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 N-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.

Durum wheat plants are extremely sensitive to drought and salinity during seedling and early development stages. Their responses to stresses have been extensively studied to provide new metabolic targets and improving the tolerance to adverse environments. Most of these studies have been performed in growth chambers under low light [300-350 µmol m s photosynthetically active radiation (PAR), LL]. However, in nature plants have to face frequent fluctuations of light intensities that often exceed their photosynthetic capacity (900-2000 µmol m s ). In this study we investigated the physiological and metabolic changes potentially involved in osmotic adjustment and antioxidant defense in durum wheat seedlings under high light (HL) and salinity. The combined application of the two stresses decreased the water potential and stomatal conductance without reducing the photosynthetic efficiency of the plants. Glycine betaine (GB) synthesis was inhibited, proline and glutamate content decreased, while γ-aminobutyric acid (GABA), amides and minor amino acids increased. The expression level and enzymatic activities of Δ1-pyrroline-5-carboxylate synthetase, asparagine synthetase and glutamate decarboxylase, as well as other enzymatic activities of nitrogen and carbon metabolism, were analyzed. Antioxidant enzymes and metabolites were also considered. The results showed that the complex interplay seen in durum wheat plants under salinity at LL was simplified: GB and antioxidants did not play a main role. On the contrary, the fine tuning of few specific primary metabolites (GABA, amides, minor amino acids and hexoses) remodeled metabolism and defense processes, playing a key role in the response to simultaneous stresses.

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Plants are currently experiencing increasing salinity problems due to irrigation with brackish water. Moreover, in fields, roots can grow in soils which show spatial variation in water content and salt concentration, also because of the type of irrigation. Salinity impairs crop growth and productivity by inhibiting many physiological and metabolic processes, in particular nitrate uptake, translocation, and assimilation. Salinity determines an increase of sap osmolality from about 305 mOsmol kg−1 in control roots to about 530 mOsmol kg−1 in roots under salinity. Root cells adapt to salinity by sequestering sodium in the vacuole, as a cheap osmoticum, and showing a rearrangement of few nitrogen-containing metabolites and sucrose in the cytosol, both for osmotic adjustment and oxidative stress protection, thus providing plant viability even at low nitrate levels. Mainly glycine betaine and sucrose at low nitrate concentration, and glycine betaine, asparagine and proline at high nitrate levels can be assumed responsible for the osmotic adjustment of the cytosol, the assimilation of the excess of ammonium and the scavenging of ROS under salinity. High nitrate plants with half of the root system under salinity accumulate proline and glutamine in both control and salt stressed split roots, revealing that osmotic adjustment is not a regional effect in plants. The expression level and enzymatic activities of asparagine synthetase and Δ1-pyrroline-5-carboxylate synthetase, as well as other enzymatic activities of nitrogen and carbon metabolism, are analyzed.

Hordeum maritimum With. (= H. marinum Huds. subsp. marinum, 2n = 14) is a wild cereal present in the saline depressions of the Soliman and Kelbia Sebkhas, which contributes significantly to annual biomass production in Tunisia. This species is able to tolerate high NaCl concentrations at the seedling stage without showing symptoms of toxicity; however, the tolerance strategy mechanisms of this plant have not yet been unravelled. Our metabolite analysis, performed on leaves of H. maritimum during extended stress in comparison with Hordeum vulgare L. cv. Lamsi, has revealed an adaptive response of the wild species based on a different temporal accumulation pattern of ions and compatible metabolites. Further, wild and cultivated genotypes with contrasting salt-tolerant behaviour display different pattern of metabolites when salt stress is prolonged over 2 weeks. In particular, when exposed to up to 3 weeks of 200 mM NaCl salt stress, H. maritimum is able to maintain lower leaf concentrations of sodium and chloride, and higher concentrations of potassium compared with H. vulgare. This likely restricts sodium entry into plants at the root level, and uses the toxic ions, glycine betaine and low levels of proline for osmotic adjustment. Under prolonged stress, the accumulation of proline increases, reaching the highest levels in concomitance with the decrease of potassium to sodium ratio, the increase of hydrogen peroxide and decrease of chlorophylls. The modulation of proline accumulation over time can be interpreted as an adaptive response to long-term salinity. Moreover, once synthetised glycine betaine is transported but not metabolised, it can contribute together with proline to osmotically balance H. maritimum leaves and protect them from oxidative stress. The 2–3 week delay of H. maritimum in showing the symptoms of stress and damages compared with H. vulgare could be important in the survival of plants when soil salinity is not a permanent condition, but just a transient state of stress.