Salicylic Acid-Dependent Plant Stress Signaling via Mitochondrial Succinate Dehydrogenase.

Mitochondria are known for their role in ATP production and generation of reactive oxygen species, but little is known about the mechanism of their early involvement in plant stress signaling. The role of mitochondrial succinate dehydrogenase (SDH) in salicylic acid (SA) signaling was analyzed using two mutants: (), which is a point mutation in SDH1 identified in a loss of SA signaling screen, and a knockdown mutant () for SDH assembly factor 2 that is required for FAD insertion into SDH1. Both mutants showed strongly decreased SA-inducible stress promoter responses and low SDH maximum capacity compared to wild type, while also showed low succinate affinity, low catalytic efficiency, and increased resistance to SDH competitive inhibitors. The SA-induced promoter responses could be partially rescued in , but not in , by supplementing the plant growth media with succinate. Kinetic characterization showed that low concentrations of either SA or ubiquinone binding site inhibitors increased SDH activity and induced mitochondrial HO production. Both and showed lower rates of SA-dependent HO production in vitro in line with their low SA-dependent stress signaling responses in vivo. This provides quantitative and kinetic evidence that SA acts at or near the ubiquinone binding site of SDH to stimulate activity and contributes to plant stress signaling by increased rates of mitochondrial HO production, leading to part of the SA-dependent transcriptional response in plant cells.