Light intensity affects chlorophyll synthesis during greening process by metabolite signal from mitochondrial alternative oxidase in Arabidopsis

Hyperspectral and Chlorophyll Fluorescence Analyses of Comparative Leaf Surfaces Reveal Cellular Influences on Leaf Optical Properties in Tradescantia Plants

The differential effects of cellular and ultrastructural characteristics on the optical properties of adaxial and abaxial leaf surfaces in the genus Tradescantia highlight the intricate relationships between cellular arrangement and pigment distribution in the plant cells. We examined hyperspectral and chlorophyll a fluorescence (ChlF) kinetics using spectroradiometers and optical and electron microscopy techniques. The leaves were analysed for their spectral properties and cellular makeup. The biochemical compounds were measured and correlated with the biophysical and ultrastructural features. The main findings showed that the top and bottom leaf surfaces had different amounts and patterns of pigments, especially anthocyanins, flavonoids, total phenolics, chlorophyll-carotenoids, and cell and organelle structures, as revealed by the hyperspectral vegetation index (HVI). These differences were further elucidated by the correlation coefficients, which influence the optical signatures of the leaves. Additionally, ChlF kinetics varied between leaf surfaces, correlating with VIS-NIR-SWIR bands through distinct cellular structures and pigment concentrations in the hypodermis cells. We confirmed that the unique optical properties of each leaf surface arise not only from pigmentation but also from complex cellular arrangements and structural adaptations. Some of the factors that affect how leaves reflect light are the arrangement of chloroplasts, thylakoid membranes, vacuoles, and the relative size of the cells themselves. These findings improve our knowledge of the biophysical and biochemical reasons for leaf optical diversity, and indicate possible implications for photosynthetic efficiency and stress adaptation under different environmental conditions in the mesophyll cells of Tradescantia plants.

The Arabidopsis leaf quantitative atlas: a cellular and subcellular mapping through unified data integration

Quantitative analyses and models are required to connect a plant's cellular organisation with its metabolism. However, quantitative data are often scattered over multiple studies, and finding such data and converting them into useful information is time-consuming. Consequently, there is a need to centralise the available data and to highlight the remaining knowledge gaps. Here, we present a step-by-step approach to manually extract quantitative data from various information sources, and to unify the data format. First, data from Arabidopsis leaf were collated, checked for consistency and correctness and curated by cross-checking sources. Second, quantitative data were combined by applying calculation rules. They were then integrated into a unique comprehensive, referenced, modifiable and reusable data compendium representing an Arabidopsis reference leaf. This atlas contains the metrics of the 15 cell types found in leaves at the cellular and subcellular levels.

Evaluation of secondary sexual dimorphism of the dioecious Amaranthus palmeri under abiotic stress

The evolution of secondary sex-specific traits of dioecious species under abiotic stress conditions has received limited research, especially in the case of Amaranthus palmeri, a fast adapting and highly competing plant. Here, we have examined the interactive effects of abiotic stress on mineral accumulation, chlorophyll a and b content, and the operating capacity of Photosystem II (PSII) in both male and female A. palmeri plants grown under three different intensities of white light, and under N, K or P deficiency. Mineral profiling of the leaves and stems (with inflorescence) highlighted intra- and intersexual differences in their accumulation pattern and mineral associations. Chlorophyll a and chlorophyll b were different between the male and the female plants, being slightly lower in the latter, at high light intensity towards maturity, or under K or P deficiency. Further, slight, although statistically significant differences were recorded in the chlorophyll a/b ratio, which was lower at the higher light intensity in the female, over that in the male, plants towards maturity. Chlorophyll fluorescence parameters, i.e., steady state and maximum fluorescence increased under high light intensity, whereas the PSII operating efficiency decreased in the female plants, indicating reduced PSII capacity. Sex-specific differences in A. palmeri showed a differential response to stressful conditions because of differences in their ontogeny and physiology, and possibly due to the cost of reproduction. We suggest that the breeding system of dioecious species has weaknesses that can be used for the ecological management of dioecious weeds without relying on the use of herbicides.

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.

Transcriptomic and metabolomic analysis of autumn leaf color change in Fraxinus angustifolia

Fraxinus angustifolia is a type of street tree and shade tree with ornamental value. It has a beautiful shape and yellow or reddish purple autumn leaves, but its leaf color formation mechanism and molecular regulation network need to be studied. In this study, we integrated the metabolomes and transcriptomes of stage 1 (green leaf) and stage 2 (red-purple leaf) leaves at two different developmental stages to screen differential candidate genes and metabolites related to leaf color variation. The results of stage 1 and stage 2 transcriptome analysis showed that a total of 5,827 genes were differentially expressed, including 2,249 upregulated genes and 3,578 downregulated genes. Through functional enrichment analysis of differentially expressed genes, we found that they were involved in flavonoid biosynthesis, phenylpropanoid biosynthesis, pigment metabolism, carotene metabolism, terpenoid biosynthesis, secondary metabolite biosynthesis, pigment accumulation, and other biological processes. By measuring the metabolites of Fraxinus angustifolia leaves, we found the metabolites closely related to the differentially expressed genes in two different periods of Fraxinus angustifolia, among which flavonoid compounds were the main differential metabolites. Through transcriptome and metabolomics data association analysis, we screened nine differentially expressed genes related to anthocyanins. Transcriptome and qRT-PCR results showed that these nine genes showed significant expression differences in different stages of the sample, and we speculate that they are likely to be the main regulatory factors in the molecular mechanism of leaf coloration. This is the first time that we have analyzed the transcriptome combination metabolome in the process of leaf coloration of Fraxinus angustifolia, which has important guiding significance for directional breeding of colored-leaf Fraxinus species and will also give new insights for enriching the landscape.

Mitochondrial alternative oxidase enhanced ABA-mediated drought tolerance in Solanum lycopersicum

The phytohormone abscisic acid (ABA) plays essential roles in modulating drought stress responses. Mitochondrial alternative oxidase (AOX) is critical for reactive oxygen species (ROS) scavenging in drought stress responses. However, whether ABA signal in concert with AOX to moderate drought stress response remains largely unclear. In our study, we uncover the positive role of AOX in ABA-mediated drought tolerance in tomato (Solanum lycopersicum). Here, we report that ABA participates in the regulation of alternative respiration, and the increased AOX was found to improve drought tolerance by reducing total ROS accumulation. We also found that transcription factor ABA response element-binding factor 1 (SlAREB1) can directly bind to the promoter of AOX1a to activate its transcription. Virus-induced gene silencing (VIGS) of SlAREB1 compromised the ABA-induced alternative respiratory pathway, disrupted redox homeostasis and decreased plant resistance to drought stress, while overexpression of AOX1a in TRV2-SlAREB1 plants partially rescued the severe drought phenotype. Taken together, our results indicated that AOX1a plays an essential role in ABA-mediated drought tolerance partially in a SlAREB1-dependent manner, providing new insights into how ABA modulates ROS levels to cope with drought stress by AOX.

