Mitochondrial complex II-derived superoxide is the primary source of mercury toxicity in barley root tip

Enhanced superoxide generation and significant inhibition of succinate dehydrogenase (SDH) activity followed by a strong reduction of root growth were detected in barley seedlings exposed to a 5 μM Hg concentration for 30 min, which increased further in an Hg dose-dependent manner. While at a 25 μM Hg concentration no cell death was detectable, a 50 μM Hg treatment triggered cell death in the root meristematic zone, which was markedly intensified after the treatment of roots with 100 μM Hg and was detectable in the whole root tips. Generation of superoxide and H2O2 was a very rapid response of root tips occurring even after 5 min of exposure to Hg. Application of an NADPH oxidase inhibitor or the inhibition of electron flow in mitochondria by the inhibition of complex I did not influence the Hg-induced H2O2 production. Treatment of roots with thenoyltrifluoroacetone, a non-competitive inhibitor of SDH, markedly reduced root growth and induced both superoxide and H2O2 production in a dose dependent manner. Similar to results obtained in intact roots, Hg strongly inhibited SDH activity in the crude mitochondrial fraction and caused a considerable increase of superoxide production, which was markedly reduced by the competitive inhibitors of SDH. These results indicate that the mitochondrial complex II-derived superoxide is the primary source of Hg toxicity in the barley root tip.