Complex II plays a central role in mitochondrialmetabolism as a component of both the electrontransport chain and the tricarboxylic acid cycle. However,the composition and function of the plant enzyme has beenelusive and differs from the well-characterised enzymes inmammals and bacteria. Herewith, we demonstrate thatmitochondrial Complex II from Arabidopsis and rice differsignificantly in several aspects: (1) Stability—Rice complexII in contrast to Arabidopsis is not stable whenresolved by native electrophoresis and activity staining. (2)Composition—Arabidopsis complex II contains 8 subunits,only 7 of which have homologs in the rice genome. SDH 1and 2 subunits display high levels of amino acid identitybetween two species, while the remainder of the subunitsare not well conserved at a sequence level, indicatingsignificant divergence. (3) Gene expression—the pairs oforthologous SDH1 and SDH2 subunits were universallyexpressed in both Arabidopsis and rice. The very divergentgenes for SDH3 and SDH4 were co-expressed in bothspecies, consistent with their functional co-ordination toform the membrane anchor. The plant-specific SDH5, 6and 7 subunits with unknown functions appeared to bedifferentially expressed in both species. (4) Biochemicalregulation -succinate-dependent O2 consumption and SDHactivity of isolated Arabidopsis mitochondria weresubstantially stimulated by ATP, but a much more minoreffect of ATP was observed for the rice enzyme. The ATPactivation of succinate-dependent reduction of DCPIP infrozen-thawed and digitonin-solubilised mitochondrialsamples, and with or without the uncoupler CCCP, indicatethat the differential ATP effect on SDH is not via theprotonmotive force but likely due to an allosteric effect onthe plant SDH enzyme itself, in contrast to the enzyme inother organisms.