Significance: For a plant to grow and develop, energy and appropriate building blocks are a fundamental requirement. Mitochondrial respiration is a vital source for both. The delicate redox processes that make up respiration are affected by the plant's changing environment. Therefore, mitochondrial regulation is critically important to maintain cellular homeostasis. This involves sensing signals from changes in mitochondrial physiology, transducing this information, and mounting tailored responses, by either adjusting mitochondrial and cellular functions directly or reprogramming gene expression. Recent Advances: Retrograde (RTG) signaling, by which mitochondrial signals control nuclear gene expression, has been a field of very active research in recent years. Nevertheless, no mitochondrial RTG-signaling pathway is yet understood in plants. This review summarizes recent advances toward elucidating redox processes and other bioenergetic factors as a part of RTG signaling of plant mitochondria. Cri...

/pdf/mitochondrial-energy-and-redox-signaling-in-plants-5bjsmezsn9.pdf

Mitochondrial Energy and Redox Signaling in Plants

Ascorbic acid (AA) currently holds a significant position in plant physiology, mainly due to its possession of antioxidant and cellular reductant etc.properties and its diverse roles in plant growth and development and the regulation of a broad spectrum of plant cellular mechanisms against environmental stresses. Some researchers suggest that endogenous AA has been implicated in the promotion of plant growth and development by involvement in a complex and enigmatic array of phytohormone-regulated signalling networks that ties together different environmental stresses. As it is evident from the present review, recent progress on AA potentiality in the tolerance of plants to environmental stresses has been impressive. Indeed, AA plays an important role in resistance to oxidative stresses such as heavy metal, saline, ultra-violet etc. Rapidly increasing evidence indicates that AA is centrally involved in several physiological processes but there has been much disagreement regarding the mechanism(s) by which AA reduces the damaging effects of such stresses in plants. Perhaps the role of AA in mediating tolerance to abiotic stress (e.g. UV, salinity and temperature, etc.) will lead to a greater research focus in the near future. In addition, AA might provide a suitably attractive target for the enhancement of crop production.

A review of ascorbic acid potentialities against oxidative stress induced in plants

Drought is one of the abiotic stresses which impairs the plant growth/development and restricts the yield of many crops throughout the world. Stomatal closure is a common adaptation response of plants to the onset of drought condition. Stomata are microscopic pores on the leaf epidermis, which regulate the transpiration/CO2 uptake by leaves. Stomatal guard cells can sense various abiotic and biotic stress stimuli from the internal and external environment and respond quickly to initiate closure under unfavorable conditions. Stomata also limit the entry of pathogens into leaves, restricting their invasion. Drought is accompanied by the production and/or mobilization of the phytohormone, abscisic acid (ABA), which is well-known for its ability to induce stomatal closure. Apart from the ABA, various other factors that accumulate during drought and affect the stomatal function are plant hormones (auxins, MJ, ethylene, brassinosteroids, and cytokinins), microbial elicitors (salicylic acid, harpin, Flg 22, and chitosan), and polyamines . The role of various signaling components/secondary messengers during stomatal opening or closure has been a matter of intense investigation. Reactive oxygen species (ROS), nitric oxide (NO), cytosolic pH, and calcium are some of the well-documented signaling components during stomatal closure. The interrelationship and interactions of these signaling components such as ROS, NO, cytosolic pH, and free Ca2+ are quite complex and need further detailed examination.

Mechanism of Stomatal Closure in Plants Exposed to Drought and Cold Stress.

L-ascorbic acid (vitamin C) is a major antioxidant in plants and plays a significant role in mitigation of excessive cellular reactive oxygen species activities caused by number of abiotic stresses. Plant ascorbate levels change differentially in response to varying environmental stress conditions, depending on the degree of stress and species sensitivity. Successful modulation of ascorbate biosynthesis through genetic manipulation of genes involved in biosynthesis, catabolism and recycling of ascorbate has been achieved. Recently, role of ascorbate in alleviating number of abiotic stresses has been highlighted in crop plants. In this article, we discuss the current understanding of ascorbate biosynthesis and its antioxidant role in order to increase our comprehension of how ascorbate helps plants to counteract or cope with various abiotic stresses.

