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Significance:
The imino acid proline is utilized by different organisms to offset cellular imbalances caused by environmental stress. The wide use in nature of proline as a stress adaptor molecule indicates that proline has a fundamental biological role in stress response. Understanding the mechanisms by which proline enhances abiotic/biotic stres...
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Context 1
... the P5CS1 knockout of Arabidopsis , significantly lower activities of ASC-GSH cycle-related enzymes, including ascorbate peroxidase, GSH peroxidase, and GSH-S-transferase, were observed under NaCl stress conditions (124). Chen et al. reported that the addition of proline in the growth medium quenched ROS as efficiently as other ROS scavengers, such as N-acetyl cysteine in the fungal pathogen Colletotrichum trifolii (17). The decrease in ROS was shown to be due to an increase in catalase activity by proline treatment (17). Altogether, different groups have reported that increased proline levels enhance antioxidant enzyme activity. Studies comparing the ability of different biological osmolytes to stabilize proteins have provided insights into the chaperone properties of proline. Proline stabilizes protein structures by driving burial of the peptide backbone and protein folding (7, 130, 143). This is different than protein folding in the absence of osmolytes, which is driven by fa- vorable burying of nonpolar side chains (7, 130, 143). Relative to other osmolytes, proline is categorized as a weak stabilizer of protein folding and ranks lower in ability to induce protein folding (7, 11). Thus, although proline helps stabilize proteins, besides facilitating protein folding, additional mechanisms likely contribute to the protective effect of proline during stress. Another mechanism by which proline protects cells against stress has been suggested to involve the chelation of metals. High proline content in metal-tolerant plants is not unusual (113). One of the major toxicities of heavy metals is pertur- bation of cellular redox balance by ROS production (114). A potent oxidizing agent of biological macromolecules in the cell is the hydroxyl radical (OH ), which is formed by the reduction of H 2 O 2 by transition metal ions such as Cu + and Fe 2 + (114). The function of proline as a metal chelator was suggested by Sharma et al. , who reported that proline can protect enzymes from zinc- and cadmium-induced inhibition by forming proline-metal complexes (115). A copper–proline complex was also reported in the copper-tolerant Armeria maritima (34). The ability of proline to directly react with ROS has been investigated by numerous laboratories (58, 114). Previous studies have shown that free and polypeptide-bound proline can react with H 2 O 2 and OH (pH 7–8) to form stable free radical adducts of proline and hydroxyproline derivatives as shown in Figure 3 ( e.g ., 4-hydroxyproline and 3-hydro- xyproline) (38, 58, 102, 106, 127). Although Floyd and Nagy (38) observed that nitroxyl radicals accumulate during the incubation of proline with H 2 O 2 , the reaction is very slow relative to that of proline and OH (5.4 · 10 8 M - 1 s - 1 ) (3). Recently, the ability of proline to scavenge H 2 O 2 was com- pared with pyruvate, a well-established scavenger of H 2 O 2 . At 30 min, H 2 O 2 levels were diminished by > 90% in cell medium supplemented with 1 m M pyruvate, whereas no significant decrease was observed with proline (5 m M ) (70). This observation further indicates that a direct reaction between H O and proline does not significantly contribute to the scavenging of cellular H 2 O 2 (48). Proline has also been shown not to directly scavenge O 2 - (58). An ROS-scavenging mechanism that is important to proline stress protection is the facile reaction of proline with singlet oxygen ( 1 O 2 ). In cultured skin fibroblasts, exogenously added proline has been shown to diminish 1 O 2 levels (141). Proline has been shown to protect human skin cells from photo-induced apoptosis, suggesting that proline suppresses photo-oxidative stress and skin carcinogenesis (141). In plants, Alia et al. reported that during strong illumination, the production of 1 O 2 in the thylakoids from