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.

Among the plant nutrients potassium (K) is one of the vital elements required for plant growth and physiology. Potassium is not only a constituent of plant structure but also plays regulatory function in several biochemical processes related to protein synthesis, carbohydrate metabolism, enzyme activation. There are several physiological processes like stomatal regulation and photosynthesis are dependent on K. In the recent decades K was found to provide abiotic stress tolerance. Under salt stress, K helps in maintaining ion homeostasis and regulation of osmotic balance. Under drought stress condition K regulates the stomatal opening and makes the plants adaptive to water deficit. Many reports provided the notion that K enhances the antioxidant defense in plants and therefore, protects the plants from oxidative stress under various environmental adversities. Also, it provides some cellular signaling alone or in association with other signaling molecules and phytohormones. Although a considerable progress in understanding K-induced abiotic stress tolerance in plants has been achieved the exact molecular mechanisms of such protections are still under research. In this review, we summarized the recent literature on the biological functions of K, its uptake, and translocation and its role in plant abiotic stress tolerance.

Hydroponically grown 12-day-old rice (Oryza sativa L. cv. BRRI dhan47) seedlings were exposed to 150 mM NaCl alone and combined with 0.5 mM MnSO 4 . Salt stress resulted in disruption of ion homeostasis by Na ? influx and K ? efflux. Higher accumulation of Na ? and water imbalance under salinity caused osmotic stress, chlorosis, and growth inhibition. Salt-induced ionic toxicity and osmotic stress consequently resulted in oxidative stress by disrupting the antioxidant defense and glyoxalase systems through overproduction of reactive oxygen species (ROS) and methylglyoxal (MG), respectively. The saltinduced damage increased with the increasing duration of stress. However, exogenous application of manganese (Mn) helped the plants to partially recover from the inhibited growth and chlorosis by improving ionic and osmotic homeostasis through decreasing Na ? influx and increasing water status, respectively. Exogenous application of Mn increased ROS detoxification by increasing the content of the phenolic compounds, flavonoids, and ascorbate (AsA), and increasing the activities of monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), superoxide dismutase (SOD), and catalase (CAT) in the salt-treated seedlings. Supplemental Mn also reinforced MG detoxification by increasing the activities of glyoxalase I (Gly I) and glyoxalase II (Gly II) in the salt-affected seedlings. Thus, exogenous application of Mn conferred salt-stress tolerance through the coordinated action of ion homeostasis and the antioxidant defense and glyoxalase systems in the salt-affected seedlings.

Nitric oxide (NO) is dynamic molecule implicated in diverse biological functions demonstrating its protective effect against damages provoked by abiotic stresses. The present study investigated that exogenous NO pretreatment (500 µM sodium nitroprusside, 24 h) prevented the adverse effect of drought stress [induced by 10% and 20% polyethylene glycol (PEG), 48 h] on rapeseed seedlings. Drought stress resulted in reduced relative water content with increased proline (Pro) level. Drought stress insisted high H 2 O 2 generation and consequently increased membrane lipid peroxidation which are clear indications of oxidative damage. Drought stress disrupted the glyoxalase system too. Exogenous NO successfully alleviated oxidative damage effects on rapeseed seedlings through improving the levels of nonenzymatic antioxidant pool and upregulating antioxidant enzymes' activities. Improvement of glyoxalase system (glyoxalase I and glyoxalase II activities) by exogenous NO was significant to improve plants' tolerance. Nonetheless, regulation of Pro level and improvement of plant-water status were vital to confer drought stress tolerance.

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 KCrO, 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 (HO) content; superoxide (O) 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.