Flexible change and cooperation between mitochondrial electron transport and cytosolic glycolysis as the basis for chilling tolerance in tomato plants

Regulation of ethylene metabolism in tomato under salinity stress involving linkages with important physiological signaling pathways

The tomato is well-known for its anti-oxidative and anti-cancer properties, and with a wide range of health benefits is an important cash crop for human well-being. However, environmental stresses (especially abiotic) are having a deleterious effect on plant growth and productivity, including tomato. In this review, authors describe how salinity stress imposes risk consequences on growth and developmental processes of tomato through toxicity by ethylene (ET) and cyanide (HCN), and ionic, oxidative, and osmotic stresses. Recent research has clarified how salinity stress induced-ACS and - β-CAS expressions stimulate the accumulation of ET and HCN, wherein the action of salicylic acid (SA),compatible solutes (CSs), polyamines (PAs) and ET inhibitors (ETIs) regulate ET and HCN metabolism. Here we emphasize how ET, SA and PA cooperates with mitochondrial alternating oxidase (AOX), salt overly sensitive (SOS) pathways and the antioxidants (ANTOX) system to better understand the salinity stress resistance mechanism. The current literature evaluated in this paper provides an overview of salinity stress resistance mechanism involving synchronized routes of ET metabolism by SA and PAs, connecting regulated network of central physiological processes governing through the action of AOX, β-CAS, SOS and ANTOX pathways, which might be crucial for the development of tomato.

Adaptive strategies of plants to conserve internal phosphorus under P deficient condition to improve P utilization efficiency

Phosphorus (P) is one of the limiting factors for plant growth and productivity due to its slow diffusion and immobilization in the soil which necessitates application of phosphatic fertilizers to meet the crop demand and obtain maximum yields. However, plants have evolved mechanisms to adapt to low P stress conditions either by increasing acquisition (alteration of belowground processes) or by internal inorganic P (Pi) utilization (cellular Pi homeostasis) or both. In this review, we have discussed the adaptive strategies that conserve the use of P and maintain cellular Pi homeostasis in the cytoplasm. These strategies involve modification in membrane lipid composition, flavanol/anthocyanin level, scavenging and reutilization of Pi adsorbed in cell wall pectin, remobilization of Pi during senescence by enzymes like RNases and purple acid phosphatases, alternative mitochondrial electron transport, and glycolytic pathways. The remobilization of Pi from senescing tissues and its internal redistribution to various cellular organelles is mediated by various Pi transporters. Although much efforts have been made to enhance P acquisition efficiency, an understanding of the physiological mechanisms conserving internal Pi and their manipulation would be useful for plants that can utilize P more efficiently to produce optimum growth per unit P uptake.

Parental drought priming enhances tolerance to low temperature in wheat (Triticum aestivum) offspring

Low temperature is one of the major environmental stresses that limit crop growth and grain yield in wheat (Triticum aestivum L.). Drought priming at the vegetative stage could enhance wheat tolerance to later cold stress; however, the transgenerational effects of drought priming on wheat offspring's cold stress tolerance remains unclear. Here, the low temperature responses of offspring were tested after the parental drought priming treatment at grain filling stage. The offspring plants from parental drought priming treatment had a higher abscisic acid (ABA) level and lower osmotic potential (Ψo) than the control plants under cold conditions. Moreover, parental drought priming increased the antioxidant enzyme activities and decreased hydrogen peroxide (H2 O2 ) accumulation in offspring. In comparison to control plants, parental drought priming plants had a higher ATP concentration and higher activities of ATPase and the enzymes involved in sucrose biosynthesis and starch metabolism. The results indicated that parental drought priming induced low temperature tolerance in offspring by regulating endogenous ABA levels and maintaining the redox homeostasis and the balance of carbohydrate metabolism, which provided a potential approach for cold resistant cultivation in wheat.

Metabolic Mechanism of Plant Defense against Rice Blast Induced by Probenazole

The probenazole fungicide is used for controlling rice blast (Magnaporthe grisea) primarily by inducing disease resistance of the plant. To investigate the mechanism of induced plant defense, rice seedlings were treated with probenazole at 15 days post emergence, and non-treated plants were used for the control. The plants were infected with M. grisea 5 days after chemical treatment and incubated in a greenhouse. After 7 days, rice seedlings were sampled. The metabolome of rice seedlings was chemically extracted and analyzed using gas chromatography and mass spectrum (GC-MS). The GC-MS data were processed using analysis of variance (ANOVA), principal component analysis (PCA) and metabolic pathway elucidation. Results showed that probenazole application significantly affected the metabolic profile of rice seedlings, and the effect was proportionally leveraged with the increase of probenazole concentration. Probenazole resulted in a change of 54 metabolites. Salicylic acid, γ-aminobutyrate, shikimate and several other primary metabolites related to plant resistance were significantly up-regulated and some metabolites such as phenylalanine, valine and proline were down-regulated in probenazole-treated seedlings. These results revealed a metabolic pathway of rice seedlings induced by probenazole treatment regarding the resistance to M. grisea infection.

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.