Maize (Zea mays L.) responses to salt stress in terms of root anatomy, respiration and antioxidative enzyme activity

BACKGROUND

Soil salt stress is a problem in the world, which turns into one of the main limiting factors hindering maize production. Salinity significantly affects root physiological processes in maize plants. There are few studies, however, that analyses the response of maize to salt stress in terms of the development of root anatomy and respiration.

RESULTS

We found that the leaf relative water content, photosynthetic characteristics, and catalase activity exhibited a significantly decrease of salt stress treatments. However, salt stress treatments caused the superoxide dismutase activity, peroxidase activity, malondialdehyde content, Na⁺ uptake and translocation rate to be higher than that of control treatments. The detrimental effect of salt stress on YY7 variety was more pronounced than that of JNY658. Under salt stress, the number of root cortical aerenchyma in salt-tolerant JNY658 plants was significantly higher than that of control, as well as a larger cortical cell size and a lower root cortical cell file number, all of which help to maintain higher biomass. The total respiration rate of two varieties exposed to salt stress was lower than that of control treatment, while the alternate oxidative respiration rate was higher, and the root response of JNY658 plants was significant. Under salt stress, the roots net Na⁺ and K⁺ efflux rates of two varieties were higher than those of the control treatment, where the strength of net Na⁺ efflux rate from the roots of JNY658 plants and the net K⁺ efflux rate from roots of YY7 plants was remarkable. The increase in efflux rates reduced the Na⁺ toxicity of the root and helped to maintain its ion balance.

CONCLUSION

These results demonstrated that salt-tolerant maize varieties incur a relatively low metabolic cost required to establish a higher root cortical aerenchyma, larger cortical cell size and lower root cortical cell file number, significantly reduced the total respiration rate, and that it also increased the alternate oxidative respiration rate, thereby counteracting the detrimental effect of oxidative damage on root respiration of root growth. In addition, Na⁺ uptake on the root surface decreased, the translocation of Na⁺ to the rest of the plant was constrained and the level of Na⁺ accumulation in leaves significantly reduced under salt stress, thus preempting salt-stress induced impediments to the formation of shoot biomass.

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.

Beneficial effects of gamma-irradiation of quinoa seeds on germination and growth

Quinoa is one of the crops well-adapted to high altitude regions that can grow relatively well under drought, humid, and high UV radiation conditions. This study was performed to investigate the effects of gamma-radiation on quinoa. Seeds were treated with various doses of 50 Gy, 100 Gy, 200 Gy, 300 Gy, 400 Gy, 600 Gy, 800 Gy, and 1000 Gy. We investigated germination, as well as plant height, chlorophyll content, and normalized difference vegetation index (NDVI) at 0, 30, 44, 58, and 88 days after transplanting (DAT) and panicle weight at 88 DAT. The plants grown from the seeds treated at radiation doses greater than 200 Gy showed reduced values in most growth and physiological characteristics. The germination rate and germination speed were higher in the 50 Gy-treated seeds than in 0 Gy-treated (control) seeds. Plant height and panicle weight were highest in the plants from 50 Gy-treated seeds. Chlorophyll content was higher in all treated samples than in the controls. NDVI value showed the highest value in 0 Gy controls and plants treated with 50 Gy. The antioxidant activity was also higher in the plants from the seeds treated with 50 Gy and 100 Gy, showing a steady increase as the radiation dose increased even at 200 Gy. The plants from seeds treated with 0 Gy showed higher expression of proteins related to photorespiration and tubulin chains. The plants from seeds treated with 50 Gy induced more stress-responsive proteins.

Performance of Red Mud/Biochar Composite Material (RMBC) as Heavy Metal Passivator in Pb-Contaminated Soil

Red mud/biochar composite material (RMBC), which was applied as heavy metal passivator in this research, was prepared with red mud (the bauxite residue) and cornstalk under anoxic sintering condition. Based on the batch experiments in Pb contaminated soil, the passivating properties of several materials, including red mud (RM), biochar (BC), RMBC and phosphate-containing RMBC (PRMBC), were investigated in comparison with each other. Some interesting results are as follows: through anoxic thermal activation, a rough and porous structure of RMBC was obtained. Substances such as Fe₃O₄ and metal-organic complexes generated in RMBC provided effective sites for Pb passivation; and the mechanisms were speculated as the precipitation between Pb²⁺ and the carbonate (or hydroxide), as well as the complexation reaction between Pb and metal organic complexes through ligand bonding. The pot experiments showed the promotion effects of four passivators on the growth of red onion were in the following order: PRMBC > RMBC > BC > RM. PRMBC stabilized Pb content in soil significantly due to the formation of insoluble substances, with the minimum transfer factor and bioconcentration factor for plant growth. The evidences above implied the composite materials (PRMBC and RMBC) would be potential passivators for heavy metal-contaminated soil.

The Lack of Alternative Oxidase 1a Restricts in vivo Respiratory Activity and Stress-Related Metabolism for Leaf Osmoprotection and Redox Balancing Under Sudden Acute Water and Salt Stress in Arabidopsis thaliana

In plants salt and water stress result in an induction of respiration and accumulation of stress-related metabolites (SRMs) with osmoregulation and osmoprotection functions that benefit photosynthesis. The synthesis of SRMs may depend on an active respiratory metabolism, which can be restricted under stress by the inhibition of the cytochrome oxidase pathway (COP), thus causing an increase in the reduction level of the ubiquinone pool. However, the activity of the alternative oxidase pathway (AOP) is thought to prevent this from occurring while at the same time, dissipates excess of reducing power from the chloroplast and thereby improves photosynthetic performance. The present research is based on the hypothesis that the accumulation of SRMs under osmotic stress will be affected by changes in folial AOP activity. To test this, the oxygen isotope-fractionation technique was used to study the in vivo respiratory activities of COP and AOP in leaves of wild-type Arabidopsis thaliana plants and of aox1a mutants under sudden acute stress conditions induced by mannitol and salt treatments. Levels of leaf primary metabolites and transcripts of respiratory-related proteins were also determined in parallel to photosynthetic analyses. The lack of in vivo AOP response in the aox1a mutants coincided with a lower leaf relative water content and a decreased accumulation of crucial osmoregulators. Additionally, levels of oxidative stress-related metabolites and transcripts encoding alternative respiratory components were increased. Coordinated changes in metabolite levels, respiratory activities and photosynthetic performance highlight the contribution of the AOP in providing flexibility to carbon metabolism for the accumulation of SRMs.

Review: the effect of light on the key pigment compounds of photosensitive etiolated tea plant

BACKGROUND

Light is the ultimate energy source of plant photosynthesis, which has an important impact on the growth, development, physiology and biochemistry of tea plant. Photosensitive etiolated tea plant belongs to a kind of colored leaf plant, which is a physiological response to light intensity. Compared with conventional green bud and leaf of tea plant, the accumulation of pigment compounds (chlorophyll and carotenoids, etc.) closely related to a series of reactions of photosynthesis in photosensitive etiolated tea plant is reduced, resulting in the difference of leaf color of tea. This specific tea resource has high application value, among which high amino acid is one of its advantages. It can be used to process high-quality green tea with delicious taste and attractive aroma, which has been widely attention. The mechanism of the color presentation of the etiolated mutant tea leaves has been given a high topic and attention, especially, what changes have taken place in the pigment compounds of tea leaves caused by light, which makes the leaves so yellow. At present, there have been a lot of research and reports.