/pdf/role-of-l-ascorbate-in-alleviating-abiotic-stresses-in-crop-4e9ne28rwv.pdf

Role of L-ascorbate in alleviating abiotic stresses in crop plants

Mitochondrial respiration provides the energy needed to drive metabolic and transport processes in cells. Mitochondria are a significant site of reactive oxygen species (ROS) production in plant cells, and redox-system components obey fine regulation mechanisms that are essential in protecting the mitochondrial integrity. In addition to ROS, there are compelling indications that nitric oxide can be generated in this organelle by both reductive and oxidative pathways. ROS and reactive nitrogen species play a key role in signaling but they can also be deleterious via oxidation of macromolecules. The high production of ROS obligates mitochondria to be provided with a set of ROS scavenging mechanisms. The first line of mitochondrial antioxidants is composed of superoxide dismutase and the enzymes of the ascorbate-glutathione cycle, which are not only able to scavenge ROS but also to repair cell damage and possibly serve as redox sensors. The dithiol-disulfide exchanges form independent signaling nodes and act as antioxidant defense mechanisms as well as sensor proteins modulating redox signaling during development and stress adaptation. The presence of thioredoxin (Trx), peroxiredoxin (Prx) and sulfiredoxin (Srx) in the mitochondria has been recently reported. Cumulative results obtained from studies in salt stress models have demonstrated that these redox proteins play a significant role in the establishment of salt tolerance. The Trx/Prx/Srx system may be subjected to a fine regulated mechanism involving post-translational modifications, among which S-glutathionylation and S-nitrosylation seem to exhibit a critical role that is just beginning to be understood. This review summarizes our current knowledge in antioxidative systems in plant mitochondria, their interrelationships, mechanisms of compensation and some unresolved questions, with special focus on their response to abiotic stress.

/pdf/dissecting-the-integrative-antioxidant-and-redox-systems-in-468f97grhi.pdf

Dissecting the integrative antioxidant and redox systems in plant mitochondria. Effect of stress and S-nitrosylation.

The possible role of L-ascorbate (AsA) as a biochemical signal during the interactions between photosynthesis and respiration was examined in leaf discs of Arabidopsis thaliana. AsA content was either decreased as in AsA-deficient vtc1 mutants or increased by treatment with L-galactono-1, 4-lactone (L-GalL, a precursor of AsA; EC 1.3.2.3). In mutants, photosynthesis was extremely sensitive to both antimycin A (inhibitor of the cytochrome c oxidase pathway [COX pathway]) and salicylhydroxamic acid (SHAM, inhibitor of the alternative pathway [AOX pathway]), particularly at high light conditions. Mitochondrial inhibitors lowered the ratio of reduced AsA to total AsA, at high light, indicating oxidative stress in leaf discs. Elevation of AsA by L-GalL decreased the sensitivity of photosynthesis at high light to antimycin A or SHAM, sustained photosynthesis at supraoptimal light and relieved the extent of photoinhibition. High ratios of reduced AsA to total AsA in L-GalL-treated leaf discs suggests that L-GalL lowers oxidative stress. The protection by L-GalL of photosynthesis against the mitochondrial inhibitors and photoinhibition was quite pronounced in vtc1 mutants. Our results suggest that the levels and redox state of AsA modify the pattern of modulation of photosynthesis by mitochondrial metabolism. The extent of the AOX pathway as a percentage of the total respiration in Arabidopsis mesophyll protoplasts was much higher in vtc1 than in wild type. We suggest that the role of AsA becomes pronounced at high light and/or when the AOX pathway is inhibited. While acknowledging the importance of the COX pathway, we hypothesize that AsA and the AOX pathway may complement each other to protect photosynthesis against photoinhibition.

Ascorbic acid is a key participant during the interactions between chloroplasts and mitochondria to optimize photosynthesis and protect against photoinhibition

Oxidative stress can occur in different parts of plant cells We employed two oxidants that induce reactive oxygen species (ROS) in different intracellular compartments: methyl viologen (MV, in chloroplasts) and menadione (MD, in mitochondria) The responses of pea (Pisum sativum) leaf discs to MV or MD after 4-h incubation in dark or moderate (300 μE m−2 s−1) or high light (1200 μE m−2 s−1) were examined Marked increase in ROS levels was observed, irrespective of compartment targeted The levels of proline, a compatible solute, increased markedly much more than that of ascorbate or glutathione during oxidative/photo-oxidative stress, emphasizing the importance of proline Further, the activities and transcripts of enzymes involved in biosynthesis or oxidation of proline were studied An upregulation of biosynthesis and downregulation of oxidation was the basis of proline accumulation Pyrroline-5-carboxylate synthetase (P5CS, involved in biosynthesis) and proline dehydrogenase (PDH, involved in oxidation) were the key enzymes regulated under oxidative stress Since these two enzymes—P5CS and PDH—are located in chloroplasts and mitochondria, respectively, we suggest that proline metabolism can help to mediate inter-organelle interactions and achieve redox homeostasis under photo-oxidative stress

Oxidative stress induced in chloroplasts or mitochondria promotes proline accumulation in leaves of pea (Pisum sativum): another example of chloroplast-mitochondria interactions.

