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Several halophytes and a few crop plants, including Poaceae, synthesize and accumulate glycine betaine (GB) in response to environmental constraints. GB plays an important role in osmoregulation, in fact, it is one of the main nitrogen-containing compatible osmolytes found in Poaceae. It can interplay with molecules and structures, preserving the activity of macromolecules, maintaining the integrity of membranes against stresses and scavenging ROS. Exogenous GB applications have been proven to induce the expression of genes involved in oxidative stress responses, with a restriction of ROS accumulation and lipid peroxidation in cultured tobacco cells under drought and salinity, and even stabilizing photosynthetic structures under stress. In the plant kingdom, GB is synthesized from choline by a two-step oxidation reaction. The first oxidation is catalyzed by choline monooxygenase (CMO) and the second oxidation is catalyzed by NAD+-dependent betaine aldehyde dehydrogenase. Moreover, in plants, the cytosolic enzyme, named -methyltransferase, catalyzes the conversion of phosphoethanolamine to phosphocholine. However, changes in CMO expression genes under abiotic stresses have been observed. GB accumulation is ontogenetically controlled since it happens in young tissues during prolonged stress, while its degradation is generally not significant in plants. This ability of plants to accumulate high levels of GB in young tissues under abiotic stress, is independent of nitrogen (N) availability and supports the view that plant N allocation is dictated primarily to supply and protect the growing tissues, even under N limitation. Indeed, the contribution of GB to osmotic adjustment and ionic and oxidative stress defense in young tissues, is much higher than that in older ones. In this review, the biosynthesis and accumulation of GB in plants, under several abiotic stresses, were analyzed focusing on all possible roles this metabolite can play, particularly in young tissues.

Nonribosomal peptide synthetases (NRPSs) and NRPS-like enzymes have diverse functions in primary and secondary metabolisms. By using a structure-guided approach, we uncovered the function of a NRPS-like enzyme with unusual domain architecture, catalyzing two sequential two-electron reductions of glycine betaine to choline. Structural analysis based on the homology model suggests cation-π interactions as the major substrate specificity determinant, which was verified using substrate analogs and inhibitors. Bioinformatic analysis indicates this NRPS-like glycine betaine reductase is highly conserved and widespread in kingdom fungi. Genetic knockout experiments confirmed its role in choline biosynthesis and maintaining glycine betaine homeostasis in fungi. Our findings demonstrate that the oxidative choline-glycine betaine degradation pathway can operate in a fully reversible fashion and provide insight in understanding fungal choline metabolism. The use of an NRPS-like enzyme for reductive choline formation is energetically efficient compared with known pathways. Our discovery also underscores the capabilities of the structure-guided approach in assigning functions of uncharacterized multidomain proteins, which can potentially aid functional discovery of new enzymes by genome mining.

The plants are exposed to different abiotic stresses, including the salinity stress (SS) that negatively affect the growth, metabolism, physiological and biochemical processes. Thus, this study investigated the effect of diverse levels of foliar-applied GB (0 control, 50 mM and 100 mM) on maize growth, membrane stability, physiological and biochemical attributes, antioxidant enzymes and nutrients accumulation under different levels of SS (i.e., control, 6 dS m-1, 12 dS m-1). Salt stress diminished the root and shoot length, root and shoot biomass, chlorophyll contents, photosynthetic rate (Pn), stomatal conductance (gs), relative water contents (RWC), soluble proteins (SP) and free amino acids; (FAA); and increased activities of antioxidant enzymes, electrical conductivity (EC) and accumulation of malondialdehyde (MDA), hydrogen peroxide (H2O2), Na+ and Cl− ions. GB application significantly increased root and shoot growth, leaves per plant, shoots length, chlorophyll contents, gs, Pn and membrane stability by reducing MDA and H2O2 accumulation. Moreover, GB also increased the SP, FAA accumulation, activities of antioxidant enzymes and Na+ and Cl- exclusion by favouring Ca2+ and K+ accumulation. In conclusion, the foliar-applied GB increased Pn, gs, ant-oxidants activities, and accumulation of SP and FAA; and reduced the accretion of Na+ and Cl− by favouring the Ca2+ and K+ accretion which in turns improved growth under SS.