PURPOSE OF THE REVIEW

We describe the metabolism and differential accumulation of key pigment compounds affecting the leaf color of photosensitive etiolated tea that are triggered by light, and discuss the different metabolism and key regulatory sites of these pigments in different light environments in order to understand the "discoloration" matrix and mechanism of etiolated tea resources, answer the scientific question between leaf color and light. It provides an important strategy for artificial intervention of discoloration of colored tea plant.

CONCLUSION

The differential accumulation of pigment compounds in tea plant can be induced phytochrome in response to the change of light signal. The synthesis of chlorophyll in photoetiolated tea plants is hindered by strong light, among which, the sites regulated by coproporphyrinogen III oxidase and chlorophyllide a oxidase is sensitive to light and can be inhibited by strong light, resulting in the aggravation of leaf etiolation. The phenomenon can be disappeared or weakened by shading or reducing light intensity, and the leaf color is greenish, but the increase of chlorophyll-b accumulation is more than that of chlorophyll-a. The synthesis of carotenoids is inhibited strong light, and high the accumulation of carotenoids is reduced by shading. Most of the genes regulating carotenoids are up-regulated by moderate shading and down-regulated by excessive shading. Therefore, the accumulation of these two types of pigments in photosensitive etiolated tea plants is closely related to the light environment, and the leaf color phenotype shape of photosensitive etiolated tea plants can be changed by different light conditions, which provides an important strategy for the production and management of tea plant.

Comparison of the Kinetic Parameters of Alternative Oxidases From Trypanosoma brucei and Arabidopsis thaliana-A Tale of Two Cavities

The alternative oxidase (AOX) is widespread in plants, fungi, and some protozoa. While the general structure of the AOX remains consistent, its overall activity, sources of kinetic activation and their sensitivity to inhibitors varies between species. In this study, the recombinant Trypanosoma brucei AOX (rTAO) and Arabidopsis thaliana AOX1A (rAtAOX1A) were expressed in the Escherichia coli ΔhemA mutant FN102, and the kinetic parameters of purified AOXs were compared. Results showed that rTAO possessed the highest V _max and K _m for quinol-1, while much lower V _max and K _m were observed in the rAtAOX1A. The catalytic efficiency (k _cat/K _m) of rTAO was higher than that of rAtAOX1A. The rTAO also displayed a higher oxygen affinity compared to rAtAOX1A. It should be noted that rAtAOX1a was sensitive to α-keto acids while rTAO was not. Nevertheless, only pyruvate and glyoxylate can fully activate Arabidopsis AOX. In addition, rTAO and rAtAOX1A showed different sensitivity to AOX inhibitors, with ascofuranone (AF) being the best inhibitor against rTAO, while colletochlorin B (CB) appeared to be the most effective inhibitor against rAtAOX1A. Octylgallate (OG) and salicylhydroxamic acid (SHAM) are less effective than the other inhibitors against protist and plant AOX. A Caver analysis indicated that the rTAO and rAtAOX1A differ with respect to the mixture of polar residues lining the hydrophobic cavity, which may account for the observed difference in kinetic and inhibitor sensitivities. The data obtained in this study are not only beneficial for our understanding of the variation in the kinetics of AOX within protozoa and plants but also contribute to the guidance for the future development of phytopathogenic fungicides.

Functional Relationship of Arabidopsis AOXs and PTOX Revealed via Transgenic Analysis

Alternative oxidase (AOX) and plastid terminal oxidase (PTOX) are terminal oxidases of electron transfer in mitochondria and chloroplasts, respectively. Here, taking advantage of the variegation phenotype of the Arabidopsis PTOX deficient mutant (im), we examined the functional relationship between PTOX and its five distantly related homologs (AOX1a, 1b, 1c, 1d, and AOX2). When engineered into chloroplasts, AOX1b, 1c, 1d, and AOX2 rescued the im defect, while AOX1a partially suppressed the mutant phenotype, indicating that AOXs could function as PQH₂ oxidases. When the full length AOXs were overexpressed in im, only AOX1b and AOX2 rescued its variegation phenotype. In vivo fluorescence analysis of GFP-tagged AOXs and subcellular fractionation assays showed that AOX1b and AOX2 could partially enter chloroplasts while AOX1c and AOX1d were exclusively present in mitochondria. Surprisingly, the subcellular fractionation, but not the fluorescence analysis of GFP-tagged AOX1a, revealed that a small portion of AOX1a could sort into chloroplasts. We further fused and expressed the targeting peptides of AOXs with the mature form of PTOX in im individually; and found that targeting peptides of AOX1a, AOX1b, and AOX2, but not that of AOX1c or AOX1d, could direct PTOX into chloroplasts. It demonstrated that chloroplast-localized AOXs, but not mitochondria-localized AOXs, can functionally compensate for the PTOX deficiency in chloroplasts, providing a direct evidence for the functional relevance of AOX and PTOX, shedding light on the interaction between mitochondria and chloroplasts and the complex mechanisms of protein dual targeting in plant cells.

Overexpression of Cerasus humilis ChAOX2 improves the tolerance of Arabidopsis to salt stress

Alternative oxidase (AOX) has a well-established involvement in plant growth and stress tolerance in many studies. In this study, we isolated and characterized the AOX2 from Cerasus humilis. The ChAOX2 Open Reading Frame (ORF) contains 1029 nucleotides and encodes 342 amino acid residues. The inferred amino acid sequence of ChAOX2 shared the highest sequence similarity with a homolog from Prunus yedoensis. The ChAOX2 transcripts were relatively abundant in the old leaves and significantly up-regulated by salt stress. Subcellular localization analysis showed that ChAOX2 was located in the mitochondria. We transformed ChAOX2 into wild-type Arabidopsis thaliana and found that compared with wild-type and aox mutant lines, heterotopic expression of ChAOX2 increased proline content, and peroxidase and superoxide dismutase activities, while decreasing relative conductivity and the reactive oxygen species level. Further, the ratio of alternate respiration to the total respiration in plants that overexpressed ChAOX2 was significantly higher than that in wild-type and mutant plants under salt stress. These results indicate that ChAOX2 plays a key role in salt tolerance.

