Jubayer Al Mahmud

Coordinated Actions of Glyoxalase and Antioxidant Defense Systems in Conferring Abiotic Stress Tolerance in Plants

Being sessile organisms, plants are frequently exposed to various environmental stresses that cause several physiological disorders and even death. Oxidative stress is one of the common consequences of abiotic stress in plants, which is caused by excess generation of reactive oxygen species (ROS). Sometimes ROS production exceeds the capacity of antioxidant defense systems, which leads to oxidative stress. In line with ROS, plants also produce a high amount of methylglyoxal (MG), which is an α-oxoaldehyde compound, highly reactive, cytotoxic, and produced via different enzymatic and non-enzymatic reactions. This MG can impair cells or cell components and can even destroy DNA or cause mutation. Under stress conditions, MG concentration in plants can be increased 2- to 6-fold compared with normal conditions depending on the plant species. However, plants have a system developed to detoxify this MG consisting of two major enzymes: glyoxalase I (Gly I) and glyoxalase II (Gly II), and hence known as the glyoxalase system. Recently, a novel glyoxalase enzyme, named glyoxalase III (Gly III), has been detected in plants, providing a shorter pathway for MG detoxification, which is also a signpost in the research of abiotic stress tolerance. Glutathione (GSH) acts as a co-factor for this system. Therefore, this system not only detoxifies MG but also plays a role in maintaining GSH homeostasis and subsequent ROS detoxification. Upregulation of both Gly I and Gly II as well as their overexpression in plant species showed enhanced tolerance to various abiotic stresses including salinity, drought, metal toxicity, and extreme temperature. In the past few decades, a considerable amount of reports have indicated that both antioxidant defense and glyoxalase systems have strong interactions in conferring abiotic stress tolerance in plants through the detoxification of ROS and MG. In this review, we will focus on the mechanisms of these interactions and the coordinated action of these systems towards stress tolerance.

γ-aminobutyric acid (GABA) confers chromium stress tolerance in Brassica juncea L. by modulating the antioxidant defense and glyoxalase systems

Chromium (Cr) toxicity is hazardous to the seed germination, growth, and development of plants. γ-aminobutyric acid (GABA) is a non-protein amino acid and is involved in stress tolerance in plants. To investigate the effects of GABA in alleviating Cr toxicity, we treated eight-d-old mustard (Brassica juncea L.) seedlings with Cr (0.15 and 0.3 mM K2CrO4, 5 days) alone and in combination with GABA (125 µM) in a semi-hydroponic medium. The roots and shoots of the seedlings accumulated Cr in a dose-dependent manner, which led to an increase in oxidative damage [lipid peroxidation; hydrogen peroxide (H2O2) content; superoxide (O2•−) generation; lipoxygenase (LOX) activity], methylglyoxal (MG) content, and disrupted antioxidant defense and glyoxalase systems. Chromium stress also reduced growth, leaf relative water content (RWC), and chlorophyll (chl) content but increased phytochelatin (PC) and proline (Pro) content. Furthermore, supplementing the Cr-treated seedlings with GABA reduced Cr uptake and upregulated the non-enzymatic antioxidants (ascorbate, AsA; glutathione, GSH) and the activities of the enzymatic antioxidants including ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR), glutathione peroxidase (GPX), superoxide dismutase (SOD), catalase (CAT), glyoxalase I (Gly I), and glyoxalase II (Gly II), and finally reduced oxidative damage. Adding GABA also increased leaf RWC and chl content, decreased Pro and PC content, and restored plant growth. These findings shed light on the effect of GABA in improving the physiological mechanisms of mustard seedlings in response to Cr stress.

Maleic acid assisted improvement of metal chelation and antioxidant metabolism confers chromium tolerance in Brassica juncea L.

