Mirza Hasanuzzaman

Physiological, Biochemical, and Molecular Mechanisms of Heat Stress Tolerance in Plants

High temperature (HT) stress is a major environmental stress that limits plant growth, metabolism, and productivity worldwide. Plant growth and development involve numerous biochemical reactions that are sensitive to temperature. Plant responses to HT vary with the degree and duration of HT and the plant type. HT is now a major concern for crop production and approaches for sustaining high yields of crop plants under HT stress are important agricultural goals. Plants possess a number of adaptive, avoidance, or acclimation mechanisms to cope with HT situations. In addition, major tolerance mechanisms that employ ion transporters, proteins, osmoprotectants, antioxidants, and other factors involved in signaling cascades and transcriptional control are activated to offset stress-induced biochemical and physiological alterations. Plant survival under HT stress depends on the ability to perceive the HT stimulus, generate and transmit the signal, and initiate appropriate physiological and biochemical changes. HT-induced gene expression and metabolite synthesis also substantially improve tolerance. The physiological and biochemical responses to heat stress are active research areas, and the molecular approaches are being adopted for developing HT tolerance in plants. This article reviews the recent findings on responses, adaptation, and tolerance to HT at the cellular, organellar, and whole plant levels and describes various approaches being taken to enhance thermotolerance in plants.

Plant Response and Tolerance to Abiotic Oxidative Stress: Antioxidant Defense Is a Key Factor

In a persistently changing environment, plants are constantly challenged by various abiotic stresses such as salinity, drought, temperature extremes, heavy metal toxicity, high-light intensity, nutrient deficiency, UV-B radiation, ozone, etc. which cause substantial losses in the yield and quality of a crop. A key sign of such stresses at the molecular level is the accelerated production of reactive oxygen species (ROS) such as singlet oxygen (1O2), superoxide (O2•−), hydrogen peroxide (H2O2) and hydroxyl radicals (OH•). ROS are extremely reactive in nature because they can interact with a number of cellular molecules and metabolites, thereby leading to irreparable metabolic dysfunction and death. Plants have well-developed enzymatic and non-enzymatic scavenging pathways or detoxification systems to counter the deleterious effects of ROS that include the enzymes superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR), glutathione S-transferase (GST), glutathione peroxidase (GPX) and peroxidases (POX) as well as non-enzymatic compounds such as ascorbate (AsA), glutathione (GSH), carotenoids and tocopherols. In plant cells, specific ROS-producing and scavenging systems are found in different organelles and the ROS-scavenging pathways from different cellular compartments are coordinated. Recent studies in plants have shown that relatively low levels of ROS act as signaling molecules that induce abiotic stress tolerance by regulating the expression of defense genes. Additionally, numerous results have shown that plants with higher levels of antioxidants, whether constitutive or induced, showed greater resistance to different types of environmental stresses. In this chapter we attempt to summarize recent researches on the mechanisms and possible regulatory roles of ROS in abiotic stress tolerance. Further, we discuss the progress made during the last few decades in improving the oxidative stress tolerance of plants through genetic engineering by different components of ROS detoxification systems in plants.

Plant Response to Salt Stress and Role of Exogenous Protectants to Mitigate Salt-Induced Damages

Plants are frequently exposed to a plethora of unfavorable or even adverse environmental conditions, termed as abiotic stresses (such as salinity, drought, heat, cold, flooding, heavy metals, ozone, UV radiation, etc.) and thus they pose serious threats to the sustainability of crop yield. Soil salinity, one of the most severe abiotic stresses, limits the production of about 6 % of the world’s total land and 20 % of irrigated land (17 % of total cultivated areas) and negatively affects crop production worldwide. On the other hand, increased salinity of agricultural land is expected to have destructive global effects, resulting in up to 50 % land loss by the next couple of decades. The adverse effects of salinity have been ascribed mainly to an increase in sodium (Na+) and chloride (Cl–) ions and hence these ions produce the critical conditions for plant survival by intercepting different plant mechanisms. Both Na+ and Cl– produce many physiological disorders in plants but Cl– is the most dangerous. A plant’s response to salt stress depends on the genotype, developmental stage, as well as the intensity and duration of the stress. Increased salinity has diverse effects on the physiology of plants grown in saline conditions and in response to major factors like osmotic stress, ion-specificity, nutritional and hormonal imbalance, and oxidative damage. In addition to upper plant parts, salinity also affects root growth and physiology and their function in nutrient uptake. The outcome of these effects may cause the disorganization of cellular membranes, inhibit photosynthesis, generate toxic metabolites and decline nutrient absorption, ultimately leading to plant death. In recent decades, exogenous protectants such as osmoprotectants, phytohormones, signaling molecules, polyamines, antioxidants and various trace elements have been found effective in plants in mitigating the salt induced damages. These protectants showed the capacity to enhance the plants’ growth, yield as well as stress tolerance under salinity. In this chapter we attempt to summarize differential responses of plants to salinity with special reference to growth, physiology and yield. Further, we have discussed the progress made in using exogenous protectants to mitigate salt-induced damages in 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.

Potential Use of Halophytes to Remediate Saline Soils

Salinity is one of the rising problems causing tremendous yield losses in many regions of the world especially in arid and semiarid regions. To maximize crop productivity, these areas should be brought under utilization where there are options for removing salinity or using the salt-tolerant crops. Use of salt-tolerant crops does not remove the salt and hence halophytes that have capacity to accumulate and exclude the salt can be an effective way. Methods for salt removal include agronomic practices or phytoremediation. The first is cost- and labor-intensive and needs some developmental strategies for implication; on the contrary, the phytoremediation by halophyte is more suitable as it can be executed very easily without those problems. Several halophyte species including grasses, shrubs, and trees can remove the salt from different kinds of salt-affected problematic soils through salt excluding, excreting, or accumulating by their morphological, anatomical, physiological adaptation in their organelle level and cellular level. Exploiting halophytes for reducing salinity can be good sources for meeting the basic needs of people in salt-affected areas as well. This review focuses on the special adaptive features of halophytic plants under saline condition and the possible ways to utilize these plants to remediate salinity.

