Naser A. Anjum

Salicylic acid-induced abiotic stress tolerance and underlying mechanisms in plants

Abiotic stresses (such as metals/metalloids, salinity, ozone, UV-B radiation, extreme temperatures, and drought) are among the most challenging threats to agricultural system and economic yield of crop plants. These stresses (in isolation and/or combination) induce numerous adverse effects in plants, impair biochemical/physiological and molecular processes, and eventually cause severe reductions in plant growth, development and overall productivity. Phytohormones have been recognized as a strong tool for sustainably alleviating adverse effects of abiotic stresses in crop plants. In particular, the significance of salicylic acid (SA) has been increasingly recognized in improved plant abiotic stress-tolerance via SA-mediated control of major plant-metabolic processes. However, the basic biochemical/physiological and molecular mechanisms that potentially underpin SA-induced plant-tolerance to major abiotic stresses remain least discussed. Based on recent reports, this paper: (a) overviews historical background and biosynthesis of SA under both optimal and stressful environments in plants; (b) critically appraises the role of SA in plants exposed to major abiotic stresses; (c) cross-talks potential mechanisms potentially governing SA-induced plant abiotic stress-tolerance; and finally (d) briefly highlights major aspects so far unexplored in the current context.

Minimising toxicity of cadmium in plants—role of plant growth regulators

A range of man-made activities promote the enrichment of world-wide agricultural soils with a myriad of chemical pollutants including cadmium (Cd). Owing to its significant toxic consequences in plants, Cd has been one of extensively studied metals. However, sustainable strategies for minimising Cd impacts in plants have been little explored. Plant growth regulators (PGRs) are known for their role in the regulation of numerous developmental processes. Among major PGRs, plant hormones (such as auxins, gibberellins, cytokinins, abscisic acid, jasmonic acid, ethylene and salicylic acid), nitric oxide (a gaseous signalling molecule), brassinosteroids (steroidal phytohormones) and polyamines (group of phytohormone-like aliphatic amine natural compounds with aliphatic nitrogen structure) have gained attention by agronomist and physiologist as a sustainable media to induce tolerance in abiotic-stressed plants. Considering recent literature, this paper: (a) overviews Cd status in soil and its toxicity in plants, (b) introduces major PGRs and overviews their signalling in Cd-exposed plants, (c) appraises mechanisms potentially involved in PGR-mediated enhanced plant tolerance to Cd and (d) highlights key aspects so far unexplored in the subject area.

Ascorbate-glutathione pathway and stress tolerance in plants

Regulatory Role of Components of Ascorbate-Glutathione Pathway in Plant Stress Tolerance.- Ascorbate and Glutathione in Organogenesis, Regeneration and Differentiation in Plant In vitro Cultures.- Role of Ascorbate Peroxidase and Glutathione Reductase in Ascorbate-Glutathione Cycle and Stress Tolerance in Plants.- The Ascorbate-Gluathione Cycle and Related Redox Signals in Plant-Pathogen Interactions.- Regulation of the Ascorbate-Glutathione Cycle in Plants Under Drought Stress.- Glutathione and Herbicide Resistance in Plants.- Ascorbate and Glutathione: Protectors of Plants in Oxidative Stress.- Changes in the Glutathione and Ascorbate Redox State Trigger Growth During Embryo Development and Meristem Reactivation at Germination.- A Winning Two Pair: Role of the Redox Pairs AsA/DHA and GSH/GSSG in Signal Transduction.- Involvement of AsA/DHA and GSH/GSSG Ratios in Gene and Protein Expression and in the Activation of Defence Mechanisms Under Abiotic Stress Conditions.- Ascorbate-Glutathione Cycle: Enzymatic and Non-enzymatic Integrated Mechanisms and Its Biomolecular Regulation.- Coordinate Role of Ascorbate-Glutathione in Response to Abiotic Stresses.- Regulation of Genes Encoding Chloroplast Antioxidant Enzymes in Comparison to Regulation of the Extra-plastidic Antioxidant Defense System.- The Peroxisomal Ascorbate-Glutathione Pathway: Molecular Identification and Insights into Its Essential Role Under Environmental Stress Conditions.- Identification of Potential Gene Targets for the Improvement of Ascorbate Contents of Genetically Modified Plants.

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.

