Mitochondrial complex II of plants: subunit composition, assembly and function in respiration and signaling
Complex II (succinate dehydrogenase [succinate‐ubiquinone oxidoreductase]; EC 18.104.22.168; SDH) is the only enzyme shared by both the electron transport chain and the tricarboxylic acid (TCA) cycle in mitochondria. Complex II in plants is considered unusual due to its accessory subunits (SDH5‐8) in addition to the catalytic subunits of SDH found in all eukaryotes (SDH1‐4). Here we review compositional and phylogenetic analysis, and biochemical dissection studies to clarify the presence and to propose the role of these subunits. We also consider the wider functional and phylogenetic evidence for SDH assembly factors and the reports from plants on the control of SDH1 flavination and SDH1‐SDH2 interaction. Plant Complex II has been shown to influence stomatal opening, the plant defence response and reactive oxygen species‐dependent stress responses. Signalling molecules such as salicyclic acid (SA) and nitric oxide (NO) are also reported to interact with the UQ binding site of SDH, influencing signalling transduction in plants. Future directions for SDH research in plants and the specific roles of its different subunits and assembly factors are suggested, including the potential for reverse electron transport to explain succinate‐dependent ROS production in plants and new avenues to explore plant mitochondrial Complex II evolution and its utility.
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