Abstract
Innate immunity signaling pathways in plants are regulated by mitogen-activated protein kinase (MAPK) cascades. Arabidopsis MEKK1, a MAPK kinase kinase (MAPKKK) has been proposed as an important factor of innate immunity signaling in the upstream of the MPK3, MPK4, and MPK6 (MAPKs). Flg22 treatment activates MPK3, MPK4, and MPK6. However, genetic evidence of MEKK1 involvement in flg22-triggered MAPK activation has not been demonstrated.
We show that loss of MEKK1 results in temperature-sensitive and tissue-specific cell death and H2O2 accumulation that are partly dependent on both RAR1, a key component in resistance (R) protein function, and SID2, an isochorismate synthase required for salicylic acid production upon pathogen infection. MEKK1 is not required for flg22-triggered activation of MPK3 and MPK6. Instead, MEKK1 is essential for activation of MPK4, which is a negative regulator of defense responses. These data suggest that MEKK1 plays an important role in negative control of defense responses.