Cadmium causes oxidative stress in mung bean by affecting the antioxidant enzyme system and ascorbate-glutathione cycle metabolism

Ascorbate (AsA)-glutathione (GSH) cycle metabolism is an essential mechanism for the resistance of plants under stress conditions. In a greenhouse pot experiment, the influence of cadmium (Cd) (25, 50, and 100 mg/kg soil) on plant dry weight and leaf area, photosynthetic parameters (net photosynthetic rate (PN) and chlorophyll (Chl) content) and oxidative stress, and the possible protective role of AsA-GSH cycle metabolism was studied in two mung bean (Vigna radiata (L.) Wilczek.) cvs. Pusa 9531 (Cd-tolerant) and PS 16 (Cd-susceptible) at 30 days after sowing. The contents of thiobarbituric acid-reactive substances (TBARS), H2O2, and the leakage of ions were the highest at 100 mg Cd/kg soil, and the effect was more pronounced in cv. PS 16 than in cv. Pusa 9531. This was concomitant with the strongest decreases in PN, plant dry weight, and leaf area. The changes in the AsA-GSH redox state and an increase in AsA-GSH-regenerating enzymes, such as glutathione reductase, monodehydroascorbate reductase, dehydroascorbate reductase, and other antioxidant enzymes, such as superoxide dismutase and ascorbate peroxidase, strongly supported over-utilization of AsA-GSH in Cd-treated plants. However, the oxidative stress caused by Cd toxicity was partially overcome by AsA-GSH-based detoxification mechanism in the two genotypes studied because an increases in lipid peroxidation (TBARS, ion leakage) and H2O2 content were accompanied by a corresponding decrease in reduced AsA and GSH pools. Thus, changes in AsA-GSH pools and the coordination between AsA-GSH-regenerating enzymes and other enzymatic antioxidants of the leaves suggest their relevance to the defense against Cd stress.

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.

Superoxide dismutase—mentor of abiotic stress tolerance in crop plants

Abiotic stresses impact growth, development, and productivity, and significantly limit the global agricultural productivity mainly by impairing cellular physiology/biochemistry via elevating reactive oxygen species (ROS) generation. If not metabolized, ROS (such as O2•−, OH•, H2O2, or 1O2) exceeds the status of antioxidants and cause damage to DNA, proteins, lipids, and other macromolecules, and finally cellular metabolism arrest. Plants are endowed with a family of enzymes called superoxide dismutases (SODs) that protects cells against potential consequences caused by cytotoxic O2•− by catalyzing its conversion to O2 and H2O2. Hence, SODs constitute the first line of defense against abiotic stress-accrued enhanced ROS and its reaction products. In the light of recent reports, the present effort: (a) overviews abiotic stresses, ROS, and their metabolism; (b) introduces and discusses SODs and their types, significance, and appraises abiotic stress-mediated modulation in plants; (c) analyzes major reports available on genetic engineering of SODs in plants; and finally, (d) highlights major aspects so far least studied in the current context. Literature appraised herein reflects clear information paucity in context with the molecular/genetic insights into the major functions (and underlying mechanisms) performed by SODs, and also with the regulation of SODs by post-translational modifications. If the previous aspects are considered in the future works, the outcome can be significant in sustainably improving plant abiotic stress tolerance and efficiently managing agricultural challenges under changing climatic conditions.

Phytohormones and Abiotic Stress Tolerance in Plants

Signal transduction of pytohormones under abiotic stresses.- Cross-talks on phytohormones signaling pathways under optimal and stressful conditions.- Phytohormones in salinity tolerance: ethylene and gibberellins cross talk.- Nitric oxide metabolism under environmental stress conditions.- Auxin as part of wound-healing response in plants.- Interaction between ethylene, auxin, light and microtubules during low-pH induced root hair formation in lettuce seedlings.- Cytokinin homeostasis.- Origin of brassinosteroids and their role in oxidative stress in plants.- Hormonal intermediates in the protective action of exogenous phytohormones in plants under salinity: A case study on wheat.- The role of phytohormones in the control of plant adaptation to oxygen depletion.- Stress hormones associated to sunflower germplasms with different sensitivity to drought.- An insight into the relationship between jasmonates and salicylic acid in salt tolerance.