Developing Climate-Resilient Chickpea Involving Physiological and Molecular Approaches With a Focus on Temperature and Drought Stresses

Chickpea is one of the most economically important food legumes, and a significant source of proteins. It is cultivated in more than 50 countries across Asia, Africa, Europe, Australia, North America, and South America. Chickpea production is limited by various abiotic stresses (cold, heat, drought, salt, etc.). Being a winter-season crop in northern south Asia and some parts of the Australia, chickpea faces low-temperature stress (0-15°C) during the reproductive stage that causes substantial loss of flowers, and thus pods, to inhibit its yield potential by 30-40%. The winter-sown chickpea in the Mediterranean, however, faces cold stress at vegetative stage. In late-sown environments, chickpea faces high-temperature stress during reproductive and pod filling stages, causing considerable yield losses. Both the low and the high temperatures reduce pollen viability, pollen germination on the stigma, and pollen tube growth resulting in poor pod set. Chickpea also experiences drought stress at various growth stages; terminal drought, along with heat stress at flowering and seed filling can reduce yields by 40-45%. In southern Australia and northern regions of south Asia, lack of chilling tolerance in cultivars delays flowering and pod set, and the crop is usually exposed to terminal drought. The incidences of temperature extremes (cold and heat) as well as inconsistent rainfall patterns are expected to increase in near future owing to climate change thereby necessitating the development of stress-tolerant and climate-resilient chickpea cultivars having region specific traits, which perform well under drought, heat, and/or low-temperature stress. Different approaches, such as genetic variability, genomic selection, molecular markers involving quantitative trait loci (QTLs), whole genome sequencing, and transcriptomics analysis have been exploited to improve chickpea production in extreme environments. Biotechnological tools have broadened our understanding of genetic basis as well as plants' responses to abiotic stresses in chickpea, and have opened opportunities to develop stress tolerant chickpea.

Cysteine and methionine contribute differentially to regulate alternative oxidase in leaves of poplar (Populus deltoides x Populus euramericana 'Nanlin 895') seedlings exposed to different salinity

The effects of different doses of NaCl on the expression profiles of genes involved in the mitochondrial electron transport chain (miETC), H₂O₂ and O₂^- levels, and antioxidant enzymes and amino acid metabolism were investigated in the leaves of poplar (Populus deltoides x Populus euramericana 'Nanlin 895'). In the miETC, complexes II and III and bypasses of the cytochrome c pathway including AOX and UCP displayed higher transcript abundance, whereas COX6b encoding cytochrome c oxidase were suppressed at 200 and 400 mM. H₂O₂ accumulated at 200 mM NaCl but O₂^- was generated at 400 mM. Accordingly, CAT was enhanced at 200 and 400 mM, while G-POD strengthened only at 400 mM. In addition, cysteine was reduced at 400 mM but did not change at 200 mM, although methionine was accumulated at 200 mM but not altered at 400 mM. Exogenous cysteine accumulated H₂S and methionine increased ACC at 200 mM NaCl. At 400 mM NaCl, cysteine elevated the expression of CGS encoding cystathionine gamma-synthase and MS2 encoding methionine synthase as well as ACC and H₂S levels, and methionine increased ACC content with repressed CGS and MS2. Moreover, exogenous KCN decreased cysteine levels, with an augment in H₂S and up-regulation of CYS C1 encoding β-cyanoalanine synthase at all salinity conditions, whereas antimycin A (AA) and salicylhydroxamic acid (SHAM) affected neither the levels of cysteine or H₂S, nor the CYS C1 expression. However, neither KCN, AA nor SHAM affected ACC content. AOX1b was induced both by exogenous cysteine and methionine as well as KCN and AA but suppressed by SHAM at 200 and 400 mM NaCl, in negative correlation with MDA content. These results suggest that poplar leaf evolved diverse strategies in amino acid metabolism of manipulating the AOX pathway to defend against different levels of salt stress.

The sphingolipid biosynthetic enzyme Sphingolipid delta8 desaturase is important for chilling resistance of tomato

The physiological functions of sphingolipids in animals have been intensively studied, while less attention has been paid to their roles in plants. Here, we reveal the involvement of sphingolipid delta8 desaturase (SlSLD) in the chilling resistance of tomato (Solanum lycopersicum cv. Micro-Tom). We used the virus-induced gene silencing (VIGS) approach to knock-down SlSLD expression in tomato leaves, and then evaluated chilling resistance. Changes in leaf cell structure under a chilling treatment were observed by transmission electron microscopy. In control plants, SlSLD was highly expressed in the fruit and leaves in response to a chilling treatment. The degree of chilling damage was greater in SlSLD-silenced plants than in control plants, indicating that SlSLD knock-down significantly reduced the chilling resistance of tomato. Compared with control plants, SlSLD-silenced plants showed higher relative electrolytic leakage and malondialdehyde content, and lower superoxide dismutase and peroxidase activities after a chilling treatment. Chilling severely damaged the chloroplasts in SlSLD-silenced plants, resulting in the disruption of chloroplast membranes, swelling of thylakoids, and reduced granal stacking. Together, these results show that SlSLD is crucial for chilling resistance in tomato.

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.

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.

Stimulated leaf dark respiration in tomato in an elevated carbon dioxide atmosphere

It is widely accepted that leaf dark respiration is a determining factor for the growth and maintenance of plant tissues and the carbon cycle. However, the underlying effect and mechanism of elevated CO2 concentrations ([CO2]) on dark respiration remain unclear. In this study, tomato plants grown at elevated [CO2] showed consistently higher leaf dark respiratory rate, as compared with ambient control plants. The increased respiratory capacity was driven by a greater abundance of proteins, carbohydrates, and transcripts involved in pathways of glycolysis carbohydrate metabolism, the tricarboxylic acid cycle, and mitochondrial electron transport energy metabolism. This study provides substantial evidence in support of the concept that leaf dark respiration is increased by elevated [CO2] in tomato plants and suggests that the increased availability of carbohydrates and the increased energy status are involved in the increased rate of dark respiration in response to elevated [CO2].

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.