the cotyledons of Brassica juncea was dramatically suppressed by proline (5). The five-membered ring of proline, pyrrolidine, has a low ionization potential that effectively quenches 1 O 2 most likely through a charge transfer mechanism in which molecular oxygen returns to the ground triplet state ( 3 O 2 ) (Fig. 3) (20, 81, 152). Alia and coworkers used irradiation of various photo- sensitizers to produce 1 O 2 , which is detected by measuring the formation of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) by EPR (4). TEMPO formation was completely inhibited by adding 20 m M proline to the reaction, indicating that proline directly scavenged or quenched 1 O 2 (4). Quenching of 1 O 2 is also documented for other secondary amine compounds as well, such as spermine (63). Due to its action as a 1 O 2 quencher, proline may help stabilize proteins, DNA, and membranes (81). Prolyl residues in proteins also provide protection against oxidative stress caused by 1 O 2 . For example, the epithelial small proline-rich protein, which is a precursor of the cornified envelope of the epidermal skin, is strongly induced after exposure of the skin to UV radiation (37, 133). Figure 3 summarizes the reactivity of proline with different ROS. In addition to the chemical properties of proline discussed earlier, changes in proline metabolic flux can also impact stress tolerance. The effects of proline metabolism on the intracellular redox state have been well studied by Phang and coworkers, who first proposed that the proline-P5C cycle can help maintain proper NADP + /NADPH levels in the cytosol and drive the oxidative pentose phosphate pathway (95). The cycling of proline and P5C via PRODH and P5CR results in the transfer of reducing equivalents from the cytosol into the mitochondria (84, 95). Electrons are passed into the mitochondrial ETC directly from PRODH via ubiquinone, leading to increases in oxidative phosphorylation and mitochondrial ROS production (84, 95). The proline-P5C cycle is, thus, thought to help maintain a proper NADP + /NADPH ratio (84, 95). The proline-P5C cycle may especially be important when increased PRODH activity is not balanced with P5CDH activity (84, 149). Currently, it is not known how P5C shuttles in/out of the mitochondria. An excellent review on the proline-P5C cycle and the wide ranging effects of proline metabolism was recently provided by Phang (97). Evidence for proline metabolic flux influencing the NADP + /NADPH ratio in plants has been reported by several groups and summarized in Figure 4 (44, 66, 112). Proline metabolic cycling was found to increase oxidation of NADPH in the soybean nodule, thereby enhancing the oxidative pentose phosphate pathway (66). Increased flux through the oxidative pentose phosphate pathway would support purine nucleotide biosynthesis during stress recovery (Fig. 4) (44, 66). Up-regulation of proline synthesis has also been proposed to maintain the NADP + /NADPH ratio at normal levels during photoinduced stress (44, 123). Significant decreases in the NADP + /NADPH ratio has been reported under different stress conditions due to decreased Calvin cycle activity (44, 123). Without sufficient levels of NADP + available for electron transfer, photosynthetic cells under stress conditions produce more 1 O 2 when exposed to high light (16, 123). Light exposure, however, promotes P5CS expression, leading to increased proline biosynthesis and NADP + levels, which ultimately diminishes 1 O 2 production in the chloroplasts (Fig. 4) (110, 124). These observations suggest a link between enhanced proline synthesis and photoinduced oxidative stress. Hare et al. suggest that the redox modulation accom- panying proline synthesis may be more important than proline accumulation (44). Manipulation of proline metabolic enzyme expression has also provided evidence for proline metabolism influencing NADP + levels in plants. A comparison of sense-orientated and antisense-orientated P5CR gene transgenic soybean plants showed that sense plants had higher NADP + levels and higher stress tolerance relative to antisense plants (25). Antisense knockdown of P5CR resulted in lower NADP + levels and higher sensitivity to stress (25). Recently, Sharma and coworkers reported that under low water stress, plants