Glutathione and proline can coordinately make plants withstand the joint attack of metal(loid) and salinity stresses

Agricultural soils in the vicinity of extensive anthropogenic activities may exhibit salinity together with high levels of metals/metalloids (hereafter termed as “metal/s”) as co-stressors. Elevated concentrations of metals (such as As, Cd, Cr, Hg, Ni, and Pb) may affect photosynthetic apparatus, electron transport chain and chlorophyll biosynthesis, induce cellular damage, impair cellular redox homeostasis, and finally cause cellular metabolic arrest (Anjum et al., 2010, 2012a; Gill and Tuteja, 2010; Talukdar, 2012; Talukdar and Talukdar, 2014). Saline soil conditions, on the other hand, can cause osmotic stress that in turn can inhibit cell expansion and cell division, impact stomatal closure, induce cell turgor via lowering water potential, and alter the normal homeostasis of cells (Miller et al., 2010). However, the generation of osmotic stress through impaired plant water relations, and oxidative stress caused by uncontrolled generation of varied reactive oxygen species (ROS; such as such as -OH, H2O2, O−2) are common in plants exposed to high levels of salinity and/or metals (Benavides et al., 2005; Anjum et al., 2010, 2012a). Diverse plant taxa have been reported to adapt metabolically to salinity and exposure to metals by enhancing synthesis of sulfur (S)-rich peptides (such as glutathione, GSH) and low-molecular-weight nitrogenous and proteogenic amino acids/osmolytes (such as proline, Pro) (Khan et al., 2009; Anjum et al., 2010, 2012a; Talukdar, 2012; Kishor and Sreenivasulu, 2014; Talukdar and Talukdar, 2014). Nevertheless, both GSH and Pro share L-glutamate as a common biosynthesis precursor (Moat et al., 2003) (Figure ​(Figure1).1). However, very little or no effort has been made so far to dissect the intricacies of potential metabolic interrelationships between the GSH and Pro induction either under salinity/osmotic or metal stress conditions. Figure 1 Schematic representation of the points of interrelationships in the major metabolic pathway of sulfur-rich peptide—glutathione (GSH) and nitrogenous and proteogenic amino acid—proline (Pro). Therefore, we discuss and interpret through this note the facts related with the mainstays (chemistry, biosynthesis, compartmentalization, significance) commonly and potentially shared by these two enigmatic compounds (GSH and Pro) in plants. The outcome of the present endeavor can be useful in designing future research aimed at sustainably alleviating isolated and/or joint impact of metal and salinity stresses in crop plants through exploiting the GSH and Pro metabolism.

Brassinosteroids make plant life easier under abiotic stresses mainly by modulating major components of antioxidant defense system

Various abiotic stress factors significantly contribute to major worldwide-losses in crop productivity by mainly impacting plant’s stress tolerance/adaptive capacity. The latter is largely governed by the efficiency of antioxidant defense system for the metabolism of elevated reactive oxygen species (ROS), caused by different abiotic stresses. Plant antioxidant defense system includes both enzymatic (such as superoxide dismutase, SOD, E.C. 1.15.1.1; catalase, CAT, E.C. 1.11.1.6; glutathione reductase, GR, E.C. 1.6.4.2; peroxidase, POD, E.C. 1.11.1.7; ascorbate peroxidase, APX, E.C. 1.11.1.11; guaiacol peroxidase, GPX, E.C. 1.11.1.7) and non-enzymatic (such as ascorbic acid, AsA; glutathione, GSH; tocopherols; phenolics, proline etc.) components. Research reports on the status of various abiotic stresses and their impact on plant growth, development and productivity are extensive. However, least information is available on sustainable strategies for the mitigation of abiotic stress-mediated major consequences in plants. Brassinosteroids (BRs) are a novel group of phytohormones with significant growth promoting nature. BRs are considered as growth regulators with pleiotropic effects, as they influence diverse physiological processes like growth, germination of seeds, rhizogenesis, senescence etc. and also confer abiotic stress resistance in plants. In the light of recent reports this paper: (a) overviews major abiotic stresses and plant antioxidant defense system, (b) introduces BRs and highlights their significance in general plant growth and development, and (c) appraises recent literature available on BRs mediated modulation of various components of antioxidant defense system in plants under major abiotic stresses including metals/metalloids, drought, salinity, and temperature regimes. The outcome can be significant in devising future research in the current direction.