CmNAC73 Mediates the Formation of Green Color in Chrysanthemum Flowers by Directly Activating the Expression of Chlorophyll Biosynthesis Genes HEMA1 and CRD1

Chrysanthemum is one of the most beautiful and popular flowers in the world, and the flower color is an important ornamental trait of chrysanthemum. Compared with other flower colors, green flowers are relatively rare. The formation of green flower color is attributed to the accumulation of chlorophyll; however, the regulatory mechanism of chlorophyll metabolism in chrysanthemum with green flowers remains largely unknown. In this study, we performed Illumina RNA sequencing on three chrysanthemum materials, Chrysanthemum vestitum and Chrysanthemum morifolium cultivars ‘Chunxiao’ and ‘Green anna’, which produce white, light green and dark green flowers, respectively. Based on the results of comparative transcriptome analysis, a gene encoding a novel NAC family transcription factor, CmNAC73, was found to be highly correlated to chlorophyll accumulation in the outer whorl of ray florets in chrysanthemum. The results of transient overexpression in chrysanthemum leaves showed that CmNAC73 acts as a positive regulator of chlorophyll biosynthesis. Furthermore, transactivation and yeast one-hybrid assays indicated that CmNAC73 directly binds to the promoters of chlorophyll synthesis-related genes HEMA1 and CRD1. Thus, this study uncovers the transcriptional regulation of chlorophyll synthesis-related genes HEMA1 and CRD1 by CmNAC73 and provides new insights into the development of green flower color in chrysanthemum and chlorophyll metabolism in plants.

Red mud based passivator reduced Cd accumulation in edible amaranth by influencing root organic matter metabolism and soil aggregate distribution

Red mud was a highly alkaline hazardous waste, and their resource utilization was a research hotspot. In this study, influencing mechanisms of red mud based passivator on the transformation of Cd fraction in acidic Cd-polluted soil, photosynthetic property, and Cd accumulation in edible amaranth were investigated based on the evaluation of Cd adsorption capacity, root metabolic response, and soil aggregate distribution. Results showed that red mud exhibited good Cd adsorption capacities at about 35 °C and pH 9 in an aqueous solution, and the adsorption behavior of red mud on Cd in rhizosphere soil solution was considered to have some similarity. In the soil-pot trial, red mud application significantly facilitated edible amaranth growth by enhancing the maximum photochemical efficiency and light energy absorption by per unit leaf area by activating more reaction centers. The main mechanisms of rhizosphere soil Cd immobilisation by red mud application included: i) the reduction of mobilized Cd caused by the increasing negative surface charge of soil and precipitation of Cd hydroxides and carbonates at high pH; ii) the increase of organics-Cd complexes caused by the increasing -OH and -COOH amounts adsorbed on the surface of rhizosphere soil after red mud application; and iii) the decrease of available Cd content in soil aggregates caused by the increasing organic matters after red mud application. This study would provide the basis for the safe utilization of red mud remediating acidic Cd-polluted soil.

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.

Comparative transcriptome analysis reveals the regulatory effects of acetylcholine on salt tolerance of Nicotiana benthamiana

Salinity is a major cause of crop losses worldwide. Acetylcholine (ACh) can ameliorate the adverse effects of abiotic stresses on plant growth, including salinity stress; however, the underlying molecular mechanisms of this process are unclear. Here, seedlings of Nicotiana benthamiana grown under normal conditions or exposed to 150 mmol L-1 NaCl salinity stress were then treated with a root application of 10 μM ACh. Exogenous ACh application resulted in the downregulation of the activity of the antioxidant enzymes, ascorbate peroxidase, and catalase. ACh-treated plants had lower levels of reactive oxygen species, including the superoxide anion radical and hydrogen peroxide. Transcriptome analysis indicated that ACh treatment under salt stress promoted the differential expression of 658 genes in leaves of N. benthamiana (527 were upregulated and 131 were downregulated). Gene ontology enrichment and Kyoto Encyclopedia of Genes and Genomes pathway analyses revealed that exogenous ACh application was associated with a substantial increase in the transcripts of genes related to cell wall peroxidases, xyloglucan endotransglucosylases or hydrolases, and expansins, indicating that ACh activates cell wall biosynthesis in salt-stressed plants. ACh also enhanced the expression of genes associated with the auxin, gibberellin, brassinosteroid, and salicylic acid signalling pathways, indicating that ACh induces the activation of these pathways under salt stress. Collectively, these findings indicate that ACh-induced salt tolerance in N. benthamiana seedlings is mediated by the inhibition of antioxidant enzymes, activation of cell wall biosynthesis, and hormone signalling pathways. Stress-induced genes involved in osmotic regulation and oxidation resistance were induced by ACh under salt stress. The genes whose transcript levels were elevated by ACh treatment in salt-stressed N. benthamiana could be used as molecular markers of the physiological status of plants under salt stress.

Photoprotection by mitochondrial alternative pathway is enhanced at heat but disabled at chilling

The mitochondrial alternative pathway (AP) represents an important photoprotective mechanism for the chloroplast, but the temperature sensitivity of its photoprotective role is unknown. In this study, using the aox1a Arabidopsis mutant, the photoprotective role of the AP was verified under various temperatures, and the mechanism underlying the temperature sensitivity of the AP's photoprotective role was clarified. It was observed that the photoprotective role of the AP increased with rising temperature but was absent at low temperature. The photoprotective role of the AP was severely reduced under non-photorespiratory conditions. Disturbance of the AP inhibited the conversion of glycine to serine in mitochondria, which may restrain upstream photorespiratory metabolism and aggravate photoinhibition. With rising temperatures, photorespiration accelerated and the restraint of photorespiration caused by disturbance of the AP also increased, determining the temperature sensitivity of the AP's photoprotective role. We also verified that not only the AP but also the cytochrome pathway in mitochondria contributes to photoprotection by maintaining photorespiration.

The alternative oxidase pathway is involved in optimizing photosynthesis in Medicago truncatula infected by Fusarium oxysporum and Rhizoctonia solani

Phytopathogen infection alters primary metabolism status and plant development. The alternative oxidase (AOX) has been hypothesized to increase under pathogen attack preventing reductions, thus optimizing photosynthesis and growth. In this study, two genotypes of Medicago truncatula, one relatively resistant (Jemalong A17) and one susceptible (TN1.11), were infected with Fusarium oxysporum and Rhizoctonia solani. The in vivo foliar respiratory activities of the cytochrome oxidase pathway (COP) and the alternative oxidase pathway (AOP) were measured using the oxygen isotope fractionation. Gas exchange and photosynthesis-related parameters were measured and calculated together with antioxidant enzymes activities and organic acids contents. Our results show that the in vivo activity of AOX (v_alt ) plays a role under fungal infection. When infected with R. solani, the increase of v_alt in A17 was concomitant to an increase in net assimilation, in mesophyll conductance, to an improvement in the maximum velocity of Rubisco carboxylation and to unchanged malate content. However, under F. oxysporum infection, the induced v_alt was accompanied by an enhancement in the antioxidant enzymes, superoxide dismutase (SOD; EC1.15.1.1), catalase (CAT; EC1.11.1.6) and guaiacol peroxidase (GPX; EC1.11.1.7), activities and to an unchanged tricarboxylic acid cycle intermediates. These results provide new insight into the role of the in vivo activity of AOX in coordinating primary metabolism interactions that, partly, modulate the relative resistance of M. truncatula to diseases caused by soil-borne pathogenic fungi.

