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Glyoxalase (GLY) system, comprising of GLYI and GLYII enzymes, has emerged as one of the primary methylglyoxal (MG) detoxification pathways with an indispensable role during abiotic and biotic stresses. MG homeostasis is indeed very closely guarded by the cell as its higher levels are cytotoxic for the organism. The dynamic responsiveness of MG-metabolizing GLY pathway to both endogenous cues such as, phytohormones, nutrient status, etc., as well as external environmental fluctuations (abiotic and biotic stresses) indicates that a tight regulation occurs in the cell to maintain physiological levels of MG in the system. Interestingly, GLY pathway is also manipulated by its substrates and reaction products. Hence, an investigation of signalling and regulatory aspects of GLY pathway would be worthwhile. Herein, we have attempted to converge all known factors acting as signals or directly regulating GLYI/II enzymes in plants. Further, we also discuss how crosstalk between these different signal molecules might facilitate the regulation of glyoxalase pathway. We believe that MG detoxification is controlled by intricate mechanisms involving a plethora of signal molecules.

Heat stress, owing to the recent climate change events, has emerged as one of the major environmental factors posing a grave threat to plant survival and crop productivity. Oxidative damage due to increased generation of reactive oxygen and dicarbonyl species in response to heat, is one of the serious consequences of high temperature stress. Methylglyoxal (MG), is one such cytotoxic dicarbonyl metabolite whose levels are known to increase in response to high temperature and various other stress conditions in plants. In fact, it is considered as a general consequence of stress in plants. The synergistic co-activation of reactive oxygen species scavenging and the MG detoxification pathways thus, plays a key role in regulation of stress responses for plant acclimation. Glyoxalase enzymes are the major MG detoxifying proteins in the living organisms. In plants, glyoxalases are encoded as multigenic families, known to be involved in plant growth and development, although, chiefly associated with multiple stress inducible behaviour and tolerance. While their role in salinity, drought and heavy metal stresses has been well-studied, their significance under high temperature stress has received limited coverage. In this review, we emphasize on the dynamic role of glyoxalases under high temperature stress for conferring thermotolerance. Through analysis of heat responsive transcriptomes and proteomes of various plants, we provide evidence for a hitherto, unexplored role of glyoxalases as a mechanism of mounting thermotolerance responses. We also, show that ‘priming’ induced stress/cross stress tolerance in plants involves the activation of glyoxalase pathway.

Serotonin (Ser) and melatonin (Mel) serve as master regulators of plant growth and development by influencing diverse cellular processes. The enzymes namely, tryptophan decarboxylase (TDC) and tryptamine 5-hydroxylase (T5H) catalyse the formation of Ser from tryptophan. Subsequently, serotonin N-acetyl transferase (SNAT) and acetyl-serotonin methyltransferase (ASMT) form Mel from Ser. Plant genomes harbour multiple genes for each of these four enzymes, all of which have not been identified. Therefore, to delineate information regarding these four gene families, we carried out a genome-wide analysis of the genes involved in Ser and Mel biosynthesis in Arabidopsis, tomato, rice and sorghum. Phylogenetic analysis unravelled distinct evolutionary relationships among these genes from different plants. Interestingly, no gene family except ASMTs showed monocot- or dicot-specific clustering of respective proteins. Further, we observed tissue-specific, developmental and stress/hormone-mediated variations in the expression of the four gene families. The light/dark cycle also affected their expression in agreement with our quantitative reverse transcriptase-PCR (qRT-PCR) analysis. Importantly, we found that miRNAs (miR6249a and miR-1846e) regulated the expression of Ser and Mel biosynthesis under light and stress by influencing the expression of OsTDC5 and OsASMT18, respectively. Thus, this study may provide opportunities for functional characterization of suitable target genes of the Ser and Mel pathway to decipher their exact roles in plant physiology.

The severe impact of climate change on crop production is already evident; there are strong chances of further changes occurring. Therefore, there is an urgent need to address agricultural adaptation more coherently. Adapting agriculture to the increasing scale of climate risks will ensure food security and sustainability. An increase in intensity and frequency of temperature extremes such as heat and cold along with other abiotic stresses such as salinity and drought are all manifestations of a global environmental change. Owing to their sessile nature, plants have developed several strategies to adapt to the adverse effects of abiotic stresses, synthesis, and build-up of osmoprotectants being one of them. Osmoprotectants are soluble organic compounds compatible with the cellular metabolism and hence are also known as compatible solutes. Among the various compatible solutes, Glycine betaine (GB) is most effective in imparting abiotic stress tolerance to plants. There is enough evidence to suggest the protective role of GB owing to its versatility and ability to accumulate under unfavourable conditions. GB, however, does not accumulate in many crop plants such as rice, tobacco, etc. Tailoring the GB biosynthetic pathway has thus been one of most promising strategies in reducing susceptibility of such crops to abiotic stresses. A number of crop plants have been engineered since then with an objective to increase the accumulation of GB to acquire improved stress tolerance. In this chapter, we shall discuss about the role of GB in ameliorating stress tolerance in plants, major breakthroughs in engineering its pathway, magnitude and mechanism of protective effects, challenges faced, strategies to overcome limitations and future prospects.