Polyamine and nitric oxide crosstalk: Antagonistic effects on cadmium toxicity in mung bean plants through upregulating the metal detoxification, antioxidant defense and methylglyoxal detoxification systems.

Cadmium (Cd) contamination is a serious agricultural and environmental hazard. The study investigates cross-protection roles of putrescine (Put, 0.2 mM) and nitric oxide (sodium nitroprusside; SNP, 1 mM) in conferring Cd (CdCl2, 1.5 mM) tolerance in mung bean (Vigna radiata L. cv. BARI Mung-2) seedlings. Cadmium stress increased root and shoot Cd content, reduced growth, destroyed chlorophyll (chl), modulated proline (Pro) and reduced leaf relative water content (RWC), increased oxidative damage [lipid peroxidation, H2O2 content, O2(∙-) generation rate, lipoxygenase (LOX) activity], methylglyoxal (MG) toxicity. Put and/or SNP reduced Cd uptake, increasd phytochelatin (PC) content, reduced oxidative damage enhancing non-enzymatic antioxidants (AsA and GSH) and activities of enzymes [superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), glutathione reductase (GR), glutathione S-transferase (GST), and glutathione peroxidase (GPX)]. Exogenous Put and/or SNP modulated endogenous polyamines, PAs (putrescine, Put; spermidine, Spd; spermine, Spm), and NO; improved glyoxalase system in detoxifying MG and improved physiology and growth where combined application showed better effects which designates possible crosstalk between NO and PAs to confer Cd-toxicity tolerance.

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.

Metal/metalloid stress tolerance in plants: role of ascorbate, its redox couple, and associated enzymes

The enhanced generation of reactive oxygen species (ROS) under metal/metalloid stress is most common in plants, and the elevated ROS must be successfully metabolized in order to maintain plant growth, development, and productivity. Ascorbate (AsA) is a highly abundant metabolite and a water-soluble antioxidant, which besides positively influencing various aspects in plants acts also as an enigmatic component of plant defense armory. As a significant component of the ascorbate-glutathione (AsA-GSH) pathway, it performs multiple vital functions in plants including growth and development by either directly or indirectly metabolizing ROS and its products. Enzymes such as monodehydroascorbate reductase (MDHAR, EC 1.6.5.4) and dehydroascorbate reductase (DHAR, EC 1.8.5.1) maintain the reduced form of AsA pool besides metabolically controlling the ratio of AsA with its oxidized form (dehydroascorbate, DHA). Ascorbate peroxidase (APX, EC 1.11.1.11) utilizes the reduced AsA pool as the specific electron donor during ROS metabolism. Thus, AsA, its redox couple (AsA/DHA), and related enzymes (MDHAR, DHAR, and APX) cumulatively form an AsA redox system to efficiently protect plants particularly against potential anomalies caused by ROS and its products. Here we present a critical assessment of the recent research reports available on metal/metalloid-accrued modulation of reduced AsA pool, AsA/DHA redox couple and AsA-related major enzymes, and the cumulative significance of these antioxidant system components in plant metal/metalloid stress tolerance.

ATP-sulfurylase, sulfur-compounds, and plant stress tolerance

Sulfur (S) stands fourth in the list of major plant nutrients after N, P, and K. Sulfate (SO42-), a form of soil-S taken up by plant roots is metabolically inert. As the first committed step of S-assimilation, ATP-sulfurylase (ATP-S) catalyzes SO42--activation and yields activated high-energy compound adenosine-5′-phosphosulfate that is reduced to sulfide (S2-) and incorporated into cysteine (Cys). In turn, Cys acts as a precursor or donor of reduced S for a range of S-compounds such as methionine (Met), glutathione (GSH), homo-GSH (h-GSH), and phytochelatins (PCs). Among S-compounds, GSH, h-GSH, and PCs are known for their involvement in plant tolerance to varied abiotic stresses, Cys is a major component of GSH, h-GSH, and PCs; whereas, several key stress-metabolites such as ethylene, are controlled by Met through its first metabolite S-adenosylmethionine. With the major aim of briefly highlighting S-compound-mediated role of ATP-S in plant stress tolerance, this paper: (a) overviews ATP-S structure/chemistry and occurrence, (b) appraises recent literature available on ATP-S roles and regulations, and underlying mechanisms in plant abiotic and biotic stress tolerance, (c) summarizes ATP-S-intrinsic regulation by major S-compounds, and (d) highlights major open-questions in the present context. Future research in the current direction can be devised based on the discussion outcomes.

Glutathione-induced drought stress tolerance in mung bean: coordinated roles of the antioxidant defence and methylglyoxal detoxification systems

Drought is considered one of the most acute abiotic stresses presently affecting agriculture. Plant tolerance to drought stress often depends on their tolerance against oxidative stress, which is acquired largely by strong antioxidant defense. In our paper we studied the glutathione-induced drought stress tolerance in Vigna radiata seedlings. Drought decreased growth, resulted in oxidative stress and increased methylglyoxal toxicity. But exogenous GSH enhanced components of the antioxidant system in drought-affected mung bean seedlings, which alleviated oxidative damage, up-regulated the glyoxalase system, reduced MG toxicity, and modulated the proline and water content, contributing to drought tolerance.