MEKK1 Is Required for MPK4 Activation and Regulates Tissue-specific and Temperature-dependent Cell Death in Arabidopsis

Abstract

Innate immunity signaling pathways in both animals and plants are regulated by mitogen-activated protein kinase (MAPK) cascades. An Arabidopsis MAPK cascade (MEKK1, MKK4/MKK5, and MPK3/MPK6) has been proposed to function downstream of the flagellin receptor FLS2 based on biochemical assays using transient overexpression of candidate components. To genetically test this model, we characterized two mekk1 mutants. We show here that MEKK1 is not required for flagellin-triggered activation of MPK3 and MPK6. Instead, MEKK1 is essential for activation of MPK4, a MAPK that negatively regulates systemic acquired resistance. We also showed that MEKK1 negatively regulates temperature-sensitive and tissue-specific cell death and H 2 O 2 accumulation that are partly dependent on both RAR1, a key component in resistance protein function, and SID2, an isochorismate synthase required for salicylic acid production upon pathogen infection.

Arabidopsis Mitogen-Activated Protein Kinase Kinases MKK1 and MKK2 Have Overlapping Functions in Defense Signaling Mediated by MEKK1, MPK4, and MKS1

“…Both mkk1/2 dwarfism and infertility were partially rescued by growth at temperatures .30°C (Supplemental Fig. S4A), as has been reported for mekk1 and mpk4 (Petersen et al, 2000;Andreasson et al, 2005;Ichimura et al, 2006;Nakagami et al, 2006;Suarez-Rodriguez et al, 2007). However, the dwarf phenotype of mekk1 is more pronounced than that of mkk1/2 and, in contrast to mekk1, the root length and branching pattern of mkk1/2 appeared normal, as did that of mpk4 (Supplemental Fig.…”

“…Mutations in MEKK1 (Ichimura et al, 2006;Nakagami et al, 2006;Suarez-Rodriguez et al, 2007) result in plants morphologically similar to mpk4 mutants, including dwarfed growth, enhanced PR1 gene expression, and reduced MPK4 activity induction by hydrogen peroxide or flagellin. mekk1 mutants show some differences from mpk4 in root phenotype and premature senescence of primary leaves, and the mekk1-1/mpk4 double mutant has a more severe phenotype than either single mutant, including an absence of root and shoot growth.…”

Systemic acquired resistance: the elusive signal(s)

“…Thus, a specific role for MPK4 in generating/transmitting the systemic SAR signal seems unlikely. However, the MAP Kinase Kinase Kinase MEKK1, which is involved in PAMP-mediated defense signaling [39,40,41], activates MPK4 in a mechanism that is independent of MEKK1 kinase activity [39,40]. Interestingly, the activities of both MPK3 and MPK6, well-established SA-mediated defense regulators, are enhanced in the mekk1 mutant [39].…”

“…However, the MAP Kinase Kinase Kinase MEKK1, which is involved in PAMP-mediated defense signaling [39,40,41], activates MPK4 in a mechanism that is independent of MEKK1 kinase activity [39,40]. Interestingly, the activities of both MPK3 and MPK6, well-established SA-mediated defense regulators, are enhanced in the mekk1 mutant [39]. Moreover, expression of MEKK1, with the exception of guard cells, localizes predominantly to the vascular tissue of Arabidopsis leaves, while (HR) cell death and hydrogen peroxide accumulation occur in the vasculature and/or guard cells of the mekk1 mutant [39].…”

“…Interestingly, the activities of both MPK3 and MPK6, well-established SA-mediated defense regulators, are enhanced in the mekk1 mutant [39]. Moreover, expression of MEKK1, with the exception of guard cells, localizes predominantly to the vascular tissue of Arabidopsis leaves, while (HR) cell death and hydrogen peroxide accumulation occur in the vasculature and/or guard cells of the mekk1 mutant [39]. Together, the data argue in favor of an antagonistic role of MEKK1 and MPK4 signaling on MPK3 and MPK6, possibly affecting SAR signal transmission through the vasculature.…”