Unravelling mitochondrial retrograde regulation in the abiotic stress induction of rice ALTERNATIVE OXIDASE 1 genes

Overexpression of Abscisic Acid Biosynthesis Gene OsNCED3 Enhances Survival Rate and Tolerance to Alkaline Stress in Rice Seedlings

Alkaline stress with high pH levels could significantly influence plant growth and survival. The enzyme 9-cis-epoxycarotenoid dioxygenase (NCED) serves as a critical bottleneck in the biosynthesis of abscisic acid (ABA), making it essential for regulating stress tolerance. Here, we show that OsNCED3-overexpressing rice lines have increased ABA content by up to 50.90% and improved transcription levels of numerous genes involved in stress responses that significantly enhance seedling survival rates. Overexpression of OsNCED3 increased the dry weight contents of the total chlorophyll, proline, soluble sugar, starch, and the activities of antioxidant enzymes of rice seedlings, while reducing the contents of O2·-, H2O2, and malondialdehyde under hydroponic alkaline stress conditions simulated by 10, 15, and 20 mmol L-1 of Na2CO3. Additionally, the OsNCED3-overexpressing rice lines exhibited a notable increase in the expression of OsNCED3; ABA response-related genes OsSalT and OsWsi18; ion homeostasis-related genes OsAKT1, OsHKT1;5, OsSOS1, and OsNHX5; and ROS scavenging-related genes OsCu/Zn-SOD, OsFe-SOD, OsPOX1, OsCATA, OsCATB, and OsAPX1 in rice seedling leaves. The results of these findings suggest that overexpression of OsNCED3 upregulates endogenous ABA levels and the expression of stress response genes, which represents an innovative molecular approach for enhancing the alkaline tolerance of rice seedlings.

Acclimation of intertidal macroalgae Ulva prolifera to UVB radiation: the important role of alternative oxidase

BACKGROUND

Solar radiation is primarily composed of ultraviolet radiation (UVR, 200 - 400 nm) and photosynthetically active radiation (PAR, 400 - 700 nm). Ultraviolet-B (UVB) radiation accounts for only a small proportion of sunlight, and it is the primary cause of plant photodamage. The use of chlorofluorocarbons (CFCs) as refrigerants caused serious ozone depletion in the 1980s, and this had led to an increase in UVB. Although CFC emissions have significantly decreased in recent years, UVB radiation still remains at a high intensity. UVB radiation increase is an important factor that influences plant physiological processes. Ulva prolifera, a type of macroalga found in the intertidal zone, is intermittently exposed to UVB. Alternative oxidase (AOX) plays an important role in plants under stresses. This research examines the changes in AOX activity and the relationships among AOX, photosynthesis, and reactive oxygen species (ROS) homeostasis in U. prolifera under changes in UVB and photosynthetically active radiation (PAR).

RESULTS

UVB was the main component of solar radiation impacting the typical intertidal green macroalgae U. prolifera. AOX was found to be important during the process of photosynthesis optimization of U. prolifera due to a synergistic effect with non-photochemical quenching (NPQ) under UVB radiation. AOX and glycolate oxidase (GO) worked together to achieve NADPH homeostasis to achieve photosynthesis optimization under changes in PAR + UVB. The synergism of AOX with superoxide dismutase (SOD) and catalase (CAT) was important during the process of ROS homeostasis under PAR + UVB.

CONCLUSIONS

AOX plays an important role in the process of photosynthesis optimization and ROS homeostasis in U. prolifera under UVB radiation. This study provides further insights into the response of intertidal macroalgae to solar light changes.

Cytokinins act synergistically with heat acclimation to enhance rice thermotolerance affecting hormonal dynamics, gene expression and volatile emission

Heat stress is a frequent environmental constraint. Phytohormones can significantly affect plant thermotolerance. This study compares the effects of exogenous cytokinin meta-topolin-9-(tetrahydropyran-2-yl)purine (mT9THP) on rice (Oryza sativa) under control conditions, after acclimation by moderate temperature (A; 37 °C, 2h), heat stress (HS; 45 °C, 6h) and their combination (AHS). mT9THP is a stable cytokinin derivative that releases active meta-topolin gradually, preventing the rapid deactivation reported after exogenous cytokinin application. Under control conditions, mT9THP negatively affected jasmonic acid in leaves and abscisic and salicylic acids in crowns (meristematic tissue crucial for tillering). Exogenous cytokinin stimulated the emission of volatile organic compounds (VOC), especially 2,3-butanediol. Acclimation upregulated trans-zeatin, expression of stress- and hormone-related genes, and VOC emission. The combination of acclimation and mT9THP promoted the expression of stress markers and antioxidant enzymes and moderately increased VOC emission, including 2-ethylhexyl salicylate or furanones. AHS and HS responses shared some common features, namely, increase of ethylene precursor aminocyclopropane-1-carboxylic acid (ACC), cis-zeatin and cytokinin methylthio derivatives, as well as the expression of heat shock proteins, alternative oxidases, and superoxide dismutases. AHS specifically induced jasmonic acid and auxin indole-3-acetic acid levels, diacylglycerolipids with fewer double bonds, and VOC emissions [e.g., acetamide, lipoxygenase (LOX)-derived volatiles]. Under direct HS, exogenous cytokinin mimicked some positive acclimation effects. The combination of mT9THP and AHS had the strongest thermo-protective effect, including a strong stimulation of VOC emissions (including LOX-derived ones). These results demonstrate for the first time the crucial contribution of volatiles to the beneficial effects of cytokinin and AHS on rice thermotolerance.

Genome-wide identification of AOX family genes in Moso bamboo and functional analysis of PeAOX1b_2 in drought and salinity stress tolerance

Five PeAOX genes from Moso bamboo genome were identified. PeAOX1b_2-OE improved tolerance to drought and salinity stress in Arabidopsis, indicating it is involved in positive regulation of abiotic stress response. Mitochondrial alternative oxidase (AOX), the important respiratory terminal oxidase in organisms, catalyzes the energy wasteful cyanide (CN)-resistant respiration, which can improve abiotic stresses tolerance and is considered as one of the functional markers for plant resistance breeding. Here, a total of five putative AOX genes (PeAOXs) were identified and characterized in a monocotyledonous woody grass Moso bamboo (Phyllostachys edulis). Phylogenetic analysis revealed that PeAOXs belonged to AOX1 subfamily, and were named PeAOX1a_1, PeAOX1a_2, PeAOX1b_1, PeAOX1b_2 and PeAOX1c, respectively. Evolutionary and divergence patterns analysis revealed that the PeAOX, OsAOX, and BdAOX families experienced positive purifying selection and may have undergone a large-scale duplication event roughly 1.35-155.90 million years ago. Additionally, the organ-specific expression analysis showed that 80% of PeAOX members were mainly expressed in leaf. Promoter sequence analysis of PeAOXs revealed cis-acting regulatory elements (CAREs) responding to abiotic stress. Most PeAOX genes were significantly upregulated after methyl jasmonate (MeJA) and abscisic acid (ABA) treatment. Moreover, under salinity and drought stresses, the ectopic overexpression of PeAOX1b_2 in Arabidopsis enhanced seed germination and seedling establishment, increased the total respiratory rate and the proportion of AOX respiratory pathway in leaf, and enhanced antioxidant ability, suggesting that PeAOX1b_2 is crucial for abiotic stress resistance in Moso bamboo.