deficient in P5CS1 or PRODH1 exhibit a lower NADP + / NADPH ratio than wild-type plants (112). In addition, L- proline catabolism was suggested to be important for maintaining the NADP + /NADPH ratio, as the NADP + /NADPH ratio is significantly lower in the prodh mutant of Arabidopsis than wild-type Arabidopsis (112). Although PRODH1 activity apparently declines during stress, a low level of cycling between proline and P5C may be enough to support the main- tenance of proper NADP + /NADPH (44). Different studies have shown that proline addition to the cell medium and up-regulation of endogenous proline biosynthesis leads to increased total GSH and protection of intracellular reduced GSH (47, 118, 144). Guarding reduced GSH is especially important in heavy metal stress, as heavy metal ion toxicity is often associated with depletion of GSH (114). The ability of proline to protect GSH during metal ion stress was tested in Chlamydomonas reinhardtii in which transgenic algae expressing the mothbean P5CS gene had 80% higher intracellular proline levels relative to wild- type algae (118). After exposing cells to 50 l M Cd 2 + , the GSH:0.5GSSG ratio was four-fold higher in transgenic algae relative to wild-type cells, indicating that proline prevents GSH depletion during heavy metal stress (118). The higher GSH levels in the P5CS transgenic algae were suggested to increase phytochelatin synthesis and the formation of Cd- thiolate complexes in the vacuole, thereby protecting against heavy metal stress (118). The manner in which proline protects the GSH pool is not clear, but it has been proposed that proline directly scavenges OH and 1 O 2 generated by heavy metal stress and helps stabilize ROS detoxifying enzymes (47, 118, 144). The proline and GSH synthesis pathways share the intermediate, c -glutamyl phosphate, suggesting possible crosstalk ...
Context 2
... of NADP + /NADPH and GSH/GSSG), and cellular signaling promoted by proline metabolism. The potential mechanisms by which proline provides stress protection are discussed next. Proline is one of the several small molecules classified as an osmolyte or an osmoprotectant (22). Other biologically important osmolytes are glycerol, trehalose, sorbitol, sucrose, taurine, sarcosine, glycine betaine, and trimethylamine N oxide (145). These osmolytes are accumulated in response to conditions of drought, salt, and temperature extremes. Osmolytes help mitigate water stress and balance turgor pressure during stress (22). Osmolytes are also excellent cryoprotectants (92). For example, proline has been shown to increase the freeze tolerance of yeast (85, 125) and plants (45, 123, 151), and to be a useful cryoprotector of protein crystals (92), fly larvae (67, 68), plant cells (140), and human stem cells (39). Thus, as an osmolyte, proline is an important molecule that is employed by various organisms to combat stress. Proline has been shown to act as a chemical protein chaperone and to prevent protein aggregation (71, 107). A ther- mosensitive dnaK -mutant strain of E. coli was rescued by increased intracellular proline levels using a GK variant that is insensitive to proline inhibition (15). The higher proline levels reduced protein aggregation and thermodenaturation. In an in vitro experiment, proline (1 M ) protected nitrate reductase under osmotic, metal, and H 2 O 2 stress (111). Ignatova et al. reported that proline can prevent the aggregation of P39A cellular retinoic acid-binding protein (an aggregation prone protein) under salt stress (51). Proline also diminished the aggregation of a cellular retinoic acid-binding protein that is fused to a pathogenic polyglutamine repeat of the human huntingtin protein (51). Due to its chaperone properties, proline protection against oxidative stress has been proposed to involve enhancement and stabilization of redox enzymes. Exogenous application of proline to cell cultures has been found to increase the activity of different antioxidant enzymes under salt (47), cadmium (53, 54, 144), and oxidative stress (17), resulting in increased stress tolerance. These enzymes include superoxide dis- mutase (53, 144), catalase (17, 48, 53, 144), and GSH related or ascorbate (ASC)-GSH cycle-related enzymes (47, 54). All these are important antioxidant