Chloroplast‐generated reactive oxygen species are involved in hypersensitive response‐like cell death mediated by a mitogen‐activated protein kinase cascade

Abstract

SummaryPlant defense against pathogens often includes rapid programmed cell death known as the hypersensitive response (HR). Recent genetic studies have demonstrated the involvement of a specific mitogen-activated protein kinase (MAPK) cascade consisting of three tobacco MAPKs, SIPK, Ntf4 and WIPK, and their common upstream MAPK kinase (MAPKK or MEK), NtMEK2. Potential upstream MAPKK kinases (MAPKKKs or MEKKs) in this cascade include the orthologs of Arabidopsis MEKK1 and tomato MAPKKKa. Activation of the SIPK/Ntf4/WIPK pathway induces cell death with phenotypes identical to pathogen-induced HR at macroscopic, microscopic and physiological levels, including loss of membrane potential, electrolyte leakage and rapid dehydration. Loss of membrane potential in NtMEK2 DD plants is associated with the generation of reactive oxygen species (ROS), which is preceded by disruption of metabolic activities in chloroplasts and mitochondria. We observed rapid shutdown of carbon fixation in chloroplasts after SIPK/Ntf4/WIPK activation, which can lead to the generation of ROS in chloroplasts under illumination. Consistent with a role of chloroplast-generated ROS in MAPK-mediated cell death, plants kept in the dark do not accumulate H 2 O 2 in chloroplasts after MAPK activation, and cell death is significantly delayed. Similar light dependency was observed in HR cell death induced by tobacco mosaic virus, which is known to activate the same MAPK pathway in an N-gene-dependent manner. These results suggest that activation of the SIPK/Ntf4/WIPK cascade by pathogens actively promotes the generation of ROS in chloroplasts, which plays an important role in the signaling for and/or execution of HR cell death in plants.

Singlet Oxygen Is the Major Reactive Oxygen Species Involved in Photooxidative Damage to Plants

“…Our data suggest that 1 O 2 production prevails over FR production and hence 1 O 2 appears as the most relevant ROS with respect to the execution of cell death in leaf tissues. An example of ROSdependent plant cell death is also the hypersensitive response (Torres et al, 2006) during which light seems to be required both to initiate a cell death program, early on in the infection, as well as at later stages of cell death execution (Zeier et al, 2004;Liu et al, 2007). Interestingly, 1 O 2 HOTE markers significantly increased in Arabidopsis leaves challenged with the avirulent strain DC3000 of Pseudomonas syringae (Grun et al, 2007).…”

“…Interestingly, 1 O 2 HOTE markers significantly increased in Arabidopsis leaves challenged with the avirulent strain DC3000 of Pseudomonas syringae (Grun et al, 2007). In tobacco (Nicotiana tabacum), a pathogeninduced mitogen-activated protein kinase cascade disrupts chloroplast metabolism and leads to a rapid shutdown of CO 2 fixation, a phenomenon that is expected to favor light-dependent excess excitation energy conditions in thylakoids and ROS production (Liu et al, 2007). It is also worth mentioning that compromised linear electron transport in transgenic tobacco plants with reduced ferredoxin-NADP(H) reductase levels also led to increased generation of 1 O 2 , eventually followed by cell death (Palatnik et al, 2003).…”

Light Regulation and Daytime Dependency of Inducible Plant Defenses in Arabidopsis: Phytochrome Signaling Controls Systemic Acquired Resistance Rather Than Local Defense

“…As SA biosynthesis via isochorismate synthase occurs in plastids (Strawn et al, 2007), photosynthetic activity might be required to supply reducing equivalents and energy for SA accumulation. At least for HR execution, intact chloroplasts and associated ROS production seem to play an important role (Genoud et al, 2002;Liu et al, 2007). The impact of carbohydrate status and chloroplast function on pathogen-induced defense activation, however, requires further attention.…”

Oxidative Stress: Antagonistic Signaling for Acclimation or Cell Death?

“…The balance between the accumulation of specific ROS and their scavenging by enhanced antioxidant defenses is controlled by EDS1 and NUDT7 and decides the fate of the cell by pushing it toward death or toward resistance. Very few details of linking steps in these pathways have been identified, but there is strong evidence for the involvement of mitogen-activated protein kinases in transducing signals derived from chloroplast-sourced ROS (Liu et al, 2007).…”