Tahsina Sharmin Hoque

Methylglyoxal: An Emerging Signaling Molecule in Plant Abiotic Stress Responses and Tolerance

The oxygenated short aldehyde methylglyoxal (MG) is produced in plants as a by-product of a number of metabolic reactions, including elimination of phosphate groups from glycolysis intermediates dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. MG is mostly detoxified by the combined actions of the enzymes glyoxalase I and glyoxalase II that together with glutathione make up the glyoxalase system. Under normal growth conditions, basal levels of MG remain low in plants; however, when plants are exposed to abiotic stress, MG can accumulate to much higher levels. Stress-induced MG functions as a toxic molecule, inhibiting different developmental processes, including seed germination, photosynthesis and root growth, whereas MG, at low levels, acts as an important signaling molecule, involved in regulating diverse events, such as cell proliferation and survival, control of the redox status of cells, and many other aspects of general metabolism and cellular homeostases. MG can modulate plant stress responses by regulating stomatal opening and closure, the production of reactive oxygen species, cytosolic calcium ion concentrations, the activation of inward rectifying potassium channels and the expression of many stress-responsive genes. MG appears to play important roles in signal transduction by transmitting and amplifying cellular signals and functions that promote adaptation of plants growing under adverse environmental conditions. Thus, MG is now considered as a potential biochemical marker for plant abiotic stress tolerance, and is receiving considerable attention by the scientific community. In this review, we will summarize recent findings regarding MG metabolism in plants under abiotic stress, and evaluate the concept of MG signaling. In addition, we will demonstrate the importance of giving consideration to MG metabolism and the glyoxalase system, when investigating plant adaptation and responses to various environmental stresses.

Heat or cold priming-induced cross-tolerance to abiotic stresses in plants: key regulators and possible mechanisms

Plants growing under field conditions are constantly exposed, either simultaneously or sequentially, to more than one abiotic stress factor. Plants have evolved sophisticated sensory systems to perceive a number of stress signals that allow them to activate the most adequate response to grow and survive in a given environment. Recently, cross-stress tolerance (i.e. tolerance to a second, strong stress after a different type of mild primary stress) has gained attention as a potential means of producing stress-resistant crops to aid with global food security. Heat or cold priming-induced cross-tolerance is very common in plants and often results from the synergistic co-activation of multiple stress signalling pathways, which involve reactive nitrogen species (RNS), reactive oxygen species (ROS), reactive carbonyl species (RCS), plant hormones and transcription factors. Recent studies have shown that the signalling functions of ROS, RNS and RCS, most particularly hydrogen peroxide, nitric oxide (NO) and methylglyoxal (MG), provide resistance to abiotic stresses and underpin cross-stress tolerance in plants by modulating the expression of genes as well as the post-translational modification of proteins. The current review highlights the key regulators and mechanisms underlying heat or cold priming-induced cross-stress tolerance in plants, with a focus on ROS, MG and NO signalling, as well as on the role of antioxidant and glyoxalase systems, osmolytes, heat-shock proteins (HSPs) and hormones. Our aim is also to provide a comprehensive idea on the topic for researchers using heat or cold priming-induced cross-tolerance as a mechanism to improve crop yields under multiple abiotic stresses.

Methylglyoxal induces inhibition of growth, accumulation of anthocyanin, and activation of glyoxalase I and II in Arabidopsis thaliana

Methylglyoxal (MG) is a highly reactive stress‐related α‐ketoaldehyde and a physiological metabolite of glycolysis, which is accumulated in ample amount under stressful conditions. In the present study, the effect of different doses of MG on growth, anthocyanin production, MG contents, and activities of two types of glyoxalases (glyoxalase I and glyoxalase II) were examined in Arabidopsis seedlings. MG at 0.1 mM dose did not affect seedling growth, anthocyanin accumulation, MG contents, or activities of glyoxalases, whereas MG at 0.5 mM and 1 mM inhibited seedling growth and induced anthocyanin accumulation, MG accumulation, and glyoxalase (both I and II) activation. Therefore, MG can reduce plant growth as a toxic molecule and can stimulate stress responses as a signal molecule under stress conditions.

