Dhriti Kapoor

Redox homeostasis in plants under abiotic stress: role of electron carriers, energy metabolism mediators and proteinaceous thiols

Contemporaneous presence of both oxidized and reduced forms of electron carriers is mandatory in efficient flux by plant electron transport cascades. This requirement is considered as redox poising that involves the movement of electron from multiple sites in respiratory and photosynthetic electron transport chains to molecular oxygen. This flux triggers the formation of superoxide, consequently give rise to other reactive oxygen species (ROS) under adverse environmental conditions like drought, high or low temperature, heavy metal stress etc. that plants owing during their life span. Plant cells synthesize ascorbate, an additional hydrophilic redox buffer, which protect the plants against oxidative challenge. Large pools of antioxidants also preside over the redox homeostasis. Besides, tocopherol is a liposoluble redox buffer, which efficiently scavenges the ROS like singlet oxygen. In addition, proteinaceous thiol members such as thioredoxin, peroxiredoxin and glutaredoxin, electron carriers and energy metabolism mediators phosphorylated (NADP) and non-phosphorylated (NAD+) coenzyme forms interact with ROS, metabolize and maintain redox homeostasis.

Physiological and Biochemical Changes in Brassica juncea Plants under Cd-Induced Stress

Plants of Brassica juncea L. var. RLC-1 were exposed for 30 days to different concentrations (0, 0.2, 0.4, and 0.6 mM) of cadmium (Cd) to analyze the Cd uptake, H2O2 content, hormonal profiling, level of photosynthetic pigments (chlorophyll, carotenoid, and flavonoid), gaseous exchange parameters (photosynthetic rate, vapour pressure deficit, intercellular CO2 concentration, and intrinsic mesophyll rate), antioxidative enzymes (superoxide dismutase, polyphenol oxidase, glutathione-S transferase, and glutathione peroxidase), antioxidant assays (DPPH, ABTS, and total phenolic content), and polyphenols. Results of the present study revealed the increased H2O2 content and Cd uptake with increasing metal doses. UPLC analysis of plants showed the presence of various polyphenols. Gaseous exchange measurements were done by infrared gas analyzer (IRGA), which was negatively affected by metal treatment. In addition, LC/MS study showed the variation in the expression of plant hormones. Level of photosynthetic pigments and activities of antioxidative enzymes were altered significantly in response to metal treatment. In conclusion, the antioxidative defence system of plants got activated due to heavy metal stress, which protects the plants by scavenging free radicals.

LEA Proteins in Salt Stress Tolerance

In late embryogenesis, the water content of living cell is reduced tremendously that leads to a state of dehydration and thus, might impose severe irreparable damage to cellular and macromolecular structures. However, the mature orthodox seeds can withstand severe desiccation due to role of osmoprotectants viz., reducing sugars, prolines, glycinebetaines or Late Embryogenesis Abundant (LEA) proteins. These operate on the virtue of intrinsic molecular mechanisms that alleviate multiple abiotic stresses in plants such as protein desiccation, membrane degradation, salt stress and cold and chilling stress. The LEA proteins are a group of versatile, adaptive, hydrophilic proteins considerably defined as ‘molecular shields’ for their anti-stress properties attributable to partial or complete structural randomness. On the basis of their amino acid composition and sequencing, LEA proteins have been clubbed into seven groups that are further sub divided into a number of protein sub families. Out of these, Group 2 LEA proteins called the ‘Dehydrins’ are of prime importance in the plant kingdom. The latent and unique stress remediating characteristics of this class of proteins has been further enhanced by transgenic studies, wherein the target LEA genes have been identified, sequenced to understand their molecular role in plants. Further investigations into the behavior of LEA proteins and mode of their regulation in stressed plants will facilitate in elucidating the function of LEA proteins. The present chapter reviews the versatility and role of LEA proteins in plant stress protection.

Aquaporins: Role Under Salt Stress in Plants

In living cells, water flows via apoplastic path (across cell walls), symplastic path (from cell to cell by plasmodesmata) or transcellular path (traversing the cell membranes). Detailed studies on water relations revealed the essential class of water channel proteins known as aquaporins (AQPs) that facilitate water-conductance. These are membrane channels with a conserved structure and play a crucial role in transport of water, solutes such as urea, boric acid, silicic acid or gases (ammonia, carbon dioxide). AQPs exhibit a high isoform multiplicity, and transport activity can be regulated by multiple mechanisms viz. protein abundance, regulation of transcript, subcellular trafficking or cytosolic protons. Besides, AQPs mediate the regulation of water transport in response to various environmental cues and hence play an important role in stressed conditions thereby making them an essential class of plant proteins. The present review highlights the structural diversity, regulation of AQPs and physiological roles of AQPs in plant stress tolerance to environmental stimuli.

Selenium: An Antioxidative Protectant in Plants Under Stress

Abstract Selenium (Se), the sister element of sulfur, has gained importance recently as an essential trace element in the plant systems. It uses the transporters of sulfur for its uptake and similar biochemical pathways for its metabolism, which leads to its incorporation in various biomolecules. The wide range of beneficial effects of Se on plants has been established. Low concentrations of Se have been proven to enhance seed germination, growth, photosynthesis, respiratory potential, yield, etc. Its role in protecting the plants against various types of biotic and abiotic stresses has become an area of active research. Various studies have revealed its direct effect on antioxidative defense system thereby increasing the potential of the plants to combat the stressful conditions. Despite of its well documented positive effects, Se still is an intermediate between being beneficial or harmful because of its toxic effects at higher concentrations. Therefore, concentration of Se to be used, is still a matter of contemplation and research.

