Sarvajeet Singh Gill

Reactive oxygen species and antioxidant machinery in abiotic stress tolerance in crop plants

Various abiotic stresses lead to the overproduction of reactive oxygen species (ROS) in plants which are highly reactive and toxic and cause damage to proteins, lipids, carbohydrates and DNA which ultimately results in oxidative stress. The ROS comprises both free radical (O(2)(-), superoxide radicals; OH, hydroxyl radical; HO(2), perhydroxy radical and RO, alkoxy radicals) and non-radical (molecular) forms (H(2)O(2), hydrogen peroxide and (1)O(2), singlet oxygen). In chloroplasts, photosystem I and II (PSI and PSII) are the major sites for the production of (1)O(2) and O(2)(-). In mitochondria, complex I, ubiquinone and complex III of electron transport chain (ETC) are the major sites for the generation of O(2)(-). The antioxidant defense machinery protects plants against oxidative stress damages. Plants possess very efficient enzymatic (superoxide dismutase, SOD; catalase, CAT; ascorbate peroxidase, APX; glutathione reductase, GR; monodehydroascorbate reductase, MDHAR; dehydroascorbate reductase, DHAR; glutathione peroxidase, GPX; guaicol peroxidase, GOPX and glutathione-S- transferase, GST) and non-enzymatic (ascorbic acid, ASH; glutathione, GSH; phenolic compounds, alkaloids, non-protein amino acids and α-tocopherols) antioxidant defense systems which work in concert to control the cascades of uncontrolled oxidation and protect plant cells from oxidative damage by scavenging of ROS. ROS also influence the expression of a number of genes and therefore control the many processes like growth, cell cycle, programmed cell death (PCD), abiotic stress responses, pathogen defense, systemic signaling and development. In this review, we describe the biochemistry of ROS and their production sites, and ROS scavenging antioxidant defense machinery.

Polyamines and abiotic stress tolerance in plants

Environmental stresses including climate change, especially global warming, are severely affecting plant growth and productivity worldwide. It has been estimated that two-thirds of the yield potential of major crops are routinely lost due to the unfavorable environmental factors. On the other hand, the world population is estimated to reach about 10 billion by 2050, which will witness serious food shortages. Therefore, crops with enhanced vigour and high tolerance to various environmental factors should be developed to feed the increasing world population. Maintaining crop yields under adverse environmental stresses is probably the major challenge facing modern agriculture where polyamines can play important role. Polyamines (putrescine, spermidine and spermine) are group of phytohormone-like aliphatic amine natural compounds with aliphatic nitrogen structure and present in almost all living organisms including plants. Evidences showed that polyamines are involved in many physiological processes, such as cell growth and development and respond to stress tolerance to various environmental factors. In many cases the relationship of plant stress tolerance was noted with the production of conjugated and bound polyamines as well as stimulation of polyamine oxidation. Therefore, genetic manipulation of crop plants with genes encoding enzymes of polyamine biosynthetic pathways may provide better stress tolerance to crop plants. Furthermore, the exogenous application of PAs is also another option for increasing the stress tolerance potential in plants. Here, we have described the synthesis and role of various polyamines in abiotic stress tolerance in plants.

Cadmium stress tolerance in crop plants: probing the role of sulfur

Plants can’t move away and are therefore continuously confronted with unfavorable environmental conditions (such as soil salinity, drought, heat, cold, flooding and heavy metal contamination). Among heavy metals, cadmium (Cd) is a non-essential and toxic metal, rapidly taken up by roots and accumulated in various plant tissues which hamper the crop growth and productivity worldwide. Plants employ various strategies to counteract the inhibitory effect of Cd, among which nutrient management is one of a possible way to overcome Cd toxicity. Sulfur (S) uptake and assimilation are crucial for determining crop yield and resistance to Cd stress. Cd affects S assimilation pathway which leads to the activation of pathway responsible for the synthesis of cysteine (Cys), a precursor of glutathione (GSH) biosynthesis. GSH, a non-protein thiol acts as an important antioxidant in mitigating Cd-induced oxidative stress. It also plays an important role in phytochelatins (PCs) synthesis, which has a proven role in Cd detoxification. Therefore, S assimilation is considered a crucial step for plant survival under Cd stress. The aim of this review is to discuss the regulatory mechanism of S uptake and assimilation, GSH and PC synthesis for Cd stress tolerance in crop plants.

Unraveling the role of fungal symbionts in plant abiotic stress tolerance

Fungal symbionts have been found associated with every plant studied in natural ecosystem, where they colonize and reside entirely in the internal tissues of their host plant or partially. Fungal endophytes can express/form a range of different lifestyle/relationships with different host including symbiotic, mutualistic, commensalistic and parasitic in response to host genotype and environmental factors. In mutualistic association fungal endophyte can enhance growth, increase reproductive success and confer biotic and abiotic stress tolerance to its host plant. Since abiotic stress such as, drought, high soil salinity, heat, cold, oxidative stress, heavy metal toxicity is the common adverse environmental conditions that affect and limit crop productivity worldwide. It may be a promising alternative strategy to exploit fungal endophytes to overcome the limitations to crop production brought by abiotic stress. There is increasing interest in developing the potential biotechnological applications of fungal endophytes for improving plant stress tolerance and sustainable production of food crops. Here we have described the fungal symbioses, fungal symbionts and their role in abiotic stress tolerance. A putative mechanism of stress tolerance by symbionts has also been covered.

