Suppression of mitochondrial alternative oxidase can result in upregulation of the ROS scavenging network: some possible mechanisms underlying the compensation effect

Mitochondrial alternative oxidase is an important protein involved in maintaining cellular metabolic and energy balance, especially under stress conditions. AOX genes knockout is aimed at revealing the functions of AOX genes. Under unfavourable conditions, AOX-suppressed plants (mainly based on Arabidopsis AOX1a-knockout lines) usually experience strong oxidative stress. However, a compensation effect, which consists of the absence of AOX1a leading to an increase in defence response mechanisms, concomitant with a decrease in ROS content, has also been demonstrated. This review briefly describes the possible mechanisms underlying the compensation effect upon the suppression of AOX1a. Information about mitochondrial retrograde regulation of AOX is given. The importance of ROS and mitochondrial membrane potential in triggering the signal transmission from mitochondria in the absence of AOX or disturbance of mitochondrial electron transport chain functions is indicated. The few available data on the response of the cell to the absence of AOX at the level of changes in the hormonal balance and the reactions of chloroplasts are presented. The decrease in the relative amount of reduced ascorbate at stable ROS levels as a result of compensation in AOX1a-suppressed plants is proposed as a sign of stress development. Obtaining direct evidence on the mechanisms and signalling pathways involved in AOX modulation in the genome should facilitate a deeper understanding of the role of AOX in the integration of cellular signalling pathways.

AOX1a Expression in Arabidopsis thaliana Affects the State of Chloroplast Photoprotective Systems under Moderately High Light Conditions

Alternative oxidase (AOX) in the mitochondrial electron transport chain is considered important for sustaining photosynthesis under high light conditions. Here, we examined the effects of the AOX pathway on the state of chloroplast photoprotective systems. Arabidopsis thaliana plants (4 weeks old), comprising three genotypes (wild type [WT], overexpressing [XX-2] and antisense [AS-12] lines for AOX1a), were exposed to moderately high light conditions (MHL, 400 μmol m^-2 s^-1) in a short-term experiment (8 h). After 8 h of MHL, the WT and XX-2 plants showed stable non-photochemical quenching (qN) and violaxanthin cycle activity. Antisense plants displayed the lowest level of qN and a lower de-epoxidation state (DEPS) relative to plants of the same line after 4-6 h MHL, as well as compared to WT and XX-2 plants after 8 h MHL. The decline in DEPS in AS-12 plants was attributed to an insufficient violaxanthin de-epoxidase activity, which in turn was associated with a decrease in reduced ascorbate levels in the chloroplasts and leaves. Simultaneously, gene expression and the activity of ascorbate peroxidase in the antisense line increased after 8 h of MHL, supporting the compensatory effect of the antioxidant system when AOX1a expression is suppressed. This study emphasizes the role played by AOX in modulating the photoprotection processes and in the maintenance of relationships between mitochondria and chloroplasts by influencing ascorbate content.

Responses of genes of DNA repair, alternative oxidase, and pro-/antioxidant state in Arabidopsis thaliana with altered expression of AOX1a to gamma irradiation

PURPOSE

High doses of gamma (γ) irradiation cause oxidative stress and DNA damage. Alternative oxidase (AOX) catalyzes the energy-dissipating cyanide-resistant alternative pathway in plant mitochondria and is an important part of the cellular defense network under stress conditions. In this study, Arabidopsis thaliana plants with an altered expression of the AOX1a gene were exposed by high dose-rate ionizing radiation to assess the expression of genes of DNA repair and pro-/antioxidant states to elucidate the functional significance of AOX in plant stress response.

MATERIALS AND METHODS

Five-week-old A. thaliana plants, either with basal AOX1a gene expression (wild-type Colombia-0 (Col-0)), antisense silencing of AOX1a (AS-12), and overexpression of the gene (XX-2), were γ-irradiated at a dose of 200 Gy. Gene expression and biochemical analyses were performed 12 h after irradiation.

RESULTS

Acute γ-irradiation caused different responses between the genotypes. XX-2 plants, either control or irradiated, showed the highest expression of AOX1a gene and AOX protein, and the lowest expression of DNA repair genes. Wild type and AS-12 plants exposed to γ-irradiation upregulated another stress-induced gene, AOX1d, and DNA repair genes. Furthermore, a higher activity of Mn-dependent superoxide dismutase (Mn-SOD) was observed in the irradiated AS-12 plants than in the untreated plants of this line. However, AS-12 plants were less effective than Col-0 plants in controlling the accumulation of the superoxide anion. XX-2 plants had the lowest reactive oxygen species (ROS) levels among the genotypes.

CONCLUSIONS

AS-12 plants display a compensatory mechanism by increasing the expression of AOX1d and the synthesis of the AOX protein, as well as by Mn-SOD activation. However, these were insufficient to maintain the background level of embryonic lethal mutations, and thereby the reproductive capacity. These results highlight the importance of AOX in the successful adaptation of plants to acute γ-irradiation, and indicate that AOX1a plays a key role in the regulation of the stress response.