Main conclusion
Phyto-S1P and S1P induced stomatal closure in epidermis of pea (Pisum sativum) by raising the levels of NO and pH in guard cells.

Phosphosphingolipids, such as phytosphingosine-1-phosphate (phyto-S1P) and sphingosine-1-phosphate (S1P), are important signaling components during drought stress. The biosynthesis of phyto-S1P or S1P is mediated by sphingosine kinases (SPHKs). Although phyto-S1P and S1P are known to be signaling components in higher plants, their ability to induce stomatal closure has been ambiguous. We evaluated in detail the effects of phyto-S1P, S1P and SPHK inhibitors on signaling events leading to stomatal closure in the epidermis of Pisum sativum. Phyto-S1P or S1P induced stomatal closure, along with a marked rise in nitric oxide (NO) and cytoplasmic pH of guard cells, as in case of ABA. Two SPHK inhibitors, DL-threo dihydrosphingosine and N’,N’-dimethylsphingosine, restricted ABA-induced stomatal closure and prevented the increase of NO or pH by ABA. Modulators of NO or pH impaired both stomatal closure and increase in NO or pH by phyto-S1P/S1P. The stomatal closure by phyto-S1P/S1P was mediated by phospholipase D and phosphatidic acid (PA). When present, PA elevated the levels of pH, but not NO of guard cells. Our results demonstrate that stomatal closure induced by phyto-S1P and S1P depends on rise in pH as well as NO of guard cells. A scheme of signaling events initiated by phyto-S1P/S1P, and converging to cause stomatal closure, is proposed.

Stomatal closure induced by phytosphingosine-1-phosphate and sphingosine-1-phosphate depends on nitric oxide and pH of guard cells in Pisum sativum.

Photorespiration, an essential component of plant metabolism, is concerted across four subcellular compartments, namely, chloroplast, peroxisome, mitochondrion, and the cytoplasm. It is unclear how the pathway located in different subcellular compartments respond to stress occurring exclusively in one of those. We attempted to assess the inter-organelle interaction during the photorespiratory pathway. For that purpose, we induced oxidative stress by menadione (MD) in mitochondria and photo-oxidative stress (high light) in chloroplasts. Subsequently, we examined the changes in selected photorespiratory enzymes, known to be located in other subcellular compartments. The presence of MD upregulated the transcript and protein levels of five chosen photorespiratory enzymes in both normal and high light. Peroxisomal glycolate oxidase and catalase activities increased by 50% and 25%, respectively, while chloroplastic glycerate kinase and phosphoglycolate phosphatase increased by ~30%. The effect of MD was maximum in high light, indicating photo-oxidative stress was an influential factor to regulate photorespiration. Oxidative stress created in mitochondria caused a coordinative upregulation of photorespiration in other organelles. We provided evidence that reactive oxygen species are important signals for inter-organelle communication during photorespiration. Thus, MD can be a valuable tool to modulate the redox state in plant cells to study the metabolic consequences across membranes.

Pidakala Rajsheel

Papers

Ascorbic acid is a key participant during the interactions between chloroplasts and mitochondria to optimize photosynthesis and protect against photoinhibition

Oxidative stress induced in chloroplasts or mitochondria promotes proline accumulation in leaves of pea (Pisum sativum): another example of chloroplast-mitochondria interactions.

Stomatal closure induced by phytosphingosine-1-phosphate and sphingosine-1-phosphate depends on nitric oxide and pH of guard cells in Pisum sativum.

Modulation of Photorespiratory Enzymes by Oxidative and Photo-Oxidative Stress Induced by Menadione in Leaves of Pea (Pisum sativum).

Photosynthesis is sensitive to nitric oxide and respiration sensitive to hydrogen peroxide: Studies with pea mesophyll protoplasts.