Unexpected biomagnifications and bioaccumulation of heavy metals (HMs) in the surrounding environment has become a predicament for all living organisms together with plants. Excessive release of HMs from industrial discharge and other anthropogenic activities has threatened sustainable agricultural practices and limited the overall profitable yield of different plants species. Heavy metals at toxic levels interact with cellular molecules, leading towards the unnecessary generation of reactive oxygen species (ROS), restricting productivity and growth of the plants. The application of various osmoprotectants is a renowned approach to mitigate the harmful effects of HMs on plants. In this review, the effective role of glycine betaine (GB) in alleviation of HM stress is summarized. Glycine betaine is very important osmoregulator, and its level varies considerably among different plants. Application of GB on plants under HMs stress successfully improves growth, photosynthesis, antioxidant enzymes activities, nutrients uptake, and minimizes excessive heavy metal uptake and oxidative stress. Moreover, GB activates the adjustment of glutathione reductase (GR), ascorbic acid (AsA) and glutathione (GSH) contents in plants under HM stress. Excessive accumulation of GB through the utilization of a genetic engineering approach can successfully enhance tolerance against stress, which is considered an important feature that needs to be investigated in depth.

Background The imbalance between Egypt's water requirements and supply necessitates the use of unconventional water sources, such as treated sewage water (TSW) and agricultural drainage water (ADW), to combat water scarcity. This study investigated the effects of foliar glycine betaine (GB) on vegetative growth parameters, physiological characteristics, photosynthetic pigments, leaf element contents, anatomical leaf structures, and antioxidant activity. The experiment was conducted in two successive seasons (2021/2022 and 2022/2023) using Kapok seedlings irrigated with ADW and TSW at different mixing ratios with normal irrigation water (NIW) (25%, 50%, 75%, and 100%), combined with foliar spraying of GB at concentrations of 0.0 and 50 mM. Results The results revealed that irrigation with 100% TSW or ADW significantly decreased vegetative growth parameters, physiological characteristics, photosynthetic pigments, leaf element contents, leaf thickness, and the contents of the leaf mid-vein, N, P, K, and Ca. In contrast, the levels of free proline, total phenolic content, Na, Cu, Ni, Mn, Zn, Pb, and antioxidant activity increased. Additionally, GB significantly improved all parameters, while reducing the contents of Na, Cu, Ni, Mn, Zn, and Pb in the leaves. Conclusions Irrigation of Kapok seedlings with TSW or ADW mixed with NIW at 25% and 50% resulted in better performance, similar to irrigation with NIW alone for most parameters. Combining GB and water treatments by mixing TSW or ADW with NIW at a 50:50 ratio and spraying with 50 mM GB produced better results than control seedlings irrigated with 100% NIW. Antioxidants also play a defensive role in plants against various stress factors. Therefore, GB may have a protective effect on peroxidation-linked membrane deterioration, scavenge free radicals, and provide osmotic protection.

Osmotic adaptation and accumulation of compatible solutes is a key process for life at high osmotic pressure and elevated salt concentrations. Most important solutes that can protect cell structures and metabolic processes at high salt concentrations are glycine betaine and ectoine. The genome analysis of more than 130 phototrophic bacteria shows that biosynthesis of glycine betaine is common among marine and halophilic phototrophic Proteobacteria and their chemotrophic relatives, as well as in representatives of Pirellulaceae and Actinobacteria, but are also found in halophilic Cyanobacteria and Chloroherpeton thalassium. This ability correlates well with the successful toleration of extreme salt concentrations. Freshwater bacteria in general lack the possibilities to synthesize and often also to take up these compounds. The biosynthesis of ectoine is found in the phylogenetic lines of phototrophic Alpha- and Gammaproteobacteria, most prominent in the Halorhodospira species and a number of Rhodobacteraceae. It is also common among Streptomycetes and Bacilli. The phylogeny of glycine-sarcosine methyltransferase (GMT) and diaminobutyrate-pyruvate aminotransferase (EctB) sequences correlate well with otherwise established phylogenetic groups. Most significantly, GMT sequences of cyanobacteria form two major phylogenetic branches and the branch of Halorhodospira species is distinct from all other Ectothiorhodospiraceae. A variety of transport systems for osmolytes are present in the studied bacteria.

Transgenic tobacco plants with additional enzymes producing hydrogen peroxide (H2O2)—superoxide dismutase from Arabisopsis thaliana and choline oxidase from Arthrobacter globiformis—have increased resistance to stress factors, which was demonstrated previously, but their reproductive potential has not been studied to date. Superoxide dismutase converts superoxide radical into H2O2, and choline oxidase catalyzes the oxidation reaction of choline to form betaine aldehyde, which is subsequently converted into glycine betaine and H2O2. We found that the addition of both exogenous genes stimulated growth of the floral organs: petals, styles, and stamens. However, the reproductive potential of the transgenic plants was different. Thus, the introduction of the superoxide dismutase gene FeSOD significantly increased pollen germination in vitro, in vivo, the size of fruits, and the number of seeds. At the same time, the insertion of the CodA gene resulted in the production of abnormal pollen with low germination in vitro. The female reproductive potential system in these plants was not affected. Thus, shifting the ROS balance towards hydrogen peroxide not only increases tobacco stress resistance but also stimulates reproductive success. Glycine betaine production disrupts pollen formation, although such plants show increased resistance to osmotic stress.