Enhancing biomass and lipid productivity of a green microalga Parachlorella kessleri for biodiesel production using rapid mutation of atmospheric and room temperature plasma

BACKGROUND

Microalgae, with their high adaptability to various stress conditions and rapid growth, are considered excellent biomass resources for lipid production and biodiesel feedstocks. However, lipid yield and productivity of the natural strains are common bottlenecks in their large-scale use for lipid production, which can be overcome by evolving new strains using conventional and advanced mutagenic techniques. It is challenging to generate microalgae strains capable of high lipid synthesis through natural selection. As a result, random mutagenesis is currently considered a viable option in many scenarios. The objective of this study was to explore atmospheric and room temperature plasma (ARTP) as a random mutagenesis technique to obtain high lipid-accumulating mutants of a green microalga for improved biodiesel production.

RESULTS

A green microalgal species was isolated from the Chinese Yellow Sea and identified as Parachlorella kessleri (OM758328). The isolated microalga was subsequently mutated by ARTP to obtain high lipid-accumulating mutants. Based on the growth rate and lipid content, 5 mutants (named M1, M2, M4, M5, and M8) were selected from 15 pre-selected mutants. These five mutants varied in their growth rate from 0.33 to 0.68 day^-1, with the lipid content varying between 0.25 g/L in M2 to 0.30 g/L in M8 at 10th day of cultivation. Among the mutants, M8 showed the maximum biomass productivity (0.046 g/L/day) and lipid productivity (20.19 mg/L/day), which were 75% and 44% higher than the wild strain, respectively. The triglyceride (TAG) content of M8 was found to be 0.56 g/L at 16th day of cultivation, which was 1.77-fold higher than that of the wild strain. Furthermore, M8 had the highest saturated fatty acids (C16-18) with the lowermost polyunsaturated fatty acid content, which are favorable properties of a biodiesel feedstock according to international standards.

CONCLUSION

The mutant strain of P. kessleri developed by the ARTP technique exhibited significant improvements in biomass productivity, lipid content, and biodiesel quality. Therefore, the biomass of this mutant microalga could be a potential feedstock for biodiesel production.

Transcriptome Analyses in a Selected Gene Set Indicate Alternative Oxidase (AOX) and Early Enhanced Fermentation as Critical for Salinity Tolerance in Rice

Plants subjected to stress need to respond rapidly and efficiently to acclimatize and survive. In this paper, we investigated a selected gene set potentially involved in early cell reprogramming in two rice genotypes with contrasting salinity tolerance (Pokkali tolerant and IR29 susceptible) in order to advance knowledge of early molecular mechanisms of rice in dealing with salt stress. Selected genes were evaluated in available transcriptomic data over a short period of 24 h and involved enzymes that avoid ROS formation (AOX, UCP and PTOX), impact ATP production (PFK, ADH and COX) or relate to the antioxidant system. Higher transcript accumulation of AOX (ROS balancing), PFK and ADH (alcohol fermentation) was detected in the tolerant genotype, while the sensitive genotype revealed higher UCP and PTOX transcript levels, indicating a predominant role for early transcription of AOX and fermentation in conferring salt stress tolerance to rice. Antioxidant gene analyses supported higher oxidative stress in IR29, with transcript increases of cytosolic CAT and SOD from all cell compartments (cytoplasm, peroxisome, chloroplast and mitochondria). In contrast, Pokkali increased mRNA levels from the AsA-GSH cycle as cytosolic/mitochondrial DHAR was involved in ascorbate recovery. In addition, these responses occurred from 2 h in IR29 and 10 h in Pokkali, indicating early but ineffective antioxidant activity in the susceptible genotype. Overall, our data suggest that AOX and ADH can play a critical role during early cell reprogramming for improving salt stress tolerance by efficiently controlling ROS formation in mitochondria. We discuss our results in relation to gene engineering and editing approaches to develop salinity-tolerant crops.

Total and Mitochondrial Transcriptomic and Proteomic Insights into Regulation of Bioenergetic Processes for Shoot Fast-Growth Initiation in Moso Bamboo

As a fast-growing, woody grass plant, Moso bamboo (Phyllostachys edulis) can supply edible shoots, building materials, fibrous raw material, raw materials for crafts and furniture and so on within a relatively short time. Rapid growth of Moso bamboo occurs after the young bamboo shoots are covered with a shell and emerge from the ground. However, the molecular reactions of bioenergetic processes essential for fast growth remain undefined. Herein, total and mitochondrial transcriptomes and proteomes were compared between spring and winter shoots. Numerous key genes and proteins responsible for energy metabolism were significantly upregulated in spring shoots, including those involved in starch and sucrose catabolism, glycolysis, the pentose phosphate pathway, the tricarboxylic acid cycle and oxidative phosphorylation. Accordingly, significant decreases in starch and soluble sugar, higher ATP content and higher rates of respiration and glycolysis were identified in spring shoots. Further, the upregulated genes and proteins related to mitochondrial fission significantly increased the number of mitochondria, indirectly promoting intracellular energy metabolism. Moreover, enhanced alternate-oxidase and uncoupled-protein pathways in winter shoots showed that an efficient energy-dissipating system was important for winter shoots to adapt to the low-temperature environment. Heterologous expression of PeAOX1b in Arabidopsis significantly affected seedling growth and enhanced cold-stress tolerance. Overall, this study highlights the power of comparing total and mitochondrial omics and integrating physiochemical data to understand how bamboo initiates fast growth through modulating bioenergetic processes.

Effect of sterile cytoplasms on content of absolutely dry matter in the grain sorghum hybrids

Analysis of absolutely dry matter (ADM) accumulation is important indicator of productivity of crop plants. For three seasons, we investigated the effect of sterile cytoplasms on content of ADM in the F1 hybrids of grain sorghum obtained on the basis of two series of alloplasmic iso-nuclear CMS-lines: (1) with A3, A4, and 9E cytoplasms and (2) with 9E and M35-1A cytoplasms. For the first time, the effect of the type of sterile cytoplasm on content of ADM in the F1 sorghum hybrids was shown. In each season, A3 cytoplasm reduced ADM in the F1 hybrids at the “tillering – heading” stage, whereas 9E cytoplasm increased ADM at the “heading – complete maturity” stage. The most significant differences were observed under drought conditions. These data indicate the genetic influence of cytoplasm on assimilation capacity of sorghum hybrids and tolerance to drought stress.