Distinct roles of alternative oxidase pathway during the greening process of etiolated algae

The vital function of mitochondrial alternative oxidase (AOX) pathway in optimizing photosynthesis during plant de-etiolation has been well recognized. However, whether and how AOX impacts the chloroplast biogenesis in algal cells remains unclear. In the present study, the role of AOX in regulating the reassembly of chloroplast in algal cells was investigated by treating Auxenochlorella protothecoides with salicylhydroxamic acid (SHAM), the specific inhibitor to AOX, in the heterotrophy to autotrophy transition process. Several lines of evidences including delayed chlorophyll accumulation, lagged reorganization of chloroplast structure, altered PSI/PSII stoichiometry, and declined photosynthetic activities in SHAM treated cells indicated that the impairment in AOX activity dramatically hindered the development of functioning chloroplast in algal cells. Besides, the cellular ROS levels and activities of antioxidant enzymes were increased by SHAM treatment, and the perturbation on the balance of NAD⁺/NADH and NADP⁺/NADPH ratios was also observed in A. protothecoides lacking AOX activity, indicating that AOX was essential in promoting ROS scavenging and keeping the redox homeostasis for algal chloroplast development during greening. Overall, our study revealed the essentiality of mitochondrial AOX pathway in sustaining algal photosynthetic performance and provided novel insights into the physiological roles of AOX on the biogenesis of photosynthetic organelle in algae.

Response of young Nerium oleander plants to long-term non-ionizing radiation

Although exposure to low frequency electromagnetic radiation is harmful to plants, LF-EM irradiated Nerium oleander seedlings exhibited enhanced development and growth, probably taking advantage of defined structural leaf deformations. Currently, evidence supports the undesirable, often destructive impact of low frequency electromagnetic (LF-EM) radiation on plants. The response of plants to LF-EM radiation often entails induction in the biosynthesis of secondary metabolites, a subject matter that is well documented. Nerium oleander is a Mediterranean plant species, which evolved remarkable resistance to various environmental stress conditions. In the current investigation, cultivated N. oleander plants, following their long-term exposure to LF-EM radiation, exhibited major structural modifications as the flattening of crypts, the elimination of trichomes and the reduction of the layers of the epidermal cells. These changes co-existed with an oxidative stress response manifested by a significant increase in reactive oxygen species at both the roots and the above ground parts, a decline in the absorbance of light by photosynthetic pigments and the substantially increased biosynthesis of L-Dopa decarboxylase (DDC), an enzyme catalyzing the production of secondary metabolites that alleviate stress. The exposed plants exhibited greater primary plant productivity, despite a manifested photosynthetic pigment limitation and the severe oxidative stress. This unique response of N. oleander to severe abiotic stress conditions may be owed to the advantage offered by a structural change consistent to an easier diffusion of CO₂ within the leaves. A major plant response to an emerging "pollutant" was documented.

Overexpressed β-cyanoalanine synthase functions with alternative oxidase to improve tobacco resistance to salt stress by alleviating oxidative damage

β-Cyanoalanine synthase (β-CAS) is an enzyme involved in cyanide detoxification. However, little information is available regarding the effects of β-CAS activity changes on plant resistance to environmental stress. Here, we found that β-CAS overexpression (CAS-OE) improves the resistance of tobacco plants to salt stress, whereas plants with β-CAS silencing suffer more oxidative damage than wild-type plants. Notably, blocking respiration by the alternative oxidase (AOX) pathway significantly aggravates stress injury and impairs the salt stress tolerance mediated by CAS-OE. These findings present novel insights into the synergistic effect between β-CAS and AOX in protecting plants from salt stress, where β-CAS plays a vital role in restraining cyanide accumulation, and AOX helps to alleviate the toxic effect of cyanide.

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.

RNASeq analysis of giant cane reveals the leaf transcriptome dynamics under long-term salt stress

BACKGROUND

To compensate for the lack of information about the molecular mechanism involved in Arundo donax L. response to salt stress, we de novo sequenced, assembled and analyzed the A. donax leaf transcriptome subjected to two levels of long-term salt stress (namely, S3 severe and S4 extreme).

RESULTS

The picture that emerges from the identification of differentially expressed genes is consistent with a salt dose-dependent response. Hence, a deeper re-programming of the gene expression occurs in those plants grew at extreme salt level than in those subjected to severe salt stress, probably representing for them an "emergency" state. In particular, we analyzed clusters related to salt sensory and signaling, transcription factors, hormone regulation, Reactive Oxygen Species (ROS) scavenging, osmolyte biosynthesis and biomass production, all of them showing different regulation either versus untreated plants or between the two treatments. Importantly, the photosynthesis is strongly impaired in samples treated with both levels of salinity stress. However, in extreme salt conditions, a dramatic switch from C3 Calvin cycle to C4 photosynthesis is likely to occur, this probably being the more impressive finding of our work.

CONCLUSIONS

Considered the distinct response to salt doses, genes either involved in severe or in extreme salt response could constitute useful markers of the physiological status of A. donax to deepen our understanding of its biology and productivity in salinized soil. Finally, many of the unigenes identified in the present study have the potential to be used for the development of A. donax varieties with improved productivity and stress tolerance, in particular the knock out of the GTL1 gene acting as negative regulator of water use efficiency has been proposed as good target for genome editing.

A Functional Alternative Oxidase Modulates Plant Salt Tolerance in Physcomitrella patens

Alternative oxidase (AOX) has been reported to be involved in mitochondrial function and redox homeostasis, thus playing an essential role in plant growth as well as stress responses. However, its biological functions in nonseed plants have not been well characterized. Here, we report that AOX participates in plant salt tolerance regulation in moss Physcomitrella patens (P. patens). AOX is highly conserved and localizes to mitochondria in P. patens. We observed that PpAOX rescued the impaired cyanide (CN)-resistant alternative (Alt) respiratory pathway in Arabidopsis thaliana (Arabidopsis) aox1a mutant. PpAOX transcription and Alt respiration were induced upon salt stress in P. patens. Using homologous recombination, we generated PpAOX-overexpressing lines (PpAOX OX). PpAOX OX plants exhibited higher Alt respiration and lower total reactive oxygen species accumulation under salt stress condition. Strikingly, we observed that PpAOX OX plants displayed decreased salt tolerance. Overexpression of PpAOX disturbed redox homeostasis in chloroplasts. Meanwhile, chloroplast structure was adversely affected in PpAOX OX plants in contrast to wild-type (WT) P. patens. We found that photosynthetic activity in PpAOX OX plants was also lower compared with that in WT. Together, our work revealed that AOX participates in plant salt tolerance in P. patens and there is a functional link between mitochondria and chloroplast under challenging conditions.