Chromium (Cr) is a highly toxic environmental pollutant that negatively affects plant growth and development. Thus, remediating Cr from soil or increasing plant tolerance against Cr stress is urgent. Organic acids are recognized as agents of phytoremediation and as exogenous protectants, but using maleic acid (MA) to attain these results has not yet been studied. Therefore, our study investigated the effects of MA on Cr uptake and mitigation of Cr toxicity. We treated 8-d-old Indian mustard (Brassica juncea L.) seedlings with Cr (0.15mM and 0.3mM K2CrO4, 5 days) alone and in combination with MA (0.25mM) in a semi-hydroponic medium. Under Cr stress, plants accumulated Cr in both the roots and shoots in a dose-dependent manner, where the roots showed higher accumulation. Chromium stress reduced the growth and biomass of the Indian mustard plants by reducing water status and photosynthetic pigments, and increased oxidative damage due to generation of toxic reactive oxygen species (ROS) and methylglyoxal (MG). Chromium stress also interfered with the function of the antioxidant defense and glyoxalase systems. However, using MA in the Cr-stressed plants further increased Cr uptake in the roots, but it slightly reduced the translocation of Cr from the roots to the shoots at a lower dose of Cr and significantly at a higher dose. Moreover, MA also increased the other non-protein thiols (NPTs) containing phytochelatin (PC) content of the seedlings, which reduced Cr toxicity. Supplementing the stressed plants with MA upregulated the non-enzymatic antioxidants (ascorbate, AsA; glutathione, GSH); the activities of the enzymatic antioxidants including ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR), glutathione peroxidase (GPX), superoxide dismutase (SOD), and catalase (CAT); and the enzymes of the glyoxalase system including glyoxalase I (Gly I) and glyoxalase II (Gly II); and finally reduced oxidative damage and increased the chlorophyll content and water status as well the growth and biomass of the plants. Our findings suggested two potential uses of MA: first, enhancing phytoremediation, principally phytostabilization and second, working as an exogenous protectant to enhance Cr tolerance.

Insights into citric acid-induced cadmium tolerance and phytoremediation in Brassica juncea L.: Coordinated functions of metal chelation, antioxidant defense and glyoxalase systems

Cadmium (Cd) is a serious environmental threat because it accumulates in plants from soil and is subsequently transported into the food cycle. Increased Cd uptake in plants disrupts plant metabolism and hampers crop growth and development. Therefore, remediation of Cd from soil and enhancing plant tolerance to metal toxicity is vital. In the present study, we investigated the function of different doses of citric acid (CA) on Cd toxicity in terms of metal accumulation and stress tolerance in mustard (Brassica juncea L.). Brassica juncea seedlings (12-day-old) were treated with Cd (0.5mMCd and 1.0mM CdCl2) alone and in combination with CA (0.5mM and 1.0mM) in a semi-hydroponic medium for three days. Cadmium accumulation in the roots and shoots of the mustard seedlings increased in a dose-dependent manner and was higher in the roots. Increasing the Cd concentration led to reduced growth, biomass, water status, and chlorophyll (chl) content resulting from increased oxidative damage (elevated malondialdehyde, MDA content; hydrogen peroxide, H2O2 level; superoxide, O2•- generation; lipoxygenase, LOX activity; and methylglyoxal, MG content) and downregulating of the major enzymes of the antioxidant defense and glyoxalase systems. Under Cd stress, both doses of CA improved the growth of the plants by enhancing leaf relative water content (RWC) and chl content; reducing oxidative damage; enhancing the pool of ascorbate (AsA) and glutathione (GSH) and the activities of the antioxidant enzymes (ascorbate peroxidase, APX; monodehydroascorbate reductase, MDHAR; dehydroascorbate reductase, DHAR; glutathione reductase, GR; glutathione peroxidase, GPX; superoxide dismutase, SOD; catalase, CAT); improving the performance of the glyoxalase system (glyoxalase I, Gly I and glyoxalase II, Gly II activity); and increasing the phytochelatin (PC) content. Exogenous CA also increased the root and shoot Cd content and Cd translocation from the roots to the shoots in a dose-dependent manner. Our findings suggest that CA plays a dual role in mustard seedlings by increasing phytoremediation and enhancing stress tolerance through upregulating the antioxidant defense and glyoxalase systems.

Actions of Biological Trace Elements in Plant Abiotic Stress Tolerance

With the increase of global population, the demand for food crops, oil, fiber and other by-product yielding crops is increasing. In contrast to this increasing demand, abiotic stresses hinder the productivity of plants. Abiotic stresses sometimes reduce more than half of the crop yields. To attain global food security, understanding of plant responses to abiotic stresses is crucial because this is the prerequisite for developing approaches/tools for improving plant stress tolerance. Trace elements are nutrients required in small quantities to facilitate a range of physiological functions. These elements stimulate growth but are not essential. Some are essential only for certain plant species or required under a given condition. Trace elements not only improve plant physiological processes and growth but play roles in improving plant stress tolerance. However, the actual physiological functions of trace elements in conferring abiotic stress tolerance are still under study. This chapter focuses on the roles of trace elements emphasizing especially the recent advances on the actions of biological trace elements in plant abiotic stress tolerance.