Redox and low-oxygen stress: signal integration and interplay

Plants are aerobic organisms relying on oxygen to serve their energy needs. The amount of oxygen available to sustain plant growth can vary significantly due to environmental constraints or developmental programs. In particular, flooding stress, which negatively impacts crop productivity, is characterized by a decline in oxygen availability. Oxygen fluctuations result in an altered redox balance and the formation of reactive oxygen/nitrogen species (ROS/RNS) during the onset of hypoxia and upon re-oxygenation. In this update, we provide an overview of the current understanding of the impact of redox and ROS/RNS on low-oxygen signaling and adaptation. We first focus on the formation of ROS and RNS during low-oxygen conditions. Following this, we examine the impact of hypoxia on cellular and organellar redox systems. Finally, we describe how redox and ROS/RNS participate in signaling events during hypoxia through potential post-translational modifications (PTMs) of hypoxia-relevant proteins. The aim of this update is to define our current understanding of the field and to provide avenues for future research directions.

The response mechanism to salt stress in Arabidopsis transgenic lines over-expressing of GmG6PD

Glucose-6-phosphate dehydrogenase (G6PD or G6PDH) plays an important role in response to salt stress in plants. However, much less is known about G6PD proteins in soybean (Glycine max L.). Here, we found that a soybean cytosolic G6PD gene, GmG6PD7, was induced by NaCl. We generated Arabidopsis transgenic lines overexpressing GmG6PD7. The seed germination rate and primary root length of Arabidopsis thaliana over-expressing GmG6PD7 under NaCl treatment were enhanced. Salt stress induced an obvious increase of the total and cytosolic G6PD activity and the marked decrease of ROS levels in the transgenic plants. At the same time, over-expressing GmG6PD7 in Arabidopsis affected the glutathione and NADPH level and activated ROS scavengers, suggesting that GmG6PD7 contributes to increase salinity tolerance by decreasing ROS accumulation. What's more, we found GmG6PD7 overexpression led to the up-regulation of abscisic acid (ABA) degradation gene and the down-regulation of ABA synthesis and ABA-responsive genes, which finally reduced ABA content to improve seed germination rate under salinity stress. It was noteworthy that GmG6PD7 can rescue the seed and root phenotype of Arabidopsis cytosolic G6PD mutant (Atg6pd5 and Atg6pd6) under salt stress, suggesting cytosolic G6PD may have a conserved function in soybean and Arabidopsis.

Rice FLOURY SHRUNKEN ENDOSPERM 5 Encodes a Putative Plant Organelle RNA Recognition Protein that Is Required for cis-Splicing of Mitochondrial nad4 Intron 1

BACKGROUND

The sequences of several important mitochondrion-encoded genes involved in respiration in higher plants are interrupted by introns. Many nuclear-encoded factors are involved in splicing these introns, but the mechanisms underlying this splicing remain unknown.

RESULTS

We isolated and characterized a rice mutant named floury shrunken endosperm 5 (fse5). In addition to having floury shrunken endosperm, the fse5 seeds either failed to germinate or produced seedlings which grew slowly and died ultimately. Fse5 encodes a putative plant organelle RNA recognition (PORR) protein targeted to mitochondria. Mutation of Fse5 hindered the splicing of the first intron of nad4, which encodes an essential subunit of mitochondrial NADH dehydrogenase complex I. The assembly and NADH dehydrogenase activity of complex I were subsequently disrupted by this mutation, and the structure of the mitochondria was abnormal in the fse5 mutant. The FSE5 protein was shown to interact with mitochondrial intron splicing factor 68 (MISF68), which is also a splicing factor for nad4 intron 1 identified previously via yeast two-hybrid (Y2H) assays.

CONCLUSION

Fse5 which encodes a PORR domain-containing protein, is essential for the splicing of nad4 intron 1, and loss of Fse5 function affects seed development and seedling growth.

Desert truffle genomes reveal their reproductive modes and new insights into plant-fungal interaction and ectendomycorrhizal lifestyle

Desert truffles are edible hypogeous fungi forming ectendomycorrhizal symbiosis with plants of Cistaceae family. Knowledge about the reproductive modes of these fungi and the molecular mechanisms driving the ectendomycorrhizal interaction is lacking. Genomes of the highly appreciated edible desert truffles Terfezia claveryi Chatin and Tirmania nivea Trappe have been sequenced and compared with other Pezizomycetes. Transcriptomes of T. claveryi × Helianthemum almeriense mycorrhiza from well-watered and drought-stressed plants, when intracellular colonizations is promoted, were investigated. We have identified the fungal genes related to sexual reproduction in desert truffles and desert-truffles-specific genomic and secretomic features with respect to other Pezizomycetes, such as the expansion of a large set of gene families with unknown Pfam domains and a number of species or desert-truffle-specific small secreted proteins differentially regulated in symbiosis. A core set of plant genes, including carbohydrate, lipid-metabolism, and defence-related genes, differentially expressed in mycorrhiza under both conditions was found. Our results highlight the singularities of desert truffles with respect to other mycorrhizal fungi while providing a first glimpse on plant and fungal determinants involved in ecto to endo symbiotic switch that occurs in desert truffle under dry conditions.

Heat Stress Targeting Individual Organs Reveals the Central Role of Roots and Crowns in Rice Stress Responses

Inter-organ communication and the heat stress (HS; 45°C, 6 h) responses of organs exposed and not directly exposed to HS were evaluated in rice (Oryza sativa) by comparing the impact of HS applied either to whole plants, or only to shoots or roots. Whole-plant HS reduced photosynthetic activity (F _v /F _m and QY_{_Lss} ), but this effect was alleviated by prior acclimation (37°C, 2 h). Dynamics of HSFA2d, HSP90.2, HSP90.3, and SIG5 expression revealed high protection of crowns and roots. Additionally, HSP26.2 was strongly expressed in leaves. Whole-plant HS increased levels of jasmonic acid (JA) and cytokinin cis-zeatin in leaves, while up-regulating auxin indole-3-acetic acid and down-regulating trans-zeatin in leaves and crowns. Ascorbate peroxidase activity and expression of alternative oxidases (AOX) increased in leaves and crowns. HS targeted to leaves elevated levels of JA in roots, cis-zeatin in crowns, and ascorbate peroxidase activity in crowns and roots. HS targeted to roots increased levels of abscisic acid and auxin in leaves and crowns, cis-zeatin in leaves, and JA in crowns, while reducing trans-zeatin levels. The weaker protection of leaves reflects the growth strategy of rice. HS treatment of individual organs induced changes in phytohormone levels and antioxidant enzyme activity in non-exposed organs, in order to enhance plant stress tolerance.

Beyond Ethylene: New Insights Regarding the Role of Alternative Oxidase in the Respiratory Climacteric

Climacteric fruits are characterized by a dramatic increase in autocatalytic ethylene production that is accompanied by a spike in respiration at the onset of ripening. The change in the mode of ethylene production from autoinhibitory to autostimulatory is known as the System 1 (S1) to System 2 (S2) transition. Existing physiological models explain the basic and overarching genetic, hormonal, and transcriptional regulatory mechanisms governing the S1 to S2 transition of climacteric fruit. However, the links between ethylene and respiration, the two main factors that characterize the respiratory climacteric, have not been examined in detail at the molecular level. Results of recent studies indicate that the alternative oxidase (AOX) respiratory pathway may play an essential role in mediating cross-talk between ethylene response, carbon metabolism, ATP production, and ROS signaling during climacteric ripening. New genomic, metabolic, and epigenetic information sheds light on the interconnectedness of ripening metabolic pathways, necessitating an expansion of the current, ethylene-centric physiological models. Understanding points at which ripening responses can be manipulated may reveal key, species- and cultivar-specific targets for regulation of ripening, enabling superior strategies for reducing postharvest wastage.

