Effect of salicylic acid on the metabolic activity of plant mitochondria

The effects of salicylic acid (SA) on mitochondrial respiration and generation of membrane potential across the inner membrane of mitochondria isolated from stored taproots of sugar beet (Beta vulgaris L.) and etiolated seedling cotyledons of yellow lupine (Lupinus luteus L.) were studied. When malate was oxidized in the presence of glutamate, low SA concentrations (lower than 1.0 mM) exerted predominantly uncoupling action on the respiration of taproot mitochondria: they activated the rate of oxygen uptake in State 4 (in the absence of ADP) and did not affect oxidation in State 3 (in the presence of ADP). In contrast, in lupine cotyledon mitochondria these SA concentrations inhibited oxygen uptake in the presence of ADP and much weaker activated substrate oxidation in State 4. Thus, SA (0.5 mM) reduced the respiratory control ratio according to Chance (RCR) by 25% in the taproots and 35% in cotyledons. When the concentration of phytohormone was increased (above 1.0 mM), malate oxidation in State 3 was inhibited and in State 4 — activated independently of the plant material used. In this case, the values of RCR and ADP/O were reduced by 50–60%. The effect of high SA concentrations (2 mM and higher) on malate oxidation depended on the duration of incubation and had a biphasic pattern: the initial activation of oxygen uptake was later replaced by its inhibition. The parallel studying the SA effect on the generation of membrane potential (ΔΨ) at malate oxidation in the mitochondria of beet taproots and lupine cotyledons showed that ΔΨ dissipation was observed because of SA uncoupling and inhibiting action on respiration. The degree of ΔΨ dissipation depended on the phytohormone concentration and duration on mitochondria treatment, especially at its high concentrations. In general, a correlation was found between the effects of SA on mitochondrial respiration and ΔΨ values in the coupling membranes. Furthermore, these results show that the responses of mitochondria to SA were determined not only by its concentration but also by treatment duration and evidently by the sensitivity to the phytohormone of mitochondria isolated from different plant tissues.