Drought Stress Tolerance in Wheat: Omics Approaches in Understanding and Enhancing Antioxidant Defense

Plants face various kinds of stresses in the changing environment. Among the environmental stresses, drought is one of the most devastating stressors due to its diverse negative effects on crop plants. Drought stress in plants is very complex as it occurs due to varying environmental conditions such as soil water scarcity, soil salinity, and high temperature. The latter ones are termed as physiological drought. Bread wheat (Triticum aestivum L.) ranks first in the world’s grain production and is consumed as staple food by more than 36% of the world population. Wheat plant is highly sensitive to drought, especially at flowering and grain filling stages. Growth, photosynthesis, metabolic processes, nutrient assimilation, and yield of wheat plants remarkably decrease under drought. The responses of wheat to drought are varied at morphological, physiological, molecular, and biochemical levels. One of the most common consequences of drought is the disturbance of the balance between production of reactive oxygen species (ROS) and antioxidant defense causing overaccumulation of ROS which induces oxidative stress. This happens due to closure of the stomata, CO2 influx, and decrease of leaf internal CO2 which direct more electrons to form ROS and enhance photorespiration. These ROS can incur direct damage to protein, lipid, and nucleic acid which can ultimately cause plant cell death. Enhancing the antioxidant defense system to mitigate the oxidative stress is one of the effective strategies to make the wheat plants tolerant to drought. It appears that plants synthesize or activate several molecules like osmoprotectants, phytohormones, signaling molecules, and antioxidants to protect themselves from drought-induced oxidative damages. Novel approaches for enhancing the antioxidant defense system to minimize the impacts of drought-induced damage in plants are prime targets of plant biologists. Several genes and their overexpression were found to confer drought tolerance in plants. Application of plant probiotic bacteria also enhances tolerance of wheat plants to drought. Recent advances in genomic, transcriptomic, proteomic, and metabolomic studies on wheat under varying levels of drought generate useful information for designing drought-tolerant wheat. This chapter comprehensively reviews and updates our understanding on molecular mechanisms of adaptation of wheat plants to drought stress with special emphasis to antioxidant defense systems.

Salt Stress Tolerance in Rice: Emerging Role of Exogenous Phytoprotectants

Excess salinity in soil is one of the major environmental factors that limit plant growth and yield of a wide variety of crops including rice. On the basis of tolerance ability toward salinity, rice is considered as salt-sensitive crop, and growth and yield of rice are greatly affected by salinity. In general, rice can tolerate a small amount of saltwater without compromising the growth and yield. However, it greatly depends on the types and species of rice and their growth stage. Salinity-induced ionic and osmotic stresses reduce rate of photosynthesis and consequently cause oxidative stress, which is also responsible for growth reduction. The negative effects of salt stress that mentioned ultimately reduced yield of most crops including rice, except some halophytes. In recent decades, researchers have developed various approaches toward making salt-tolerant rice varieties. Using phytoprotectants is found to be effective in conferring salt tolerance to rice plants. In this chapter, we reviewed the recent reports on different aspects on salt stress tolerance strategies in light of using phytoprotectants.

The Role of Sulfur in Plant Abiotic Stress Tolerance: Molecular Interactions and Defense Mechanisms

Sulfur (S) is an essential macronutrient in plants that serves numerous plant functions and is vital for the metabolic processes. Moreover, it is the constituent of some essential amino acids and metabolites. Recent studies have provided the notion that S not only improves the productivity of plants under normal condition but also protects them from abiotic stresses like salinity, drought, and toxic metals/metalloids. Different S compounds directly act as antioxidants or modulate antioxidant defense system. Among them, glutathione (GSH) is regarded as one of the powerful antioxidants and stress protectors. Interactions of S with other biological molecules afford stress signaling to provide defense against environmental stresses. However, the S uptake, translocation, and mechanisms of action in plants under stressful conditions are still under research. The recent progress on the roles of S in conferring abiotic stresses and related literature is presented in this chapter.

Responses, Adaptation, and ROS Metabolism in Plants Exposed to Waterlogging Stress

Waterlogging condition imposes serious threat to plant survival by disturbing normal growth and development by hampering different physiological and metabolic activities, which includes reduction in stomatal conductance, CO2 assimilation rate, photosynthesis rate, and nutritional imbalance resulting in crop yield loss. However, plant develops different morphological, anatomical, and physiological adaptive features, for instance, formation of adventitious roots and aerenchyma, enhances the production of ethylene, and increases ADH activities and proline content under flooding condition. Under flooding, plants show oxidative damage condition due to the excess ROS production. ROS interferes with normal metabolism through oxidation of protein, nucleic acid, and DNA and reduces membrane integrity. Under this condition, plants itself develop antioxidant defense system including both enzymatic and non-enzymatic antioxidants which scavenges excess ROS. In this chapter, we tried to focus on different types of strategies such as hormonal regulation, osmoprotectants, and mineral nutrients to cope up with excess ROS production and suggested the future need of research in hormonal interaction and significance of osmolytes in protecting crop plants under waterlogging environment.