Cadmium at high dose perturbs growth, photosynthesis and nitrogen metabolism while at low dose it up regulates sulfur assimilation and antioxidant machinery in garden cress (Lepidium sativum L.).

Metal contamination of soils has become a worldwide problem and great environmental threat, as these metals accumulate in soils and plants in excess, and enter the food chain. Increased cadmium (Cd) uptake from contaminated soils leads to altered plant metabolism and limits the crop productivity. The experimental crop, Lepidium sativum L. (Garden Cress, Family: Brassicaceae) is a medicinally and economically important plant. An experiment was conducted to examine the effect of different concentrations of Cd (0, 25, 50 or 100 mg kg(-1) soil) on the performance of L. sativum. Cd accumulation in roots and leaves (roots>leaves) increased with the increaseing Cd concentration in soil. High Cd concentration (100mg Cd kg(-1) soil) inhibited the leaf area and plant dry mass and significant decline in net photosynthetic rate (P(N)), stomatal conductance (gs), intercellular CO(2) (Ci), chlorophyll (Chl a, Chl b, total Chl) content, carbonic anhydrase (CA; E.C. 4.2.1.1) activity, nitrate reductase (NR; E.C. 1.6.6.1) activity and nitrogen (N) content was also observed. However, ATP-sulfurylase (ATP-S; EC. 2.7.7.4) activity, sulfur (S) content and activities of antioxidant enzymes such as superoxide dismutase (SOD; E.C. 1.15.1.1); catalase (CAT; E.C. 1.11.1.6); ascorbate peroxidase (APX; E.C. 1.11.1.11) and glutathione reductase (GR; E.C. 1.6.4.2) and glutathione (GSH) content were increased. Specifically, the decrease in NR activity and N content showed that Cd affects N metabolism negatively; whereas, the increase in ATP-S activity and S content suggests the up-regulation of S assimilation pathway for possible Cd tolerance in coordination with enhanced activities of antioxidant enzymes and GSH. High Cd concentration (100mg Cd kg(-1) soil) perturbs the L. sativum growth by interfering with the photosynthetic machinery and disrupting the coordination between carbon, N and S metabolism. On the other hand, at low Cd concentration (25mg Cd kg(-1) soil) co-ordination of S and N metabolism complemented to the antioxidant machinery to protect the growth and photosynthesis of L. sativum plants.

Importance of nitric oxide in cadmium stress tolerance in crop plants

Cadmium (Cd(2+)) is a widespread heavy metal pollutant in the environment with a long biological half-life, originating mainly from industrial processes and phosphate fertilizers. It is easily taken up by plants, resulting in toxicity symptoms, such as chlorosis, wilting, growth reduction, and cell death. This cellular toxicity might result from interactions with vital metabolic pathways, carboxyl or thiol groups of proteins and reactive oxygen species (ROS) burst in plants. Plant exposure even to low concentrations of Cd may lead to cell death but the mechanism of its toxicity is still debatable. Therefore, exploring various ways to improve crop productivity and/or alleviate Cd stress effects is one of the major areas of concern. Nitric oxide (NO) is a hydrophobic gaseous molecule involved in various physiological processes such as germination, root growth, stomatal closure, control of the flowering timing etc. NO also functions as cell signaling molecule in plants and play important roles in the regulation of plant responses to both abiotic and biotic stress conditions. At the molecular level, NO signaling includes protein modification by binding to critical cysteine residues, heme or iron-sulfur centers and tyrosine residue nitration via peroxynitrite formation (ONOO(-)), mobilization of secondary messengers (Ca(2+), cyclic GMP and cyclic ADP-Rib) and modulation of protein kinase activities. Significant research had been done to understand the NO biosynthesis and signaling in plants under stress, but several questions still need to be answered. The present review is focused specifically on the importance of NO as Cd stress modulator in crop plants.

Eutrophication: causes, consequences and control

1. Eutrophication: Challenges and Solutions.- 2. Eutrophication: Global Scenario and Local Threat to Dynamics of Aquatic Ecosystems.- 3. Effects of Eutrophication.- 4. The Economics of Eutrophication.- 5. Eutrophication of Lakes.- 6. Lake Eutrophication and Plankton Food Webs.- 7. Environmental Impacts of Tourism on Lakes.- 8. Eutrophication in the Great Lakes of the Chinese Pacific Drainage Basin: Changes, Trends and Management.- 9. Photoautotrophic Productivity in Eutrophic Ecosystems.- 10. Seasonal and Spatial Nutrient Dynamics in Saronikos Gulf: The Impact of Sewage Effluents from Athens Sewage Treatment Plant.- 11. Eutrophication Impacts on Salt Marshes Natural Metal Remediation.- 12. Household Detergents Causing Eutrophication in Freshwater Ecosystems.- 13. Estimating Fish Production in the Itaipu Reservoir (Brazil): The Relationship between Fish Trophic Guilds, Limnology and Application of Morphoedaphic Index.- 14. Phytoplankton Assemblages as an Indicator of Water Quality in Seven Temperate Estuarine Lakes in South-East Australia.- 15. Biogeochemical Indicators of Nutrient Enrichments in Wetlands: The Microbial Response as a Sensitive Indicator of Wetland Eutrophication.- 16. Task of Mineral Nutrients in Eutrophication.- 17. Phytoremediation Systems for the Recovery of Nutrients from Eutrophic Waters.- 18. Ultra Violet Radiation and Bromide as Limiting Factors in Eutrophication Processes in Semi-Arid Climate Zones.