A mitochondria-targeted coenzyme Q peptoid induces superoxide dismutase and alleviates salinity stress in plant cells

Salinity is a serious challenge to global agriculture and threatens human food security. Plant cells can respond to salt stress either by activation of adaptive responses, or by programmed cell death. The mechanisms deciding the respective response are far from understood, but seem to depend on the degree, to which mitochondria can maintain oxidative homeostasis. Using plant PeptoQ, a Trojan Peptoid, as vehicle, it is possible to transport a coenzyme Q10 (CoQ10) derivative into plant mitochondria. We show that salinity stress in tobacco BY-2 cells (Nicotiana tabacum L. cv Bright Yellow-2) can be mitigated by pretreatment with plant PeptoQ with respect to numerous aspects including proliferation, expansion, redox homeostasis, and programmed cell death. We tested the salinity response for transcripts from nine salt-stress related-genes representing different adaptive responses. While most did not show any significant response, the salt response of the transcription factor NtNAC, probably involved in mitochondrial retrograde signaling, was significantly modulated by the plant PeptoQ. Most strikingly, transcripts for the mitochondrial, Mn-dependent Superoxide Dismutase were rapidly and drastically upregulated in presence of the peptoid, and this response was disappearing in presence of salt. The same pattern, albeit at lower amplitude, was seen for the sodium exporter SOS1. The findings are discussed by a model, where plant PeptoQ modulates retrograde signalling to the nucleus leading to a strong expression of mitochondrial SOD, what renders mitochondria more resilient to perturbations of oxidative balance, such that cells escape salt induced cell death and remain viable.

Photosynthesis, respiration and growth: A carbon and energy balancing act for alternative oxidase

This review summarizes knowledge of alternative oxidase, a mitochondrial electron transport chain component that lowers the ATP yield of plant respiration. Analysis of mutant and transgenic plants has established that alternative oxidase activity supports leaf photosynthesis. The interaction of alternative oxidase respiration with chloroplast metabolism is important under conditions that challenge energy and/or carbon balance in the photosynthetic cell. Under such conditions, alternative oxidase provides an extra-chloroplastic means to optimize the status of chloroplast energy pools (ATP, NADPH) and to manage cellular carbohydrate pools in response to changing rates of carbon fixation and carbon demand for growth and maintenance. Transcriptional and post-translational mechanisms ensure that alternative oxidase can respond effectively when carbon and energy balance are being challenged. This function appears particularly significant under abiotic stress conditions such as water deficit, high salinity, or temperature extremes. Under such conditions, alternative oxidase respiration positively affects growth and stress tolerance, despite it lowering the energy yield and carbon use efficiency of respiration. In part, this beneficial effect relates to the ability of alternative oxidase respiration to prevent excessive reactive oxygen species generation in both mitochondria and chloroplasts. Recent evidence suggests that alternative oxidase respiration is an interesting target for crop improvement.

Heavy metal induced stress on wheat: phytotoxicity and microbiological management

Among many soil problems, heavy metal accumulation is one of the major agronomic challenges that has seriously threatened food safety. Due to these problems, soil biologists/agronomists in recent times have also raised concerns over heavy metal pollution, which indeed are unpleasantly affecting agro-ecosystems and crop production. The toxic heavy metals once deposited beyond certain permissible limits, obnoxiously affect the density, composition and physiological activities of microbiota, dynamics and fertility of soil leading eventually to reduction in wheat production and via food chain, human and animal health. Therefore, the metal induced phytotoxicity problems warrant urgent and immediate attention so that the physiological activities of microbes, nutrient pool of soils and concurrently the production of wheat are preserved and maintained in a constantly deteriorating environment. To mitigate the magnitude of metal induced changes, certain microorganisms have been identified, especially those belonging to the plant growth promoting rhizobacteria (PGPR) group endowed with the distinctive property of heavy metal tolerance and exhibiting unique plant growth promoting potentials. When applied, such metal-tolerant PGPR have shown variable positive impact on wheat production, even in soils contaminated with metals, by supplying macro and micro nutrients and secreting active biomolecules like EPS, melanins and metallothionein (MTs). Despite some reports here and there, the phytotoxicity of metals to wheat and how wheat production in metal-stressed soil can be enhanced is poorly explained. Thus, an attempt is made in this review to better understand the mechanistic basis of metal toxicity to wheat, and how such phytotoxicity can be mitigated by incorporating microbiological remediation strategies in wheat cultivation practices. The information provided here is likely to benefit wheat growers and consequently optimize wheat production inexpensively under stressed soils.

Among many soil problems, heavy metal accumulation is one of the major agronomic challenges that has seriously threatened food safety.

Morphological, physiochemical and antioxidant responses of Maclura pomifera to drought stress

Drought is one of the most important environmental factor limiting the growth of woody and non woody plants. In the present paper, we aimed to explore the performance of Maclura pomifera under a prolonged drought period followed by re-watering. M. pomifera plants were exposed to four different watering regimes (100%, 75%, 50% and 30% of the field capacity (FC)) for three weeks and then rewatered. The exposure to drought affected physiological, morphological and biochemical traits of M. pomifera. Leaf area, relative water content and water potential of leaf decreased in parallel with increased water deficit. Malondialdehyde content increased along with the drought stress experiment. Soluble carbohydrates (sucrose, glucose and fructose) accumulated during drought stress, but decreased after 22 days of water deficit in severe stressed plants (30% FC). Proline and mannitol, two compatible osmolytes, were higher in drought stresses plants than in control plants. Additionally the activity of antioxidant enzymes (SOD, APX, DHAR and GR) resulted affected by drought stress. In the recovery period, the physiological parameters as well as the proline content recovered at control levels, whereas soluble sugars, mannitol and total activity of antioxidant enzymes remained slight higher than in control plants, presumably to allow plants a complete recovery after stress. Our results suggest that M. pomifera has a good adaptive response to drought stress, probably corresponded to decreasing oxidative injury by induction of the antioxidant system and accumulation of stable and protective osmolytes such as proline and mannitol at higher rates.

Overexpression of BnaAOX1b Confers Tolerance to Osmotic and Salt Stress in Rapeseed

Alternative oxidases (AOXs) are the terminal oxidase in the cyanide-resistant respiration pathway in plant mitochondria, which play an important role in abiotic stress and are proposed as a functional marker for high tolerant breeding. In this study, ten AOX genes (BnaAOXs) were identified, and CysI and CysII of AOX isoforms were highly conserved in rapeseed. Among them, Bna.AOX1b was mainly expressed in the ovule and displayed varying expression between rapeseed cultivars which showed different salt resistance in seed germination. We identified its mitochondrial localization of this gene. To investigate the function of BnaAOX1b in rapeseed, transgenic rapeseed lines with overexpressed BnaAOX1b were created and seed germination and seedling establishment assays were performed under osmotic, salt, and ABA treatment. The results indicated that overexpression of BnaAOX1b significantly improved seed germination under osmotic and salt stress and weakened ABA sensitivity. In addition, post-germination seedling growth was improved under high salt condition, but showed hypersensitivity to ABA. RNA-sequencing analysis indicated that the genes involved in electron transport or energy pathway were induced and a number of gene responses to salt stress and ABA were regulated in Bna.AOX1b overexpressing seeds. Taken together, our results imply that Bna.AOX1b confers tolerance to osmotic and salt stress in terms of seed germination and seedling establishment by regulating stress responsive genes and the response to ABA, and could be utilized as a candidate gene in transgenic breeding.

