase cascades: Negative regulatory roles turn out positive

Mitogen-activated protein (MAP) kinase cascades are a standard player in the signal transduction literature for diverse organisms (1–3). Previous work has shown that specific MAP kinases become activated during plant responses to pathogens, suggesting a role for MAP kinase cascades in disease resistance. However, no function-blocking experiments had demonstrated a causal role until the work of Frye et al. (4) on the Arabidopsis EDR1 gene that was presented in a recent issue of PNAS. In a related development, Petersen et al. (5) have just published work showing that disruption of the Arabidopsis MPK4 MAP kinase gene also alters plant defense activation. It is surprising that, for all plant species, these are only the second and third characterized mutants known to carry a disrupted kinase from a MAP kinase cascade. Equally striking, both papers revisit a theme established by the first known plant MAP kinase cascade mutant and reinforced in a separate study (6, 7): the MAP kinase cascade as a negative regulator of the relevant biological response. However, the probability of crosstalk among MAP kinase cascades, discussed below, suggests that this negative regulatory activity of MAP kinase cascades could be misleading.