Silver nanoparticles in soil–plant systems

Silver nanoparticles (AgNPs) have broad spectrum antimicrobial/biocidal properties against all classes of microorganisms and possess numerous distinctive physico-chemical properties compared to bulk Ag. Hence, AgNPs are among the most widely used engineered NPs in a wide range of consumer products and are expected to enter natural ecosystems including soil via diverse pathways. However, despite: (i) soil has been considered as a critical pathway for NPs environmental fate, (ii) plants (essential base component of all ecosystems) have been strongly recommended to be included for the development of a comprehensive toxicity profile for rapidly mounting NPs in varied environmental compartments, and (iii) the occurrence of an intricate relationship between “soil–plant systems” where any change in soil chemical/biological properties is bound to have impact on plant system, the knowledge about AgNPs in soils and investigations on AgNPs–plants interaction is still rare and in its rudimentary stage. To this end, the current paper: (a) overviews sources, status, fate, and chemistry of AgNPs in soils, AgNPs-impact on soil biota, (b) critically discusses terrestrial plant responses to AgNPs exposure, and (c) illustrates the knowledge-gaps in the current perspective. Based on the available literature critically appraised herein, a multidisciplinary integrated approach is strongly recommended for future research in the current direction aimed at unveiling the rapidly mounting AgNPs-fate, transformation, accumulation, and toxicity potential in “soil–plant systems,” and their cumulative impact on environmental and human health.

Differential cadmium stress tolerance in five indian mustard (Brassica juncea L.) cultivars: An evaluation of the role of antioxidant machinery

The presence of Cadmium (Cd) in the agricultural soils affects horticultural cultivars and constrains the crop productivity. A pot experiment was performed using five cultivars of mustard (Brassica juncea L.) to evaluate the difference in their response to Cd toxicity under greenhouse conditions. The pots containing reconstituted soil were supplied with different concentration of CdCl2 (0, 25, 50, 100 or 150 mg Cd kg-1 soil). Increasing concentration of Cd in the soil resulted in decreased growth, photosynthesis and yield. Maximum significant reduction in growth, photosynthesis and yield were observed with 150 mg Cd kg-1 soil in all the cultivars. Our results indicate that the cultivar Alankar is found to be more tolerant to Cd stress, recording higher plant dry mass, net photosynthesis rate, associated with high antioxidant activity and low Cd content in the plant leaves and thus less oxidative damage. Cultivar RH30 experienced maximum damage in terms of reduction in growth, photosynthesis, yield characteristics and oxidative damage and emerged as sensitive cultivar. The data of tolerance index of Alankar were found to be higher among all tested mustard cultivars which indicate its higher tolerance to Cd. Better coordination of antioxidants protected Alankar from Cd toxicity, whereas lesser antioxidant activity in RH30 resulted in maximum damage. Cultivars of mustard were ranked with respect to their tolerance to Cd: Alankar>Varuna>Pusa Bold>Sakha>RH30, respectively.

Lipids and proteins—major targets of oxidative modifications in abiotic stressed plants

Stress factors provoke enhanced production of reactive oxygen species (ROS) in plants. ROS that escape antioxidant-mediated scavenging/detoxification react with biomolecules such as cellular lipids and proteins and cause irreversible damage to the structure of these molecules, initiate their oxidation, and subsequently inactivate key cellular functions. The lipid- and protein-oxidation products are considered as the significant oxidative stress biomarkers in stressed plants. Also, there exists an abundance of information on the abiotic stress-mediated elevations in the generation of ROS, and the modulation of lipid and protein oxidation in abiotic stressed plants. However, the available literature reflects a wide information gap on the mechanisms underlying lipid- and protein-oxidation processes, major techniques for the determination of lipid- and protein-oxidation products, and on critical cross-talks among these aspects. Based on recent reports, this article (a) introduces ROS and highlights their relationship with abiotic stress-caused consequences in crop plants, (b) examines critically the various physiological/biochemical aspects of oxidative damage to lipids (membrane lipids) and proteins in stressed crop plants, (c) summarizes the principles of current technologies used to evaluate the extent of lipid and protein oxidation, (d) synthesizes major outcomes of studies on lipid and protein oxidation in plants under abiotic stress, and finally, (e) considers a brief cross-talk on the ROS-accrued lipid and protein oxidation, pointing to the aspects unexplored so far.