Alleviation of Cadmium and Mercury Stress by Supplementation of Steroid Hormone to Raphanus sativus Seedlings

The present work was undertaken to study the effects of 24-epibrassinolide (24-EBL) on percent germination, growth (root length, shoot length, fresh weight and dry weight), lipid peroxidation, sodium and potassium ion concentrations, proline content, total osmolyte content, level of antioxidants (ascorbic acid, tocopherol and glutathione) in 7-day old seedlings of Raphanus sativus exposed to cadmium and mercury toxicity. Results of present study revealed that growth of seedlings was enhanced with the treatment of 24-EBL. In addition, 24-EBL was proved effective to overcome cadmium and mercury stress by altering the level of ions, osmoprotectants and antioxidants of plant.

Brassinosteroids: Improving Crop Productivity and Abiotic Stress Tolerance

Brassinosteroids (BRs), a group of phyto-steroidal hormones, play crucial role in a wide spectrum of biochemical, physiological, growth, and developmental processes in plants. Recent molecular and physiological studies have further revealed the efficacy of BRs in improving the crop yield and productivity by controlling several genes regulated at cellular and sub-cellular levels. BRs also regulate several important agronomic traits like rhizogenesis, senescence, abscission, flower and fruit development, fruit ripening, flower sex expression, yield and quality of seed, grain, and fiber in horticultural and cash crops. These eco-friendly, nontoxic, and biosafe steroidal phytohormones when applied at specific dose at specific stage of development of specific crop enhanced quantity and quality of field-grown crops. Moreover, BRs also possess anti-ecdysteroidal, antiviral, and antifungal properties, and thus are considered as potential substitute to conventional pesticide, insecticide, and herbicide. Therefore, the present book chapter will update the knowledge of BRs in improving crop yield by ameliorating abiotic stresses such as salinity, drought, extreme temperatures, heavy metals, and pesticides. Besides, the current research has highlighted BRs-mediated cell elongation in vegetative organs as well as meristem homeostasis in plants. Various studies on BRs-related mutants indicated that steady BRs signaling is required for the optimal root growth, which further emphasized the indispensable roles of BRs in the regulation of the cell-cycle progression and differentiation in plants. Furthermore, the current chapter focuses on the exogenous application of effective doses of BRs to stress-affected plants as better and simple alternative approach for protecting plants from environmental stresses in comparison to the regular plant breeding practices.

Salicylic Acid: It’s Physiological Role and Interactions

Salicylic acid is a lipophillic monohydroxybenzoic acid, a type of phenolic acid and a beta hydroxyl acid (BHA) with formula C7H6O3. This colourless crystalline organic acid is widely used in organic synthesis and functions as growth hormone. The Salicylic acid plays a vital role in the regulation of plant growth, developmental, and other physiological activities. It provides a defense response towards the biotic and abiotic stresses. Its role is evident in seed germination, glycolysis, flowering, fruit yield, ion uptake, photosynthetic rate, stomatal conductance, transpiration and thermo-tolerance. Plants that are treated with SA exhibit high level of pigments which help in photosynthesis than those which are under control or salt stress. Salicylic acid action includes the development of antistress system in plants and acceleration of the normal growth of plants after the removal of stress factors. It plays an important role in boosting of antioxidative defence system in plants.

Cd-Metal Stress Protection in B. juncea triggered by the Synthesis of Osmolytes, Plant Growth Regulators and Antioxidants

The present piece of work was undertaken to determine the influence of cadmium (Cd) metal on H2O2, level of antioxidants (ascorbic acid and tocopherol), antioxidant assays (DPPH and ABTS), antioxidative enzyme activities (POD, APOX and GR), total osmolytes content, photosynthetic pigments (carotenoid and flavonoid contents), gaseous exchange parameters (photosynthetic rate and water use efficiency), level of Na+ and K+ ions and hormone profiling of 60 days old plants of Brassica juncea. This study demonstrated that Cd metal led to increase in the level of hydrogen peroxide. There was a reduction in the level of photosynthetic pigments, photosynthetic rate and water use efficiency due to Cd toxicity. However, enhancement in the antioxidant potential and level of osmolytes had been noticed in Cd treated plants. In addition, Cd toxicity caused the alteration in the content of ions and hormones.

Characterization and Antioxidant Activity of oil Extract of a Gymnosperm-Araucaria cunninghamii Aiton ex D. Don

The objectives of the present study were the investigation of antioxidant activity of oil extract of Araucaria cunninghamii Aiton ex D. Don and identification of classes of antioxidants with HPLC analysis. The antioxidant potential of the oil extract of Araucaria cunninghamii Aiton ex D. Don was evaluated by DPPH assay, reducing power assay and nitric oxide scavenging assay. Total antioxidant capacity, total phenolic and flavonoid content were also calculated. The oil extract of Araucaria cunninghamii Aiton ex D. Don was investigated for polyphenols qualitatively as well as quantitatively using High-performance liquid chromatography (HPLC). Oil extract of Araucaria cunninghamii Aiton ex D. Don showed antioxidant activity in different assays in a dose dependent manner. A positive correlation between the antioxidant activity and phenolic content was observed. HPLC studies revealed the presence of Gallic acid, catechin, chlorogenic acid, epicatechin, caffeic acid, umbelliferone, rutin, ellagic acid, quercetin, kaempferol in the oil extract. Results indicate that the oil extract of Araucaria cunninghamii Aiton ex D. Don possess antioxidant activities which can be correlated with the polyphenolic compounds identified by HPLC analysis.