enzymes (52, 87) and in plants, the ASC-GSH cycle is especially critical for mitigating ROS (87). In the P5CS1 knockout of Arabidopsis , significantly lower activities of ASC-GSH cycle-related enzymes, including ascorbate peroxidase, GSH peroxidase, and GSH-S-transferase, were observed under NaCl stress conditions (124). Chen et al. reported that the addition of proline in the growth medium quenched ROS as efficiently as other ROS scavengers, such as N-acetyl cysteine in the fungal pathogen Colletotrichum trifolii (17). The decrease in ROS was shown to be due to an increase in catalase activity by proline treatment (17). Altogether, different groups have reported that increased proline levels enhance antioxidant enzyme activity. Studies comparing the ability of different biological osmolytes to stabilize proteins have provided insights into the chaperone properties of proline. Proline stabilizes protein structures by driving burial of the peptide backbone and protein folding (7, 130, 143). This is different than protein folding in the absence of osmolytes, which is driven by fa- vorable burying of nonpolar side chains (7, 130, 143). Relative to other osmolytes, proline is categorized as a weak stabilizer of protein folding and ranks lower in ability to induce protein folding (7, 11). Thus, although proline helps stabilize proteins, besides facilitating protein folding, additional mechanisms likely contribute to the protective effect of proline during stress. Another mechanism by which proline protects cells against stress has been suggested to involve the chelation of metals. High proline content in metal-tolerant plants is not unusual (113). One of the major toxicities of heavy metals is pertur- bation of cellular redox balance by ROS production (114). A potent oxidizing agent of biological macromolecules in the cell is the hydroxyl radical (OH ), which is formed by the reduction of H 2 O 2 by transition metal ions such as Cu + and Fe 2 + (114). The function of proline as a metal chelator was suggested by Sharma et al. , who reported that proline can protect enzymes from zinc- and cadmium-induced inhibition by forming proline-metal complexes (115). A copper–proline complex was also reported in the copper-tolerant Armeria maritima (34). The ability of proline to directly react with ROS has been investigated by numerous laboratories (58, 114). Previous studies have shown that free and polypeptide-bound proline can react with H 2 O 2 and OH (pH 7–8) to form stable free radical adducts of proline and hydroxyproline derivatives as shown in Figure 3 ( e.g ., 4-hydroxyproline and 3-hydro- xyproline) (38, 58, 102, 106, 127). Although Floyd and Nagy (38) observed that nitroxyl radicals accumulate during the incubation of proline with H 2 O 2 , the reaction is very slow relative to that of proline and OH (5.4 · 10 8 M - 1 s - 1 ) (3). Recently, the ability of proline to scavenge H 2 O 2 was com- pared with pyruvate, a well-established scavenger of H 2 O 2 . At 30 min, H 2 O 2 levels were diminished by > 90% in cell medium supplemented with 1 m M pyruvate, whereas no significant decrease was observed with proline (5 m M ) (70). This observation further indicates that a direct reaction between H O and proline does not significantly contribute to the scavenging of cellular H 2 O 2 (48). Proline has also been shown not to directly scavenge O 2 - (58). An ROS-scavenging mechanism that is important to proline stress protection is the facile reaction of proline with singlet oxygen ( 1 O 2 ). In cultured skin fibroblasts, exogenously added proline has been shown to diminish 1 O 2 levels (141). Proline has been shown to protect human skin cells from photo-induced apoptosis, suggesting that proline suppresses photo-oxidative stress and skin carcinogenesis (141). In plants, Alia et al. reported that during strong illumination, the production of 1 O 2 in the thylakoids from the cotyledons of Brassica juncea was dramatically suppressed by proline (5). The five-membered ring of proline, pyrrolidine, has a low ionization potential that effectively quenches 1 O 2 most likely through a charge transfer mechanism in which molecular oxygen returns to the ground triplet state ( 3 O 2 ) (Fig. 3) (20, 81, 152). Alia and coworkers used irradiation