Hydrogen peroxide modulate photosynthesis and antioxidant systems in tomato (Solanum lycopersicum L.) plants under copper stress

Plant growth and development could be modulated by minute concentrations of hydrogen peroxide (H₂O₂) which serves as a signaling molecule for various processes. The present work was conducted with an aim that H₂O₂ could also modify root morphology, morphology and movement of stomata, photosynthetic responses, activity of carbonic anhydrase, and antioxidant systems in tomato (Solanum lycopersicum L.) plants under copper stress (Cu; 10 or 100 mg kg^-1 soil). Roots of 20 d old plants were dipped in 0.1 or 0.5 mM of H₂O₂ solution for 4 h and then transplanted to the soil filled in earthen pots. High Cu stress (100 mg kg^-1 soil) altered root morphology, reduced chlorophyll content and photosynthetic capacity and also affected movement of stomata and generation of antioxidant species at 40 d after transplantation. Further, root dipping treatment of H₂O₂ to plants under stress and stress-free conditions enhanced accumulation of proline and activity of catalase, peroxidase, and superoxide dismutase, whereas production of superoxide radical (O₂^•¯) and H₂O₂ were decreased. Overall, H₂O₂ treatment improved growth, photosynthesis, metabolic state of the plants which provided tolerance and helped the plants to cope well under Cu stress.

Meta-analysis of drought-tolerant genotypes in Oryza sativa: A network-based approach

BACKGROUND

Drought is a severe environmental stress. It is estimated that about 50% of the world rice production is affected mainly by drought. Apart from conventional breeding strategies to develop drought-tolerant crops, innovative computational approaches may provide insights into the underlying molecular mechanisms of stress response and identify drought-responsive markers. Here we propose a network-based computational approach involving a meta-analytic study of seven drought-tolerant rice genotypes under drought stress.

RESULTS

Co-expression networks enable large-scale analysis of gene-pair associations and tightly coupled clusters that may represent coordinated biological processes. Considering differentially expressed genes in the co-expressed modules and supplementing external information such as resistance/tolerance QTLs, transcription factors, network-based topological measures, we identify and prioritize drought-adaptive co-expressed gene modules and potential candidate genes. Using the candidate genes that are well-represented across the datasets as 'seed' genes, two drought-specific protein-protein interaction networks (PPINs) are constructed with up- and down-regulated genes. Cluster analysis of the up-regulated PPIN revealed ABA signalling pathway as a central process in drought response with a probable crosstalk with energy metabolic processes. Tightly coupled gene clusters representing up-regulation of core cellular respiratory processes and enhanced degradation of branched chain amino acids and cell wall metabolism are identified. Cluster analysis of down-regulated PPIN provides a snapshot of major processes associated with photosynthesis, growth, development and protein synthesis, most of which are shut down during drought. Differential regulation of phytohormones, e.g., jasmonic acid, cell wall metabolism, signalling and posttranslational modifications associated with biotic stress are elucidated. Functional characterization of topologically important, drought-responsive uncharacterized genes that may play a role in important processes such as ABA signalling, calcium signalling, photosynthesis and cell wall metabolism is discussed. Further transgenic studies on these genes may help in elucidating their biological role under stress conditions.

CONCLUSION

Currently, a large number of resources for rice functional genomics exist which are mostly underutilized by the scientific community. In this study, a computational approach integrating information from various resources such as gene co-expression networks, protein-protein interactions and pathway-level information is proposed to provide a systems-level view of complex drought-responsive processes across the drought-tolerant genotypes.

iTRAQ-Based Protein Profiling and Biochemical Analysis of Two Contrasting Rice Genotypes Revealed Their Differential Responses to Salt Stress

Salt stress is one of the key abiotic stresses causing huge productivity losses in rice. In addition, the differential sensitivity to salinity of different rice genotypes during different growth stages is a major issue in mitigating salt stress in rice. Further, information on quantitative proteomics in rice addressing such an issue is scarce. In the present study, an isobaric tags for relative and absolute quantitation (iTRAQ)-based comparative protein quantification was carried out to investigate the salinity-responsive proteins and related biochemical features of two contrasting rice genotypes—Nipponbare (NPBA, japonica) and Liangyoupeijiu (LYP9, indica), at the maximum tillering stage. The rice genotypes were exposed to four levels of salinity: 0 (control; CK), 1.5 (low salt stress; LS), 4.5 (moderate salt stress; MS), and 7.5 g of NaCl/kg dry soil (high salt stress, HS). The iTRAQ protein profiling under different salinity conditions identified a total of 5340 proteins with 1% FDR in both rice genotypes. In LYP9, comparisons of LS, MS, and HS compared with CK revealed the up-regulation of 28, 368, and 491 proteins, respectively. On the other hand, in NPBA, 239 and 337 proteins were differentially upregulated in LS and MS compared with CK, respectively. Functional characterization by KEGG and COG, along with the GO enrichment results, suggests that the differentially expressed proteins are mainly involved in regulation of salt stress responses, oxidation-reduction responses, photosynthesis, and carbohydrate metabolism. Biochemical analysis of the rice genotypes revealed that the Na⁺ and Cl⁻ uptake from soil to the leaves via the roots was increased with increasing salt stress levels in both rice genotypes. Further, increasing the salinity levels resulted in increased cell membrane injury in both rice cultivars, however more severely in NPBA. Moreover, the rice root activity was found to be higher in LYP9 roots compared with NPBA under salt stress conditions, suggesting the positive role of rice root activity in mitigating salinity. Overall, the results from the study add further insights into the differential proteome dynamics in two contrasting rice genotypes with respect to salt tolerance, and imply the candidature of LYP9 to be a greater salt tolerant genotype over NPBA.

