Pegulating Of Stomatal Opening And Closure By A MAPK Cascade In Plant Immunity

Plant defense against invading pathogens is initiated by the recognition of pathogen-associated molecular patterns (PAMPs) through the cell surface-localized pattern-recognition receptors (PRRs), known as PAMP-triggered immunity (PTI). Plants also possess a second layer of defense through the recognition of pathogen-derived effectors by the intracellular R proteins, known as effector-triggered immunity (ETI). Both PAMPs and effectors can induce rapid activation of mitogen-activated protein kinases (MAPKs or MPKs). In Arabidopsis, three MPKs, MPK3, MPK6, and MPK4, are involved in plant defense responses. They play important roles in stomatal closure, ROS generation, activation the resistance genes, phytoalexin accumulation, and HR cell death. Recently, we found that the loss of function of both MPK3 and MPK6 severely compromises Arabidopsis resistance against Pseudomonas syringae pv. tomato, and the compromised resistance is associated with failure of stomatal closure. In this project, we analyzed in detail the function of MPK3/MPK6 in the stomatal closure in plant immunity and the relationship between plant MAPK signal pathway and ABA pathway. Following are the main results and conclusions:1. Arabidopsis MPK3 and MPK6 play redundant function in PAMP-triggered stomatal closure. Treatment with flg22 induces stomatal closure in mpk3 single and mpk6 single mutant, but not the mpk3 mpk6 double mutant. Similarly, flg22 fails to close the stomata of mkk4 mkk5 double mutant, suggesting that MKK4/MKK5-MPK3/MPK6 module plays an important role in plant stomatal immunity.2. Treatment with flg22 also fails to induce stomatal closure in abil-11,aba2-1, ost1 or slac1-3 mutant, suggesting that ABA synthesis/signaling are also essential to PAMP-triggered stomatal closure.3. Gain-of-function activation of MPK3/MPK6 in the DD plants is sufficient to induce stomatal closure, which is independent of ABA synthesis or signaling pathways based on the analysis of DD/abil-11, DD/aba2-1, DD/ost1 and DD/slac1-3 double mutants.4. Surprisingly, although flg22 fails to close the stomata in the loss-of-function mpk3 mpk6 double mutant, exogenously added ABA can, suggesting that MPK3/MPK6 and ABA are independent pathways in, but both are required for, the stomatal closure during plant immunity.