Redox Homeostasis in Photosynthetic Organisms: Novel and Established Thiol-Based Molecular Mechanisms

Significance: Redox homeostasis consists of an intricate network of reactions in which reactive molecular species, redox modifications, and redox proteins act in concert to allow both physiological responses and adaptation to stress conditions. Recent Advances: This review highlights established and novel thiol-based regulatory pathways underlying the functional facets and significance of redox biology in photosynthetic organisms. In the last decades, the field of redox regulation has largely expanded and this work is aimed at giving the right credit to the importance of thiol-based regulatory and signaling mechanisms in plants. Critical Issues: This cannot be all-encompassing, but is intended to provide a comprehensive overview on the structural/molecular mechanisms governing the most relevant thiol switching modifications with emphasis on the large genetic and functional diversity of redox controllers (i.e., redoxins). We also summarize the different proteomic-based approaches aimed at investigating the dynamics of redox modifications and the recent evidence that extends the possibility to monitor the cellular redox state in vivo. The physiological relevance of redox transitions is discussed based on reverse genetic studies confirming the importance of redox homeostasis in plant growth, development, and stress responses. Future Directions: In conclusion, we can firmly assume that redox biology has acquired an established significance that virtually infiltrates all aspects of plant physiology.

Alternative respiration pathway is involved in the response of highland barley to salt stress

Alternative respiration pathway is involved in the response of highland barley to salt stress. The response of two barley seedlings to salt stress was investigated. Results showed that the growth of highland barley (Kunlun 14) and barley (Ganpi 6) had no obvious difference under low concentrations (50, 100 and 200 mM) of NaCl treatment. However, high concentrations of NaCl treatment (300 and 400 mM) severely affected the growth of two barley cultivars. Under 300 mM NaCl treatment, the fresh weight, relative water content (RWC), pigments and K⁺ content reduced more in Ganpi 6 than in Kunlun 14. In contrast, the electrolyte leakage and the content of MDA, Na⁺, H₂O₂ and O₂^- increased more in Ganpi 6 than in Kunlun 14. The gene expression of AOX1a, HvNHX1, HvNHX3, HvHVP1, HvHVA, H⁺-ATPase, the alternative respiration capacity (V_alt) and the enzymatic activity of SOD, POD, CAT, APX and H⁺-ATPase increased more in Kunlun14 than in Ganpi6 under 300 mM NaCl treatment, whereas the cytochrome respiration capacity (V_cyt) decreased similarly in both barley cultivars. Western blot analysis showed that the protein level of the alternative oxidase (AOX) increased more in Kunlun 14 than in Ganpi 6 under 300 mM NaCl treatment. Inhibition of the alternative respiration by salicylhydroxamic acid (SHAM) decreased the fresh weight, K⁺ content, V_alt, H⁺-ATPase activity and the gene expression of AOX1a, HvNHX1, HvNHX3, HvHVP1, HvHVA, H⁺-ATPase, but increased the electrolyte leakage, MDA and Na⁺ content in both cultivars under 300 mM NaCl treatment. In short, alternative respiration is involved in the tolerance of highland barley to salt stress.

Hydrogen Peroxide and Nitric Oxide Crosstalk Mediates Brassinosteroids Induced Cold Stress Tolerance in Medicago truncatula

Brassinosteroids (BRs) play pivotal roles in modulating plant growth, development, and stress responses. In this study, a Medicago truncatula plant pretreated with brassinolide (BL, the most active BR), enhanced cold stress tolerance by regulating the expression of several cold-related genes and antioxidant enzymes activities. Previous studies reported that hydrogen peroxide (H₂O₂) and nitric oxide (NO) are involved during environmental stress conditions. However, how these two signaling molecules interact with each other in BRs-induced abiotic stress tolerance remain largely unclear. BL-pretreatment induced, while brassinazole (BRZ, a specific inhibitor of BRs biosynthesis) reduced H₂O₂ and NO production. Further, application of dimethylthiourea (DMTU, a H₂O₂ and OH- scavenger) blocked BRs-induced NO production, but BRs-induced H₂O₂ generation was not sensitive to 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO, a scavenger of NO). Moreover, pretreatment with DMTU and PTIO decreased BL-induced mitochondrial alternative oxidase (AOX) and the photosystem capacity. However, pretreatment with PTIO was found to be more effective than DMTU in reducing BRs-induced increases in Valt, Vt, and MtAOX1 gene expression. Similarly, BRs-induced photosystem II efficiency was found in NO dependent manner than H₂O₂. Finally, we conclude that H₂O₂ was involved in NO generation, whereas NO was found to be crucial in BRs-induced AOX capacity, which further contributed to the protection of the photosystem under cold stress conditions in Medicago truncatula.

Nitrogen-improved photosynthesis quantum yield is driven by increased thylakoid density, enhancing green light absorption

A nitrogen supply is necessary for all plants. The multifaceted reasons why this nutrient stimulates plant dry weight accumulation are assessed herein. We compared tomato plants grown in full sunlight and in low light environments under four N doses and evaluated plant growth, photosynthetic and calorimetric parameters, leaf anatomy, chloroplast transmission electron microscopy (TEM) and a high resolution profile of optical leaf properties. Increases in N supplies allow tomato plants to grow faster in low light environments (91.5% shading), displaying a robust light harvesting machinery and, consequently, improved light harvesting efficiency. Ultrastructurally, high N doses were associated to a high number of grana per chloroplast and greater thylakoid stacking, as well as high electrodensity by TEM. Robust photosynthetic machinery improves green light absorption, but not blue or red. In addition, low construction and dark respiration costs were related to improved total dry weight accumulation in shade conditions. By applying multivariate analyses, we conclude that improved green light absorbance, improved quantum yield and greater palisade parenchyma cell area are the primary components that drive increased plant growth under natural light-limited photosynthesis.

Nitric oxide regulates chlorophyllide biosynthesis and singlet oxygen generation differently between Arabidopsis and barley

Nitric oxide (NO) has a general inhibitory effects on chlorophyll biosynthesis, especially to the step of 5-aminolevulinic acid (ALA) biosynthesis and protochlorophyllide (Pchlide) to chlorophyllide (Chlide) conversion (responsible by the NADPH:Pchlide oxidoreductase POR). Previous study suggested that barley large POR aggregates may be generated by dithiol oxidation of cysteines of two POR monomers, which can be disconnected by some reducing agents. POR aggregate assembly may be correlated with seedling greening in barley, but not in Arabidopsis. Thus, NO may affect POR activity and seedling greening differently between Arabidopsis and barley. We proved this assumption by non-denaturing gel-analysis and reactive oxygen species (ROS) monitoring during the greening. NO treatments cause S-nitrosylation to POR cysteine residues and disassembly of POR aggregates. This modification reduces POR activity and induces Pchlide accumulation and singlet oxygen generation upon dark-to-high-light shift (and therefore inducing photobleaching lesions) in barley leaf apex, but not in Arabidopsis seedlings. ROS staining and ROS-related-gene expression detection confirmed that superoxide anion and singlet oxygen accumulated in barley etiolated seedlings after the NO treatments, when exposed to a fluctuating light. The data suggest that POR aggregate assembly may be correlated with barley chlorophyll biosynthesis and redox homeostasis during greening. Cysteine S-nitrosylation may be one of the key reasons for the NO-induced inhibition to chlorophyll biosynthetic enzymes.

