It has been shown that the Arabidopsis MEK kinase MEKK1 acts upstream of the MAP kinase MPK4 to negatively regulate salicylic acid-dependent defense-response pathways. Here, we report that the mekk1;mpk4 double-mutant combination causes seedling lethality. In addition, we demonstrate that mekk1 and mpk4 single-mutant plants have significantly different phenotypes. mekk1 plants are defective for lateral root formation, while mpk4 plants are not. In addition, treatment with elevated levels of sodium chloride improves the growth of mekk1 plants, while it inhibits the growth of mpk4 plants. Our results suggest that MEKK1 and MPK4 functions are not limited to a single, linear signaling pathway. Instead there appears to be more complexity to the signaling pathways in which these two proteins function.