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.

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.

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Methylglyoxal (MG) is a highly reactive metabolite derived from glycolysis. In this study, we examined the effect of MG on seed germination, root elongation, chlorosis and stress-responsive gene expression in Arabidopsis using an abscisic acid (ABA)-deficient mutant, aba2-2. In the wild type, 0.1 mm MG did not affect germination but delayed root elongation, whereas 1.0 mm MG inhibited germination and root elongation and induced chlorosis. MG increased transcription levels of RD29B and RAB18 in a dose-dependent manner but did not affect RD29A transcription level. In contrast, in the aba2-2 mutant, MG inhibition of seed germination at 1.0 mm and 10.0 mm and a delay of root elongation at 0.1 mm MG were mitigated, although there was no significant difference in chlorosis between the wild type and mutant. Moreover, the aba2-2 mutation impaired MG-induced RD29B and RAB18 gene expression. These observations suggest that MG not only directly inhibits germination and root elongation but also indirectly modulates these processes via endogenous ABA in Arabidopsis.

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Moringa is a valuable plant whose leaves are enriched with antioxidants, amino acids, vitamins and mineral nutrients and can be used as a bio-stimulant. A field work was conducted at the Soil Science Field Laboratory of Bangladesh Agricultural University (BAU), Mymensingh, during rabi season from November 2017 to February 2018 in order to investigate the effect of moringa leaf extract (MLE) on growth, yield and nutrient status of cabbage. The experiment was laid out in a randomized complete block design with four treatments and three replications. The treatments were T1 (control), T2 (MLE) sprayed at 2 weeks after transplanting only], T3 (MLE sprayed at 2 weeks and 4 weeks after transplanting), T4(MLE sprayed at 2 weeks after transplanting and after every two weeks thereafter). The rate of MLE application was 25 mL plant-1. All the treatments received recommended dose of N, P, K, S, Zn and B fertilizers. The application of MLE significantly improved the growth parameters, yield and yield contributing characters as well as nutrient content and uptake of cabbage. Among the parameters plant height (33.40 cm), leaf number (19.33 cm), length of the largest leaf (29.00 cm), head thickness (9.67 cm), head diameter (20.33 cm), gross yield (72.83 t ha-1) and marketable yield (48.87 t ha-1), were maximum in T4 where MLE was sprayed at 2 weeks after transplanting and after every two weeks thereafter. The lowest values of all these parameters were found in T1 where no MLE was sprayed. Foliar application of MLE also improved the concentration and uptake of macronutrients (N, P, K and S) in head of cabbage. Thus, application of MLE as a bio-stimulant has the potentiality to enhance growth, yield and nutritional quality of cabbage. Asian J. Med. Biol. Res. June 2020, 6(2): 196-203

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