The role of retrograde signals during plant stress responses

Chloroplast and mitochondria not only provide the energy to the plant cell but due to the sensitivity of organellar processes to perturbations caused by abiotic stress, they are also key cellular sensors of environmental fluctuations. Abiotic stresses result in reduced photosynthetic efficiency and thereby reduced energy supply for cellular processes. Thus, in order to acclimate to stress, plants must re-program gene expression and cellular metabolism to divert energy from growth and developmental processes to stress responses. To restore cellular energy homeostasis following exposure to stress, the activities of the organelles must be tightly co-ordinated with the transcriptional re-programming in the nucleus. Thus, communication between the organelles and the nucleus, so-called retrograde signalling, is essential to direct the energy use correctly during stress exposure. Stress-triggered retrograde signals are mediated by reactive oxygen species and metabolites including β-cyclocitral, MEcPP (2-C-methyl-d-erythritol 2,4-cyclodiphosphate), PAP (3'-phosphoadenosine 5'-phosphate), and intermediates of the tetrapyrrole biosynthesis pathway. However, for the plant cell to respond optimally to environmental stress, these stress-triggered retrograde signalling pathways must be integrated with the cytosolic stress signalling network. We hypothesize that the Mediator transcriptional co-activator complex may play a key role as a regulatory hub in the nucleus, integrating the complex stress signalling networks originating in different cellular compartments.

Import of Soluble Proteins into Chloroplasts and Potential Regulatory Mechanisms

Chloroplasts originated from an endosymbiotic event in which a free-living cyanobacterium was engulfed by an ancestral eukaryotic host. During evolution the majority of the chloroplast genetic information was transferred to the host cell nucleus. As a consequence, proteins formerly encoded by the chloroplast genome are now translated in the cytosol and must be subsequently imported into the chloroplast. This process involves three steps: (i) cytosolic sorting procedures, (ii) binding to the designated receptor-equipped target organelle and (iii) the consecutive translocation process. During import, proteins have to overcome the two barriers of the chloroplast envelope, namely the outer envelope membrane (OEM) and the inner envelope membrane (IEM). In the majority of cases, this is facilitated by two distinct multiprotein complexes, located in the OEM and IEM, respectively, designated TOC and TIC. Plants are constantly exposed to fluctuating environmental conditions such as temperature and light and must therefore regulate protein composition within the chloroplast to ensure optimal functioning of elementary processes such as photosynthesis. In this review we will discuss the recent models of each individual import stage with regard to short-term strategies that plants might use to potentially acclimate to changes in their environmental conditions and preserve the chloroplast protein homeostasis.

Coordinated regulation of photosynthetic and respiratory components is necessary to maintain chloroplast energy balance in varied growth conditions

Mitochondria have a non-energy-conserving alternative oxidase (AOX) proposed to support photosynthesis, perhaps by promoting energy balance under varying growth conditions. To investigate this, wild-type (WT) Nicotiana tabacum were compared with AOX knockdown and overexpression lines. In addition, the amount of AOX protein in WT plants was compared with that of chloroplast light-harvesting complex II (LHCB2), whose amount is known to respond to chloroplast energy status. With increased growth irradiance, WT leaves maintained higher rates of respiration in the light (RL), but no differences in RL or photosynthesis were seen between the WT and transgenic lines, suggesting that, under non-stress conditions, AOX was not critical for leaf metabolism, regardless of growth irradiance. However, under drought, the AOX amount became an important determinant of RL, which in turn was an important determinant of chloroplast energy balance (measured as photosystem II excitation pressure, EP), and photosynthetic performance. In the WT, the AOX amount increased and the LHCB2 amount decreased with increased growth irradiance or drought severity. These changes in protein amounts correlated strongly, in opposing ways, with growth EP. This suggests that a signal deriving from the photosynthetic electron transport chain status coordinately controls the amounts of AOX and LHCB2, which then both contribute to maintaining chloroplast energy balance, particularly under stress conditions.

Mg-Protoporphyrin IX Signals Enhance Plant's Tolerance to Cold Stress

The relationship between Mg-protoporphyrin IX (Mg-Proto IX) signals and plant's tolerance to cold stress is investigated. Arabidopsis seedlings grown for 3 weeks were pretreated with 2 mM glutamate (Glu) and 2 mM MgCl₂ for 48 h at room temperature to induce Mg-Proto IX accumulation. Then cold stress was performed at 4°C for additional 72 h. Glu + MgCl₂ pre-treatments alleviated the subsequent cold stress significantly by rising the leaf temperature through inducing Mg-Proto IX signals. The protective role of Glu + MgCl₂ treatment was greatly compromised in the mutants of Mg-Proto IX synthesis, Mg-Proto IX signaling, and cyanide-resistant respiration. And the enhancement of cold-responsive gene expression was greatly compromised in the mutants of Mg-Proto IX synthesis, Mg-Proto IX signaling and ABA signaling, but not in the mutant of cyanide-resistant respiration. Cold stress promoted cyanide-resistant respiration and leaf total respiration exponentially, which could be further induced by the Glu + MgCl₂ treatment. Mg-Proto IX signals also activate antioxidant enzymes and increase non-enzymatic antioxidants [glutathione but not ascorbic acid (AsA)] to maintain redox equilibrium during the cold stress.

Mitochondrial Alternative Pathway-Associated Photoprotection of Photosystem II is Related to the Photorespiratory Pathway

Respiratory electron transport has two ubiquinol-oxidizing pathways, the cytochrome pathway (CP) and the alternative pathway (AP). The AP, which is catalyzed by the alternative oxidase (AOX), is energetically wasteful but may alleviate PSII photoinhibition under light conditions excessive for photosynthesis. However, its mechanism remains unknown. We used Arabidopsis aox1a mutants lacking AOX activity and studied the mutation's effects on photoinhibition by measuring the decrease in the maximum quantum yield of PSII (F_v/F_m) after high light exposure. Since the CP compensates for the lack of AOX, we monitored the extent of photoinhibition under conditions where CP activity is partially inhibited by antimycin A. When leaves were exposed to high light at 350 µmol m^-2 s^-1, the decline in F_v/F_m was significantly faster in the aox1a mutants than in the wild type. However, under conditions where photorespiration was suppressed by high CO₂ or low O₂ levels, the decline in F_v/F_m was suppressed in the aox1a mutants, but not in the wild type, making the difference between the wild type and mutants small. Our results demonstrate that the lack of the AP causes an acceleration of PSII photoinhibition in relation to the photorespiratory pathway, suggesting that the AP can support the activity of the photorespiratory pathway under high light conditions.