The low salinity tolerance of almond cultivars can cause a significant setback in almond production. Therefore, selecting suitable cultivars and rootstocks in salinity-affected areas can facilitate sustainable crop production. In this research, the effects of two wild almond species, Badamkohiand Arjan as rootstocks on the salinity tolerance of ‘Sahand’ as a scion were investigated through in vitro culture. A factorial experiment of 2 (species) × 4 (levels of salinity) was conducted in a completely randomized design (CRD) with 4 replications. The results showed that ‘Sahand’ grafted on Badamkohi had the higher fresh and dry weight than grafted on Arjan in all level of salinity. The Na+ and Cl- ions contents inthe shoots and root of both micrografting combinations increased with increasing salinity. However, their amount in the shoot and the root of ‘Sahand’/Arjan plants were significantly higher than those ions in ‘Sahand’/Badamkohi plants at 80 and 120 mM NaCl. The amount of total chlorophyll in ‘Sahand’ grafted on Badamkohi was 0.68 mg g-1 FW which was significantly higher than the total chlorophyll of the same scion grafted on Arjan rootstock (0.51 mg g-1 FW) at 120 mM NaCl. The highest leaf cell electrical leakage occurred in ‘Sahand’ grafted on Arjan which was significantly higher than leaf electrical leakage of the same scion grafted on Badamkohi at 120 mM NaCl. The grafting combination of ‘Sahand’/Badamkohi showed a higher proline and glycine betaine content, compared to the grafting combination of ‘Sahand’/Arjan. The shoot and root antioxidant enzyme activities (SOD, POX and CAT) in micrografting combination of ‘Sahand’/ Badamkohi were also significantly higher than those in ‘Sahand’/Arjan. It can be concluded that ‘Sahand/Badamkohi combination is a suitable choice for the regions with late spring frost and saline conditions.

Glycine betaine is one of the most effective compatible solutes of the halophilic lactic acid bacterium Tetragenococcus halophilus, the transportation of which is essential for its survival under salinity stress condition. In the current study, we attempted to define a glycine betaine ABC transporter system of T. halophilus, busATha, which plays an important role in adapting to salinity condition. The expression of busATha enhanced the growth of the recombinant strain under high salinity. BusRTha, a transcription regulator that represses the expression of busATha, was characterized, and the repression was abrogated under high salinity. The binding of the regulator was demonstrated through electrophoretic mobility shift assays, and the binding sites were characterized as 5′-AAA(T/G)TGAC(C/A)(G/A)T(C/A)C-3′. This is the first studied transcription regulator of T. halophilus, and our findings provide insights into the molecular mechanism of halophilic life and tools for further application of halophiles as chassis in industrial biotechnology.

• Glycine betaine (GB) is known to accumulate in plants exposed to cold, but the underlying molecular mechanisms and associated regulatory network remain unclear. • Here, we demonstrated that PtrMYC2 of Poncirus trifoliata integrates the jasmonic acid (JA) signal to modulate cold-induced GB accumulation by directly regulating PtrBADH-l, a betaine aldehyde dehydrogenase (BADH)-like gene. • PtrBADH-l was identified based on transcriptome and expression analysis in P. trifoliata. Overexpression and VIGS (virus-induced gene silencing)-mediated knockdown showed that PtrBADH-l plays a positive role in cold tolerance and GB synthesis. Yeast one-hybrid library screening using PtrBADH-l promoter as baits unraveled PtrMYC2 as an interacting candidate. PtrMYC2 was confirmed to directly bind to two G-box cis-acting elements within PtrBADH-l promoter and acts as a transcriptional activator. In addition, PtrMYC2 functions positively in cold tolerance through modulation of GB synthesis by regulating PtrBADH-l expression. Interestingly, we found that GB accumulation under cold stress was JA-dependent and that PtrMYC2 orchestrates JA-mediated PtrBADH-l upregulation and GB accumulation. • This study sheds new light on the roles of MYC2 homolog in modulating GB synthesis. In particular, we propose a transcriptional regulatory module PtrMYC2-PtrBADH-l to advance the understanding of molecular mechanisms underlying the GB accumulation under cold stress.