Genome-wide identification and characterization of ALTERNATIVE OXIDASE genes and their response under abiotic stresses in Camellia sinensis (L.) O. Kuntze

Four typical ALTERNATIVE OXIDASE genes have been identified in tea plants, and their sequence features and gene expression profiles have provided useful information for further studies on function and regulation. Alternative oxidase (AOX) is a terminal oxidase located in the respiratory electron transport chain. AOX catalyzes the oxidation of quinol and the reduction of oxygen into water. In this study, a genome-wide search and subsequent DNA cloning were performed to identify and characterize AOX genes in tea plant (Camellia sinensis (L.) O. Kuntze cv. Longjing43). Our results showed that tea plant possesses four AOX genes, i.e., CsAOX1a, CsAOX1d, CsAOX2a and CsAOX2b. Gene structure and protein sequence analyses revealed that all CsAOXs share a four-exon/three-intron structure with highly conserved regions and amino acid residues, which are necessary for AOX secondary structures, catalytic activities and post-translational regulations. All CsAOX were shown to localize in mitochondria using the green fluorescent protein (GFP)-targeting assay. Both CsAOX1a and CsAOX1d were induced by cold, salt and drought stresses, and with different expression patterns in young and mature leaves. Reactive oxygen species (ROS) accumulated strongly after 72 and 96 h cold treatments in both young and mature leaves, while the polyphenol and total catechin decreased significantly only in mature leaves. In comparison to AtAOX1a in Arabidopsis thaliana, CsAOX1a lost almost all of the stress-responsive cis-acting regulatory elements in its promoter region (1500 bp upstream), but possesses a flavonoid biosynthesis-related MBSII cis-acting regulatory element. These results suggest a link between CsAOX1a function and the metabolism of some secondary metabolites in tea plant. Our studies provide a basis for the further elucidation of the biological function and regulation of the AOX pathway in tea plants.

Connecting salt stress signalling pathways with salinity-induced changes in mitochondrial metabolic processes in C3 plants

Salinity exerts a severe detrimental effect on crop yields globally. Growth of plants in saline soils results in physiological stress, which disrupts the essential biochemical processes of respiration, photosynthesis, and transpiration. Understanding the molecular responses of plants exposed to salinity stress can inform future strategies to reduce agricultural losses due to salinity; however, it is imperative that signalling and functional response processes are connected to tailor these strategies. Previous research has revealed the important role that plant mitochondria play in the salinity response of plants. Review of this literature shows that 2 biochemical processes required for respiratory function are affected under salinity stress: the tricarboxylic acid cycle and the transport of metabolites across the inner mitochondrial membrane. However, the mechanisms by which components of these processes are affected or react to salinity stress are still far from understood. Here, we examine recent findings on the signal transduction pathways that lead to adaptive responses of plants to salinity and discuss how they can be involved in and be affected by modulation of the machinery of energy metabolism with attention to the role of the tricarboxylic acid cycle enzymes and mitochondrial membrane transporters in this process.

Contribution of the Alternative Respiratory Pathway to PSII Photoprotection in C3 and C4 Plants

The mechanism by which the mitochondrial alternative oxidase (AOX) pathway contributes to photosystem II (PSII) photoprotection is in dispute. It was generally thought that the AOX pathway protects photosystems by dissipating excess reducing equivalents exported from chloroplasts through the malate/oxaloacetate (Mal/OAA) shuttle and thus preventing the over-reduction of chloroplasts. In this study, using the aox1a Arabidopsis mutant and nine other C3 and C4 plant species, we revealed an additional action model of the AOX pathway in PSII photoprotection. Although the AOX pathway contributes to PSII photoprotection in C3 leaves treated with high light, this contribution was observed to disappear when photorespiration was suppressed. Disruption or inhibition of the AOX pathway significantly decreased the photorespiration in C3 leaves. Moreover, the AOX pathway did not respond to high light and contributed little to PSII photoprotection in C4 leaves possessing a highly active Mal/OAA shuttle but with little photorespiration. These results demonstrate that the AOX pathway contributes to PSII photoprotection in C3 plants by maintaining photorespiration to detoxify glycolate and via the indirect export of excess reducing equivalents from chloroplasts by the Mal/OAA shuttle. This new action model explains why the AOX pathway does not contribute to PSII photoprotection in C4 plants.

The role of mitochondria in plant development and stress tolerance

Eukaryotic cells require orchestrated communication between nuclear and organellar genomes, perturbations in which are linked to stress response and disease in both animals and plants. In addition to mitochondria, which are found across eukaryotes, plant cells contain a second organelle, the plastid. Signaling both among the organelles (cytoplasmic) and between the cytoplasm and the nucleus (i.e. nuclear-cytoplasmic interactions (NCI)) is essential for proper cellular function. A deeper understanding of NCI and its impact on development, stress response, and long-term health is needed in both animal and plant systems. Here we focus on the role of plant mitochondria in development and stress response. We compare and contrast features of plant and animal mitochondrial genomes (mtDNA), particularly highlighting the large and highly dynamic nature of plant mtDNA. Plant-based tools are powerful, yet underutilized, resources for enhancing our fundamental understanding of NCI. These tools also have great potential for improving crop production. Across taxa, mitochondria are most abundant in cells that have high energy or nutrient demands as well as at key developmental time points. Although plant mitochondria act as integrators of signals involved in both development and stress response pathways, little is known about plant mtDNA diversity and its impact on these processes. In humans, there are strong correlations between particular mitotypes (and mtDNA mutations) and developmental differences (or disease). We propose that future work in plants should focus on defining mitotypes more carefully and investigating their functional implications as well as improving techniques to facilitate this research.

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.

Do Mitochondria Play a Central Role in Stress-Induced Somatic Embryogenesis?

This review highlights a four-step rational for the hypothesis that mitochondria play an upstream central role for stress-induced somatic embryogenesis (SE): (1) Initiation of SE is linked to programmed cell death (PCD) (2) Mitochondria are crucially connected to cell death (3) SE is challenged by stress per se (4) Mitochondria are centrally linked to plant stress response and its management. Additionally the review provides a rough perspective for the use of mitochondrial-derived functional marker (FM) candidates to improve SE efficiency. It is proposed to apply SE systems as phenotyping tool for identifying superior genotypes with high general plasticity under severe plant stress conditions.

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.

The effect of exogenous calcium on mitochondria, respiratory metabolism enzymes and ion transport in cucumber roots under hypoxia

Hypoxia induces plant stress, particularly in cucumber plants under hydroponic culture. In plants, calcium is involved in stress signal transmission and growth. The ultimate goal of this study was to shed light on the mechanisms underlying the effects of exogenous calcium on the mitochondrial antioxidant system, the activity of respiratory metabolism enzymes, and ion transport in cucumber (Cucumis sativus L. cv. Jinchun No. 2) roots under hypoxic conditions. Our experiments revealed that exogenous calcium reduces the level of reactive oxygen species (ROS) and increases the activity of antioxidant enzymes in mitochondria under hypoxia. Exogenous calcium also enhances the accumulation of enzymes involved in glycolysis and the tricarboxylic acid (TCA) cycle. We utilized fluorescence and ultrastructural cytochemistry methods to observe that exogenous calcium increases the concentrations of Ca(2+) and K(+) in root cells by increasing the activity of plasma membrane (PM) H(+)-ATPase and tonoplast H(+)-ATPase and H(+)-PPase. Overall, our results suggest that hypoxic stress has an immediate and substantial effect on roots. Exogenous calcium improves metabolism and ion transport in cucumber roots, thereby increasing hypoxia tolerance in cucumber.