Tuning of Redox Regulatory Mechanisms, Reactive Oxygen Species and Redox Homeostasis under Salinity Stress

Soil salinity is a crucial environmental constraint which limits biomass production at many sites on a global scale. Saline growth conditions cause osmotic and ionic imbalances, oxidative stress and perturb metabolism, e.g., the photosynthetic electron flow. The plant ability to tolerate salinity is determined by multiple biochemical and physiological mechanisms protecting cell functions, in particular by regulating proper water relations and maintaining ion homeostasis. Redox homeostasis is a fundamental cell property. Its regulation includes control of reactive oxygen species (ROS) generation, sensing deviation from and readjustment of the cellular redox state. All these redox related functions have been recognized as decisive factors in salinity acclimation and adaptation. This review focuses on the core response of plants to overcome the challenges of salinity stress through regulation of ROS generation and detoxification systems and to maintain redox homeostasis. Emphasis is given to the role of NADH oxidase (RBOH), alternative oxidase (AOX), the plastid terminal oxidase (PTOX) and the malate valve with the malate dehydrogenase isoforms under salt stress. Overwhelming evidence assigns an essential auxiliary function of ROS and redox homeostasis to salinity acclimation of plants.

Genetic engineering of AtAOX1a in Saccharomyces cerevisiae prevents oxidative damage and maintains redox homeostasis

This study aimed to validate the physiological importance of Arabidopsis thaliana alternative oxidase 1a (AtAOX1a) in alleviating oxidative stress using Saccharomyces cerevisiae as a model organism. The AOX1a transformant (pYES2AtAOX1a) showed cyanide resistant and salicylhydroxamic acid (SHAM)‐sensitive respiration, indicating functional expression of AtAOX1a in S. cerevisiae. After exposure to oxidative stress, pYES2AtAOX1a showed better survival and a decrease in reactive oxygen species (ROS) when compared to S. cerevisiae with empty vector (pYES2). Furthermore, pYES2AtAOX1a sustained growth by regulating GPX2 and/or TSA2, and cellular NAD⁺/NADH ratio. Thus, the expression of AtAOX1a in S. cerevisiae enhances its respiratory tolerance which, in turn, maintains cellular redox homeostasis and protects from oxidative damage.

The alternative respiratory pathway is involved in brassinosteroid-induced environmental stress tolerance in Nicotiana benthamiana

Brassinosteroids (BRs), plant steroid hormones, play essential roles in modulating cell elongation, vascular differentiation, senescence, and stress responses. However, the mechanisms by which BRs regulate plant mitochondria and resistance to abiotic stress remain largely unclear. Mitochondrial alternative oxidase (AOX) is involved in the plant response to a variety of environmental stresses. In this report, the role of AOX in BR-induced tolerance against cold, polyethylene glycol (PEG), and high-light stresses was investigated. Exogenous applied brassinolide (BL, the most active BR) induced, while brassinazole (BRZ, a BR biosynthesis inhibitor) reduced alternative respiration and AOX1 expression in Nicotiana benthamiana. Chemical scavenging of H2O2 and virus-induced gene silencing (VIGS) of NbRBOHB compromised the BR-induced alternative respiratory pathway, and this result was further confirmed by NbAOX1 promoter analysis. Furthermore, inhibition of AOX activity by chemical treatment or a VIGS-based approach decreased plant resistance to environmental stresses and compromised BR-induced stress tolerance. Taken together, our results indicate that BR-induced AOX capability might contribute to the avoidance of superfluous reactive oxygen species accumulation and the protection of photosystems under stress conditions in N. benthamiana.

Importance of the alternative oxidase (AOX) pathway in regulating cellular redox and ROS homeostasis to optimize photosynthesis during restriction of the cytochrome oxidase pathway in Arabidopsis thaliana

BACKGROUND AND AIMS

The importance of the alternative oxidase (AOX) pathway, particularly AOX1A, in optimizing photosynthesis during de-etiolation, under elevated CO2, low temperature, high light or combined light and drought stress is well documented. In the present study, the role of AOX1A in optimizing photosynthesis was investigated when electron transport through the cytochrome c oxidase (COX) pathway was restricted at complex III.

METHODS

Leaf discs of wild-type (WT) and aox1a knock-out mutants of Arabidopsis thaliana were treated with antimycin A (AA) under growth-light conditions. To identify the impact of AOX1A deficiency in optimizing photosynthesis, respiratory O2 uptake and photosynthesis-related parameters were measured along with changes in redox couples, reactive oxygen species (ROS), lipid peroxidation and expression levels of genes related to respiration, the malate valve and the antioxidative system.

KEY RESULTS

In the absence of AA, aox1a knock-out mutants did not show any difference in physiological, biochemical or molecular parameters compared with WT. However, after AA treatment, aox1a plants showed a significant reduction in both respiratory O2 uptake and NaHCO3-dependent O2 evolution. Chlorophyll fluorescence and P700 studies revealed that in contrast to WT, aox1a knock-out plants were incapable of maintaining electron flow in the chloroplastic electron transport chain, and thereby inefficient heat dissipation (low non-photochemical quenching) was observed. Furthermore, aox1a mutants exhibited significant disturbances in cellular redox couples of NAD(P)H and ascorbate (Asc) and consequently accumulation of ROS and malondialdehyde (MDA) content. By contrast, WT plants showed a significant increase in transcript levels of CSD1, CAT1, sAPX, COX15 and AOX1A in contrast to aox1a mutants.

CONCLUSIONS

These results suggest that AOX1A plays a significant role in sustaining the chloroplastic redox state and energization to optimize photosynthesis by regulating cellular redox homeostasis and ROS generation when electron transport through the COX pathway is disturbed at complex III.

Roles of mitochondrial energy dissipation systems in plant development and acclimation to stress

BACKGROUND

Plants are sessile organisms that have the ability to integrate external cues into metabolic and developmental signals. The cues initiate specific signal cascades that can enhance the tolerance of plants to stress, and these mechanisms are crucial to the survival and fitness of plants. The adaption of plants to stresses is a complex process that involves decoding stress inputs as energy-deficiency signals. The process functions through vast metabolic and/or transcriptional reprogramming to re-establish the cellular energy balance. Members of the mitochondrial energy dissipation pathway (MEDP), alternative oxidases (AOXs) and uncoupling proteins (UCPs), act as energy mediators and might play crucial roles in the adaption of plants to stresses. However, their roles in plant growth and development have been relatively less explored.

SCOPE

This review summarizes current knowledge about the role of members of the MEDP in plant development as well as recent advances in identifying molecular components that regulate the expression of AOXs and UCPs. Highlighted in particular is a comparative analysis of the expression, regulation and stress responses between AOXs and UCPs when plants are exposed to stresses, and a possible signal cross-talk that orchestrates the MEDP, reactive oxygen species (ROS), calcium signalling and hormone signalling.

CONCLUSIONS

The MEDP might act as a cellular energy/metabolic mediator that integrates ROS signalling, energy signalling and hormone signalling with plant development and stress accumulation. However, the regulation of MEDP members is complex and occurs at transcriptional, translational, post-translational and metabolic levels. How this regulation is linked to actual fluxes through the AOX/UCP in vivo remains elusive.