The gene expression profiles of mitochondrial respiratory components in Arabidopsis plants with differing amounts of ALTERNATIVE OXIDASE1a under high intensity light

We compared the expression of mitochondrial alternative oxidase (AOX) and other non-phosphorylating respiratory components (NPhPs) in wild type and AOX1a transgenic Arabidopsis thaliana following short-term transfer of plants to higher irradiance conditions to gain more insight into the mechanisms of AOX functioning under light. The AOX1a overexpressing line (XX-2) showed the highest amount of AOX1a transcripts and AOX1A synthesis during the entire experiment, and many NPhPs genes were down-regulated after 6-8 h under the higher light conditions. Antisense AS-12 plants displayed a compensatory effect, typically after 8 h of exposure to higher irradiance, by up-regulating their expression of the majority of genes encoding AOX and other respiratory components. In addition, AS-12 plants displayed 'overcompensation effects' prior to their transfer to high light conditions, i.e., they showed a higher expression level of certain genes. As a result, the ROS content in AS-12, as in XX-2, was consistently lower than in the wild type. All NPhPs genes share, in common with AOX1a, light- and stress-related cis-acting regulatory elements (CAREs) in their promoters. However, the expression of respiratory genes does not always depend on the level of AOX1a expression. This suggests the presence of multiple combinations of signaling pathways in gene induction. Based on our results, we outline possible directions for future research.

Role of mitochondrial alternative oxidase in the regulation of cellular homeostasis during development of photosynthetic function in greening leaves

Here, recent publications on the role of mitochondrial non-phosphorylating pathways (NPhPs) in the electron transport chain during the de-etiolation of wheat leaves are reviewed. Among NPhPs, the alternative oxidase (AOX) pathway is the most effective pathway in maintaining cellular redox and energy balance, especially under stress conditions, including light stress. AOX is considered to dissipate excess reductants produced in the chloroplasts, and thereby prevent photooxidation. However, when etiolated wheat plants were exposed to a physiologically relevant light level, AOX was rapidly induced and increased, although the etioplasts did not produce excess reductants and have their own strong photoprotective mechanisms. The present study provides further insights into the role of AOX in greening cells and highlights the importance of AOX in the integration of cellular signalling pathways.

Altered levels of AOX1a expression result in changes in metabolic pathways in Arabidopsis thaliana plants acclimated to low dose rates of ultraviolet B radiation

UV-B is a damaging component of solar radiation that inevitably reaches the Earth's surface. Plants have developed response mechanisms to adapt to UVB exposure. The alternative oxidase (AOX) catalyzes the ATP-uncoupling cyanide-resistant alternative pathway (AP) in plant mitochondria and is thought to be an important part of the cellular defense network under stress conditions. This study aimed to unravel the poorly understood functional significance of AOX1a induction in Arabidopsis thaliana leaves exposed to ecologically relevant doses of UVB radiation, by comparing wild-type (WT) plants with plants with modified expression of the AOX1a gene, either downregulated by antisense (AS-12) or overexpressed (XX-2). UVB exposure resulted in a phenotypic difference between lines. AOX1a overexpression resulted in the highest induction of AOX1A synthesis and MnSOD activity, and the lowest ROS level without pronounced changes in the phenotype relative to other genotypes. In AS-12 plants, expression of the majority of the genes encoding AOX was detected, other non-phosphorylating pathway components and antioxidant enzymes increased along with anthocyanin accumulation in leaves, and the ROS content was lower than in the WT. In addition to the expected AOX1 protein size (34 kDa), an AOX1 30 kDa band appeared under UVB exposure in all genotypes. However, in AS-12, the alterations in the transcript level and in the abundance of AOX1 protein isoforms induced by UVB could not fully functionally compensate for the lack of AOX1A. This was confirmed by the observed low AP capacity and increased levels of the oxidized form of ascorbate. These results highlight the importance of AOX in plant response to UVB for the control of a balanced metabolism, and indicate that AOX1a plays a key role in the regulation of the stress response.

Light regulation of mitochondrial alternative oxidase pathway during greening of etiolated wheat seedlings

This study deals with effects of de-etiolation (48h) of spring wheat (Triticum aestivum L., var. Irgina) seedlings on differential expression of AOX1 genes, levels of AOX protein and the alternative respiratory pathway (AP) capacity. As a result of exposure to continuous irradiation of dark-grown wheat seedlings, the respiratory activity and AP capacity in leaves significantly increased during the first 6h of studies. Expression of AOX1a was up-regulated by light and proved consistent with changes in the AP capacity. Effects on expression of AOX1c were less pronounced. Immunoblot analysis showed three distinct bands of AOX with molecular weights of 34, 36 and 38kDa, with no significant changes in the relative levels during de-etiolation. The lack of a clear correlation between AOX protein amount, AOX1a expression, and AP capacity suggests post-translational control of the enzyme activation. The AOX1a suppression and a decrease in the AP capacity correlated with the sugar pool depletion after 24h of the de-etiolation, which may mean a possible substrate dependence of the AOX activity in the green cells. More efficient malate oxidation by mitochondria as well as the higher AOX capacity during the first 6h of de-etiolation was detected, whereas respiration and AOX capacity with exogenous NADH and glycine increased after 6 and 24h, respectively. We conclude that AOX plays an important role during development of an actively photosynthesizing cell, and can rapidly adapt to changes in metabolism and photosynthesis.

[Optimization of the mineral nutrition of plants constituting the phototrophic component of closed biological life support systems]

Applicability of a new substrate for crops cultivation in bioregenerative LSSs with a high degree of mass-exchange closure was tested. Optimization of leaf cabbage nutrition by supplementing the basic substrate fabricated of plant and animal residues with ion-exchange resins proved to have a success.