Glycinebetaine-Mediated Abiotic Oxidative-Stress Tolerance in Plants: Physiological and Biochemical Mechanisms

Plants face many stressful conditions during their lifetimes and because of their sessile nature they have to adapt to these conditions in order to survive. One unfortunate and unavoidable consequence of all major biotic and abiotic stresses is the overproduction of reactive oxygen species (ROS). ROS are highly reactive and toxic chemical entities and can cause serious damage to cellular proteins, lipids, carbohydrates and DNA, leading to irreparable metabolic dysfunction and cell death. Plant cells and their organelles, particularly the chloroplasts, mitochondria and peroxisomes have antioxidant defence systems, composed of enzymatic and non-enzymatic components, to counter the deleterious effects of ROS and/or to perform signalling functions. It is an established fact that the timely induction of antioxidant defences is a key to protection of plant cells from oxidative damage due to stress. Enzymatic antioxidants include superoxide dismutase, catalase, peroxidases and glutathione reductase, while the major non-enzymatic antioxidants are compatible osmolytes (glycinebetaine, GB; and proline), ascorbic acid, reduced glutathione, α-tocopherol, amino acids and polyphenols. Stimulated biosynthesis and accumulation of low molecular weight compatible osmolytes is one of the most effective mechanisms evolved by plants to maintain their cellular integrity and ensure survival when exposed to multiple abiotic stresses. Glycinebetaine, an N-trimethyl derivative of glycine and a quaternary ammonium compound, is one of the most studied and efficient compatible solutes. Due to its unique structural features, it interacts both with the hydrophobic and hydrophilic domains of macromolecules, including enzymes and proteins. GB has been reported to protect plants from the antagonistic effects of a range of abiotic stresses, by maintaining the water balance between plant cells and environment, osmotic adjustment, protecting the thylakoid membrane system, protein stabilization, photosystem and photosynthetic electron transport chain protection and by modulating ROS detoxification. In recent years, GB has attained unprecedented attention due to its multifunctional roles in plants under stressful conditions. In this chapter, we summarize our understanding of ROS formation under abiotic stress and GB biosynthesis and accumulation, as an adaptive mechanism, with particular emphasis on the new insights into the biochemical and molecular mechanisms involved in GB-mediated abiotic oxidative stress tolerance in plants.

The Glyoxalase System: A Possible Target for Production of Salinity-Tolerant Crop Plants

Among the various abiotic stressors, soil salinity is one of the most detrimental, restricting the growth and productivity of major agricultural crops worldwide. Apart from ionic, osmotic, and oxidative stress, one of the most important biochemical impacts of salt stress on plants is overaccumulation of methylglyoxal (MG), a cytotoxic compound that can cause degradation of proteins, lipids, and nucleic acids, inactivation of antioxidant systems and, finally, the death of plants. However, plants possess a complex network of enzymatic and nonenzymatic scavenging and detoxification systems to defend against MG-induced glycation and oxidative stress. Among the various defense mechanisms employed by plants, the glyoxalase system (composed mainly of two enzymes—glyoxalase I and glyoxalase II) is the most important, playing a crucial role in detoxifying MG, as well as regulating glutathione homeostasis and reactive oxygen species metabolism. Apart from its deleterious effects on plant growth and development, MG also has important signaling roles associated with stress tolerance. Recent genetic engineering studies have shown that overexpression of glyoxalase genes confers tolerance of various abiotic stresses, including salinity stress. This chapter summarizes the current knowledge and understanding of MG and the glyoxalase pathway, with respect to salinity stress tolerance and the potential for use of genetic engineering of glyoxalase genes into crop plants to improve crop yields under salt stress.

Nutritional improvement of wheat by foliar application of moringa leaf extract

A field experiment was conducted at the Soil Science Field Laboratory of Bangladesh Agricultural University (BAU), Mymensingh, during rabi season to evaluate the effect of foliar application of moringa leaf extract on productivity and nutrient uptake efficiency of wheat plants. The experiment was laid out in a randomized complete block design with six treatments and three replications. The treatments were T1 (Control), T2 [moringa leaf extract (MLE) sprayed only at tillering stage], T3 (MLE sprayed at tillering and jointing stages), T4 (MLE sprayed at tillering, jointing and booting stages), T5 (MLE sprayed at tillering, jointing, booting and heading stages) and T6 (MLE sprayed only at heading stage). The application of moringa leaf extract significantly increased nutrient content as well as nutrient uptake by grain and straw. The content of N, P, K and S in both grain and straw of wheat was the highest in T4 treatment which produced the maximum biological yield of 9.05 t ha-1. Again, treatment T4 resulted in the highest total uptakes of N, P, K and S in wheat, which were 131.91, 15.55, 122.27, and 24.16 kg ha-1, respectively. The results of this study indicate that foliar application of MLE can potentially be a viable option to increase biological yield and nutrient uptake efficiency of wheat plants, particularly N, P, K and S. In this study, the foliar application of moringa leaf extract on tillering, jointing and booting stages of the crop showed the best performance and therefore, it might be used due to its eco-friendly nature for yield enhancement as well as nutrient enrichment in wheat.