ALTERNATIVE OXIDASE1a modulates the oxidative challenge during moderate Cd exposure in Arabidopsis thaliana leaves

This study aims to unravel the functional significance of alternative oxidase1a (AOX1a) induction in Arabidopsis thaliana leaves exposed to cadmium (Cd) by comparing wild-type (WT) plants and aox1a knockout mutants. In the absence of AOX1a, differences in stress-responsive transcript and glutathione levels suggest an increased oxidative challenge during moderate (5 µM) and prolonged (72h) Cd exposure. Nevertheless, aox1a knockout leaves showed lower hydrogen peroxide (H2O2) accumulation as compared to the WT due to both acute (24h) and prolonged (72h) exposure to 5 µM Cd, but not to 10 µM Cd. Taken together, we propose a working model where AOX1a acts early in the response to Cd and activates or maintains a mitochondrial signalling pathway impacting on cellular antioxidative defence at the post-transcriptional level. This fine-tuning pathway is suggested to function during moderate (5 µM) Cd exposure while being overwhelmed during more severe (10 µM) Cd stress. Within this framework, ethylene is required - either directly or indirectly via NADPH oxidase isoform C - to fully induce AOX1 expression. In addition, reciprocal crosstalk between these components was demonstrated in leaves of A. thaliana plants exposed to Cd.

Identification and analysis of the mechanism underlying heat-inducible expression of rice aconitase 1

Respiratory metabolism is an important though poorly understood facet of plant adaptation to stress. Posttranslational modification of aconitase, a component of the tricarboxylic acid cycle (TCA), may be involved in stress tolerance. However, such stress-related transcriptional regulation and its mechanism remain unknown. In this study, we found that expression of the rice Aconitase gene OsACO1 is induced in a time-dependent manner by heat but not other typical abiotic stresses. To analyze the transcriptional regulation mechanism underlying the response to heat, the OsACO1 promoter (POsACO1) was isolated and characterized in transgenic rice. Using qualitative and quantitative analyses, we found that the expression of the GUS reporter gene responded to heat in different tissues and at different stages of development when driven by POsACO1. A series of 5' distal deletions of POsACO1 was generated to delineate the region responsible for heat-induced gene expression. Transient expression analyses in tobacco leaves identified a 322-bp minimal region between -1386 and -1065 as being essential and sufficient for heat-induced expression by POsACO1. We screened for known heat response-related cis-elements in this 322-bp region; however, sequences correlating with heat-induced gene expression were not identified in POsACO1. Therefore, truncations and successive mutagenesis analyses were performed in this 322-bp region. By comparing the activities of promoter fragments and their derivatives, our results indicated that the heat response element resided in a 9-bp region between -1132 and -1124, a sequence that contains a W-box motif. Additional site-directed mutagenesis analyses eliminated the heat response activity of POsACO1 via the W-box element, and an electrophoretic mobility shift assay (EMSA) indicated the binding of POsACO1 by factors in the nuclear extracts of heat-stressed rice seedlings in a W-box-dependent manner. Our results illustrate the expression pattern of a key component of the TCA response to abiotic stress and establish a putative regulatory pathway in the transcriptional modulation of rice respiratory metabolism genes in response to heat.

Mitochondrial energy-dissipating systems (alternative oxidase, uncoupling proteins, and external NADH dehydrogenase) are involved in development of frost-resistance of winter wheat seedlings

Gene expression, protein synthesis, and activities of alternative oxidase (AOX), uncoupling proteins (UCP), adenine nucleotide translocator (ANT), and non-coupled NAD(P)H dehydrogenases (NDex, NDPex, and NDin) were studied in shoots of etiolated winter wheat (Triticum aestivum L.) seedlings after exposure to hardening low positive (2°C for 7 days) and freezing (-2°C for 2 days) temperatures. The cold hardening efficiently increased frost-resistance of the seedlings and decreased the generation of reactive oxygen species (ROS) during further cold shock. Functioning of mitochondrial energy-dissipating systems can represent a mechanism responsible for the decrease in ROS under these conditions. These systems are different in their response to the action of the hardening low positive and freezing temperatures. The functioning of the first system causes induction of AOX and UCP synthesis associated with an increase in electron transfer via AOX in the mitochondrial respiratory chain and also with an increase in the sensitivity of mitochondrial non-phosphorylating respiration to linoleic and palmitic acids. The increase in electron transfer via AOX upon exposure of seedlings to hardening freezing temperature is associated with retention of a high activity of NDex. It seems that NDex but not the NDPex and NDin can play an important role in maintaining the functional state of mitochondria in heterotrophic tissues of plants under the influence of freezing temperatures. The involvement of the mitochondrial energy-dissipating systems and their possible physiological role in the adaptation of winter crops to cold and frost are discussed.

ROS Regulation During Abiotic Stress Responses in Crop Plants

Abiotic stresses such as drought, cold, salt and heat cause reduction of plant growth and loss of crop yield worldwide. Reactive oxygen species (ROS) including hydrogen peroxide (H2O2), superoxide anions (O2 (•-)), hydroxyl radical (OH•) and singlet oxygen ((1)O2) are by-products of physiological metabolisms, and are precisely controlled by enzymatic and non-enzymatic antioxidant defense systems. ROS are significantly accumulated under abiotic stress conditions, which cause oxidative damage and eventually resulting in cell death. Recently, ROS have been also recognized as key players in the complex signaling network of plants stress responses. The involvement of ROS in signal transduction implies that there must be coordinated function of regulation networks to maintain ROS at non-toxic levels in a delicate balancing act between ROS production, involving ROS generating enzymes and the unavoidable production of ROS during basic cellular metabolism, and ROS-scavenging pathways. Increasing evidence showed that ROS play crucial roles in abiotic stress responses of crop plants for the activation of stress-response and defense pathways. More importantly, manipulating ROS levels provides an opportunity to enhance stress tolerances of crop plants under a variety of unfavorable environmental conditions. This review presents an overview of current knowledge about homeostasis regulation of ROS in crop plants. In particular, we summarize the essential proteins that are involved in abiotic stress tolerance of crop plants through ROS regulation. Finally, the challenges toward the improvement of abiotic stress tolerance through ROS regulation in crops are discussed.

ALT1, a Snf2 family chromatin remodeling ATPase, negatively regulates alkaline tolerance through enhanced defense against oxidative stress in rice

Alkaline salt stress adversely affects rice growth, productivity and grain quality. However, the mechanism underlying this process remains elusive. We characterized here an alkaline tolerant mutant, alt1 in rice. Map-based cloning revealed that alt1 harbors a mutation in a chromatin remodeling ATPase gene. ALT1-RNAi transgenic plants under different genetic background mimicked the alt1 phenotype, exhibiting tolerance to alkaline stress in a transcript dosage-dependent manner. The predicted ALT1 protein belonged to the Ris1 subgroup of the Snf2 family and was localized in the nucleus, and transcription of ALT1 was transiently suppressed after alkaline treatment. Although the absorption of several metal ions maintained well in the mutant under alkaline stress, expression level of the genes involved in metal ions homeostasis was not altered in the alt1 mutant. Classification of differentially expressed abiotic stress related genes, as revealed by microarray analysis, found that the majority (50/78) were involved in ROS production, ROS scavenging, and DNA repair. This finding was further confirmed by that alt1 exhibited lower levels of H₂O₂ under alkaline stress and tolerance to methyl viologen treatment. Taken together, these results suggest that ALT1 negatively functions in alkaline tolerance mainly through the defense against oxidative damage, and provide a potential two-step strategy for improving the tolerance of rice plants to alkaline stress.

