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br Introduction Methylglyoxal MG is a highly
Introduction
Methylglyoxal (MG) is a highly reactive dicarbonyl metabolite formed in cells mainly by the spontaneous degradation of triose phosphates, glyceraldehyde-3-phosphate, and dihydroxyacetone phosphate (Rabbani and Thornalley, 2012). It exists in a wide range of organisms, including protoctista (Wendler et al., 2009), bacteria (Ferguson et al., 1998), fungi (Maeta et al., 2005), plants (Singla-Pareek et al., 2003) and animals (Thornalley et al., 2003). MG shows dual role, that is, as cytotoxin at high concentration or signal molecule crosstalk with Ca2+, reactive oxygen species and 4-Aminobutyric acid at low concentration in plants (Kaur et al., 2014, Li, 2016). Thus, MG homeostasis in plant cells is important to exert its physiological function. When at high concentration, methylglyoxal irreversibly modifies amino and sulfhydryl groups in cellular proteins resulting in the formation of advanced glycation end products that are toxic and potentially mutagenic (Cantero et al., 2007). MG reacts with guanyl nucleotides in nucleic acids to form imidazopurinone adducts and also reacts with lysine and arginine residues in proteins resulting in crosslinking and the formation of imidazole derivatives (Martins et al., 2001). The protein modifications associated with MG activity often result in the dysfunction of cellular antioxidant systems.
The glyoxalase system, containing glyoxalase I (Gly I) and glyoxalase II (Gly II), is a ubiquitous enzymatic pathway that catalyzes the glutathione (GSH)-dependent detoxification of MG and the other reactive aldehydes. Detoxification of MG is accomplished by the sequential action of Gly I and Gly II. Gly I catalyzes the isomerisation of spontaneously formed hemithioacetal adducts, which form as the result of the interaction between GSH and MG, into S-D-lactoylglutathione. S-D-lactoylglutathione is then converted to D-lactate and GSH by Gly II (Thornalley, 1993, Kaur et al., 2014, Li, 2016). So the glyoxalase system plays a crucial role in the cellular defense against glycation and oxidative stress, and glyoxalases and MG were proposed as potential markers associated with plant stress responses (Thornalley, 1993, Kalapos, 2008, Kaur et al., 2014).
The function of glyoxalase in plants has been previously characterized. Gly I enzyme activity was reported to be associated with cell proliferation in soybean (Glycine max) (Paulus et al., 1993) and Amaranthus paniculatus (Chakravarty and Sopory, 1998). The genes encoding Gly I have been isolated in a few plant species, including tomato (Espartero et al., 1995), pumpkin (Hossain et al., 2009), onion (Hossain and Fujita, 2009), and wheat (Lin et al., 2010), and suggested to play a critical role in abiotic stress response. Overexpression of a Gly I gene from Brassica juncea improved stress tolerance in tobacco (Veena et al., 1999), black gram (Bhomkar et al., 2008), Arabidopsis (Roy et al., 2008), mustard (Rajwanshi et al., 2016), and rice crop (Verma et al., 2005). Gly II in rice (OsGly II) has been reported to detoxify of MG and also involved in salt stress tolerance (Yadav et al., 2007, Singla-Pareek et al., 2008, Wani and Gosal, 2011). However, functional validation of rice Gly I gene has not been previously documented. The objectives of the present work were to clone the rice gene representing Gly I (OsGly I), and overexpress OsGly I in rice (Oryza sative L. cv. Nipponbare) using Agrobacterium-mediated transformation to evaluate its physiological roles on the improvements of abiotic stress tolerance and agronomic performance in rice.
Materials and methods
Results
Discussion
Changes in cell metabolism in response to abiotic stresses often result in increased levels of MG and lipid peroxidation due to alterations in glycolysis and tricarboxylicacidcycle acid cycle (Umeda et al., 1994). Gly I, a major component of the glyoxalase system, plays an important role in regulating MG levels and thus cytotoxicity and stress tolerance (Vij and Tyagi, 2007). In the present study, various expression abundance of rice Gly I (OsGly I) was detected in different organs of rice plants (Fig. 1A), and enhanced by NaCl, ZnCl2 and mannitol treatments in seedlings (Fig. 1B). This observation is in accordance with a recent report by Wu et al. (2013) in sugar beet (Beta vulagris), who found that BvGly I was ubiquitously expressed in different tissues of sugar beet M14 line and was up-regulated in response to salt, mannitol and oxidative stresses. Veena et al. (1999) observed B. juncea Gly I was increased when subjected to salt, water and heavy metal stresses. Lin et al. (2010) also indicated that wheat Gly I was induced by NaCl and ZnCl2, as well as by inoculation of plants with pathogen Fusarium graminearum. Consistently, the expression patterns of OsGly I in the current work suggest that rice OsGly I might also be involved in abiotic stress tolerance.