Background The glyoxalase pathway is evolutionarily conserved and involved in the glutathione-dependent detoxification of methylglyoxal (MG), a cytotoxic by-product of glycolysis. It acts via two metallo-enzymes, glyoxalase I (GLYI) and glyoxalase II (GLYII), to convert MG into D-lactate, which is further metabolized to pyruvate by D-lactate dehydrogenases (D-LDH). Since D-lactate formation occurs solely by the action of glyoxalase enzymes, its metabolism may be considered as the ultimate step of MG detoxification. By maintaining steady state levels of MG and other reactive dicarbonyl compounds, the glyoxalase pathway serves as an important line of defence against glycation and oxidative stress in living organisms. Therefore, considering the general role of glyoxalases in stress adaptation and the ability of Sorghum bicolor to withstand prolonged drought, the sorghum glyoxalase pathway warrants an in-depth investigation with regard to the presence, regulation and distribution of glyoxalase and D-LDH genes. Result Through this study, we have identified 15 GLYI and 6 GLYII genes in sorghum. In addition, 4 D-LDH genes were also identified, forming the first ever report on genome-wide identification of any plant D-LDH family. Our in silico analysis indicates homology of putatively active SbGLYI, SbGLYII and SbDLDH proteins to several functionally characterised glyoxalases and D-LDHs from Arabidopsis and rice. Further, these three gene families exhibit development and tissue-specific variations in their expression patterns. Importantly, we could predict the distribution of putatively active SbGLYI, SbGLYII and SbDLDH proteins in at least four different sub-cellular compartments namely, cytoplasm, chloroplast, nucleus and mitochondria. Most of the members of the sorghum glyoxalase and D-LDH gene families are indeed found to be highly stress responsive. Conclusion This study emphasizes the role of glyoxalases as well as that of D-LDH in the complete detoxification of MG in sorghum. In particular, we propose that D-LDH which metabolizes the specific end product of glyoxalases pathway is essential for complete MG detoxification. By proposing a cellular model for detoxification of MG via glyoxalase pathway in sorghum, we suggest that different sub-cellular organelles are actively involved in MG metabolism in plants.

Additional file 4: Figure S4. Multiple sequence alignment of predicted SbGLYII proteins with GLYII proteins from rice. Conserved THHHXDH and C/GHT motifs have been highlighted in blue (for active GLYII) and green (for SDO) colours and indicated by a bar.

Additional file 9. Glyoxalase I protein sequences from various species used for the phylogenetic analysis.

Additional file 8: Table S2. List of primers used in the study.

Additional file 5: Table S1. List of FAD-binding-4 containing oxido-reductase superfamily members.

Additional file 3: Figure S3. Domain sequence alignment of predicted GLYI proteins from sorghum and rice. Conserved metal binding sites have been marked with an asterisk and region specific for Zn2+-dependent isoforms has been marked with a red line. Each domain of the two glyoxalase domain (PF00903)-containing putative GLYI proteins has been represented as a separate sequence, with domains being indicated by numbers 1 or 2 suffixed to the protein name thereby, indicating first and second domain, respectively.

Additional file 1: Figure S1. Determination of alternate splicing of putative functionally active SbGLYI transcripts. (A) Depiction of primer designing scheme. (B) Details of primers used for the determination of spliced variants of SbGLYI transcripts and respective amplification details. (C) Gel showing the amplification of SbGLYI transcripts.

Additional file 6: Figure S5. Multiple sequence alignment of predicted SbDLDH proteins with the previously characterised rice and Arabidopsis D-LDH proteins.

Additional file 7: Figure S6. Phylogenetic tree representing all putative FAD_binding_4 oxido-reductase superfamily proteins in Sorghum bicolor. Differently coloured rings in the circular tree represent different domains present in the respective proteins.

Additional file 2: Figure S2. Alignment of SbGLYI-8/8.1 with its rice and Arabidopsis orthologs. Box indicates the location of putative nuclear localisation signal (NLS) sequences.