Overexpression of ALTERNATIVE OXIDASE1a alleviates mitochondria-dependent programmed cell death induced by aluminium phytotoxicity in Arabidopsis

Alternative oxidase (AOX) is a terminal oxidase found in all plants, and functions to maintain the electron flux and reduce the production of reactive oxygen species (ROS). Our previous study demonstrated that aluminium (Al) treatment could induce increased expression of the AOX1a gene, but the mechanism of how AOX1a participates in the regulation of Al-induced programmed cell death (PCD) is still not clear. To investigate the possible mechanism, mitochondrial ROS production and the behaviour of mitochondria, as well as caspase-3-like activation were monitored under Al treatment in wild-type (WT), AOX1a-lacking (aox1a), and AOX1a-overexpressing (AOX1a-OE) Arabidopsis. Our results showed that Al treatment increased the expression of AOX1a at both the transcriptional and translational levels. Overexpression of AOX1a reduced mitochondrial ROS production by maintaining the mitochondrial electron flux, and alleviated subsequent mitochondrial dysfunction and caspase-3-like activation in Al-induced PCD. Moreover, it was found that a change in AOX1a level could influence the expression levels of downstream functional genes that play protective roles in Al-induced PCD. Experiments using mutants and inhibitors demonstrated that superoxide anion (O2 (-)) derived from mitochondria was involved in Al-induced upregulation of AOX1a gene expression. Taken together, these results indicated that overexpression of AOX1a alleviated Al-induced PCD by maintaining mitochondrial function and promoting the expression of protective functional genes, providing new insights into the signalling cascades that modulate the Al phytotoxicity mechanism.

Peroxisomal polyamine oxidase and NADPH-oxidase cross-talk for ROS homeostasis which affects respiration rate in Arabidopsis thaliana

Homeostasis of reactive oxygen species (ROS) in the intracellular compartments is of critical importance as ROS have been linked with nearly all cellular processes and more importantly with diseases and aging. PAs are nitrogenous molecules with an evolutionary conserved role in the regulation of metabolic and energetic status of cells. Recent evidence also suggests that polyamines (PA) are major regulators of ROS homeostasis. In Arabidopsis the backconversion of the PAs spermidine (Spd) and spermine to putrescine and Spd, respectively, is catalyzed by two peroxisomal PA oxidases (AtPAO). However, the physiological role of this pathway remains largely elusive. Here we explore the role of peroxisomal PA backconversion and in particular that catalyzed by the highly expressed AtPAO3 in the regulation of ROS homeostasis and mitochondrial respiratory burst. Exogenous PAs exert an NADPH-oxidase dependent stimulation of oxygen consumption, with Spd exerting the strongest effect. This increase is attenuated by treatment with the NADPH-oxidase blocker diphenyleneiodonium iodide (DPI). Loss-of-function of AtPAO3 gene results to increased NADPH-oxidase-dependent production of superoxide anions ([Formula: see text] ), but not H2O2, which activate the mitochondrial alternative oxidase pathway (AOX). On the contrary, overexpression of AtPAO3 results to an increased but balanced production of both H2O2 and [Formula: see text] . These results suggest that the ratio of [Formula: see text] /H2O2 regulates respiratory chain in mitochondria, with PA-dependent production of [Formula: see text] by NADPH-oxidase tilting the balance of electron transfer chain in favor of the AOX pathway. In addition, AtPAO3 seems to be an important component in the regulating module of ROS homeostasis, while a conserved role for PA backconversion and ROS across kingdoms is discussed.

Coexpression network analysis associated with call of rice seedlings for encountering heat stress

Coexpression network analysis is useful tool for identification of functional association of coexpressed genes. We developed a coexpression network of rice from heat stress transcriptome data. Global transcriptome of rice leaf tissues was performed by microarray at three time points--post 10 and 60 min heat stress at 42 °C and 30 min recovery at 26 °C following 60 min 42 °C heat stress to investigate specifically the early events in the heat stress and recovery response. The transcriptome profile was significantly modulated within 10 min of heat stress. Strikingly, the number of up-regulated genes was higher than the number of down-regulated genes in 10 min of heat stress. The enrichment of GO terms protein kinase activity/protein serine threonine kinase activity, response to heat and reactive oxygen species in up-regulated genes after 10 min signifies the role of signal transduction events and reactive oxygen species during early heat stress. The enrichment of transcription factor (TF) binding sites for heat shock factors, bZIPs and DREBs coupled with up-regulation of TFs of different families suggests that the heat stress response in rice involves integration of various regulatory networks. The interpretation of microarray data in the context of coexpression network analysis identified several functionally correlated genes consisting of previously documented heat upregulated genes as well as new genes that can be implicated in heat stress. Based on the findings on parallel analysis of growth of seedlings, associated changes in transcripts of selected Hsps, genome-wide microarray profiling and the coexpression network analysis, this study is a step forward in understanding heat response of rice, the world's most important food crop.

Expression and signal regulation of the alternative oxidase genes under abiotic stresses

Plants in their natural environment frequently face various abiotic stresses, such as drought, high salinity, and chilling. Plant mitochondria contain an alternative oxidase (AOX), which is encoded by a small family of nuclear genes. AOX genes have been shown to be highly responsive to abiotic stresses. Using transgenic plants with varying levels of AOX expression, it has been confirmed that AOX genes are important for abiotic stress tolerance. Although the roles of AOX under abiotic stresses have been extensively studied and there are several excellent reviews on this topic, the differential expression patterns of the AOX gene family members and the signal regulation of AOX gene(s) under abiotic stresses have not been extensively summarized. Here, we review and discuss the current progress of these two important issues.

Alternative oxidase: a mitochondrial respiratory pathway to maintain metabolic and signaling homeostasis during abiotic and biotic stress in plants

Alternative oxidase (AOX) is a non-energy conserving terminal oxidase in the plant mitochondrial electron transport chain. While respiratory carbon oxidation pathways, electron transport, and ATP turnover are tightly coupled processes, AOX provides a means to relax this coupling, thus providing a degree of metabolic homeostasis to carbon and energy metabolism. Beside their role in primary metabolism, plant mitochondria also act as "signaling organelles", able to influence processes such as nuclear gene expression. AOX activity can control the level of potential mitochondrial signaling molecules such as superoxide, nitric oxide and important redox couples. In this way, AOX also provides a degree of signaling homeostasis to the organelle. Evidence suggests that AOX function in metabolic and signaling homeostasis is particularly important during stress. These include abiotic stresses such as low temperature, drought, and nutrient deficiency, as well as biotic stresses such as bacterial infection. This review provides an introduction to the genetic and biochemical control of AOX respiration, as well as providing generalized examples of how AOX activity can provide metabolic and signaling homeostasis. This review also examines abiotic and biotic stresses in which AOX respiration has been critically evaluated, and considers the overall role of AOX in growth and stress tolerance.