Although plant cell bioenergetics is strongly affected by abiotic stresses, mitochondrial metabolism under stress is still largely unknown. Interestingly, plant mitochondria may control reactive oxygen species (ROS) generation by means of energy-dissipating systems. Therefore, mitochondria may play a central role in cell adaptation to abiotic stresses, which are known to induce oxidative stress at cellular level. With this in mind, in recent years, studies have been focused on mitochondria from durum wheat, a species well adapted to drought stress. Durum wheat mitochondria possess three energy-dissipating systems: the ATP-sensitive plant mitochondrial potassium channel (PmitoK_ATP); the plant uncoupling protein (PUCP); and the alternative oxidase (AOX). It has been shown that these systems are able to dampen mitochondrial ROS production; surprisingly, PmitoK_ATP and PUCP (but not AOX) are activated by ROS. This was found to occur in mitochondria from both control and hyperosmotic-stressed seedlings. Therefore, the hypothesis of a ‘feed-back’ mechanism operating under hyperosmotic/oxidative stress conditions was validated: stress conditions induce an increase in mitochondrial ROS production; ROS activate PmitoK_ATP and PUCP that, in turn, dissipate the mitochondrial membrane potential, thus inhibiting further large-scale ROS production. Another important aspect is the chloroplast/cytosol/mitochondrion co-operation in green tissues under stress conditions aimed at modulating cell redox homeostasis. Durum wheat mitochondria may act against chloroplast/cytosol over-reduction: the malate/oxaloacetate antiporter and the rotenone-insensitive external NAD(P)H dehydrogenases allow cytosolic NAD(P)H oxidation; under stress this may occur without high ROS production due to co-operation with AOX, which is activated by intermediates of the photorespiratory cycle.

In this study, evidence is given that a number of isolated coupled plant mitochondria (from durum wheat, bread wheat, spelt, rye, barley, potato, and spinach) can take up externally added K⁺ ions. This was observed by following mitochondrial swelling in isotonic KCl solutions and was confirmed by a novel method in which the membrane potential decrease due to externally added K⁺ is measured fluorimetrically by using safranine. A detailed investigation of K⁺ uptake by durum wheat mitochondria shows hyperbolic dependence on the ion concentration and specificity. K⁺ uptake electrogenicity and the non-competitive inhibition due to either ATP or NADH are also shown. In the whole, the experimental findings reported in this paper demonstrate the existence of the mitochondrial K⁺ _ATPchannel in plants (PmitoK_ATP). Interestingly, Mg²⁺ and glyburide, which can inhibit mammalian K⁺ channel, have no effect on PmitoK_ATP. In the presence of the superoxide anion producing system (xanthine plus xanthine oxidase), PmitoK_ATP activation was found. Moreover, an inverse relationship was found between channel activity and mitochondrial superoxide anion formation, as measured via epinephrine photometric assay. These findings strongly suggest that mitochondrial K⁺ uptake could be involved in plant defense mechanism against oxidative stress due to reactive oxygen species generation.

In order to gain a first insight into the alternative oxidase (AO) function in durum wheat mitochondria (DWM), we investigated some activation pathways of this enzyme in DWM purified from both etiolated shoots and green leaves. AO was activated when DWM were added with either pyruvate, known as an AO activator in other plant mitochondria, or alanine plus 2-oxoglutarate, which can generate intramitochondrial pyruvate and glutamate via transamination. In contrast, no AO activity was observed during oxidation of malate plus glutamate or succinate (which can generate malate). In this regard DWM differ from other plant mitochondria. Moreover, DWM were found: (i) to have a very low malic enzyme (ME) activity, (ii) to release oxaloacetate rather than pyruvate during malate oxidation and (iii) to poorly oxidise malate in the absence of glutamate, which removes oxaloacetate via transamination. Therefore, we show that, unlike other plant mitochondria, no pyruvate is generated inside DWM from malate via ME, allowing no AO activity. Other AO activators, alternative to pyruvate, were checked by evaluating the capability of several compounds to induce oxygen uptake and/or electrical membrane potential (ΔΨ) in cyanide-treated DWM. Hydroxypyruvate and glyoxylate, photorespiratory cycle intermediates, were found to be powerful AO activators, capable of inducing a maximal rate of cyanide-insensitive oxygen uptake 1.7 times and 2.3 times higher than pyruvate, respectively. These results suggest that in durum wheat a link may exist between AO activity and photorespiratory metabolism rather than malate metabolism. Moreover, we observed that AO activation resulted in both a partially coupled respiration and a reduction by half of the rate of superoxide anion generation; therefore, AO is expected to work as an antioxidative defence system when the photorespiratory cycle is highly active, as under environmental stress.