of various photo- sensitizers to produce 1 O 2 , which is detected by measuring the formation of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) by EPR (4). TEMPO formation was completely inhibited by adding 20 m M proline to the reaction, indicating that proline directly scavenged or quenched 1 O 2 (4). Quenching of 1 O 2 is also documented for other secondary amine compounds as well, such as spermine (63). Due to its action as a 1 O 2 quencher, proline may help stabilize proteins, DNA, and membranes (81). Prolyl residues in proteins also provide protection against oxidative stress caused by 1 O 2 . For example, the epithelial small proline-rich protein, which is a precursor of the cornified envelope of the epidermal skin, is strongly induced after exposure of the skin to UV radiation (37, 133). Figure 3 summarizes the reactivity of proline with different ROS. In addition to the chemical properties of proline discussed earlier, changes in proline metabolic flux can also impact stress tolerance. The effects of proline metabolism on the intracellular redox state have been well studied by Phang and coworkers, who first proposed that the proline-P5C cycle can help maintain proper NADP + /NADPH levels in the cytosol and drive the oxidative pentose phosphate pathway (95). The cycling of proline and P5C via PRODH and P5CR results in the transfer of reducing equivalents from the cytosol into the mitochondria (84, 95). Electrons are passed into the mitochondrial ETC directly from PRODH via ubiquinone, leading to increases in oxidative phosphorylation and mitochondrial ROS production (84, 95). The proline-P5C cycle is, thus, thought to help maintain a proper NADP + /NADPH ratio (84, 95). The proline-P5C cycle may especially be important when increased PRODH activity is not balanced with P5CDH activity (84, 149). Currently, it is not known how P5C shuttles in/out of the mitochondria. An excellent review on the proline-P5C cycle and the wide ranging effects of proline metabolism was recently provided by Phang (97). Evidence for proline metabolic flux influencing the NADP + /NADPH ratio in plants has been reported by several groups and summarized in Figure 4 (44, 66, 112). Proline metabolic cycling was found to increase oxidation of NADPH in the soybean nodule, thereby enhancing the oxidative pentose phosphate pathway (66). Increased flux through the oxidative pentose phosphate pathway would support purine nucleotide biosynthesis during stress recovery (Fig. 4) (44, 66). Up-regulation of proline synthesis has also been proposed to maintain the NADP + /NADPH ratio at normal levels during photoinduced stress (44, 123). Significant decreases in the NADP + /NADPH ratio has been reported under different stress conditions due to decreased Calvin cycle activity (44, 123). Without sufficient levels of NADP + available for electron transfer, photosynthetic cells under stress conditions produce more 1 O 2 when exposed to high light (16, 123). Light exposure, however, promotes P5CS expression, leading to increased proline biosynthesis and NADP + levels, which ultimately diminishes 1 O 2 production in the chloroplasts (Fig. 4) (110, 124). These observations suggest a link between enhanced proline synthesis and photoinduced oxidative stress. Hare et ...
Context 3
... stress (51). Proline also diminished the aggregation of a cellular retinoic acid-binding protein that is fused to a pathogenic polyglutamine repeat of the human huntingtin protein (51). Due to its chaperone properties, proline protection against oxidative stress has been proposed to involve enhancement and stabilization of redox enzymes. Exogenous application of proline to cell cultures has been found to increase the activity of different antioxidant enzymes under salt (47), cadmium (53, 54, 144), and oxidative stress (17), resulting in increased stress tolerance. These enzymes include superoxide dis- mutase (53, 144), catalase (17, 48, 53, 144), and GSH related or ascorbate (ASC)-GSH cycle-related enzymes (47, 54). All these are important antioxidant enzymes (52, 87) and in plants, the ASC-GSH cycle is especially critical for mitigating ROS (87). In the P5CS1 knockout of Arabidopsis , significantly lower activities of ASC-GSH cycle-related enzymes, including ascorbate peroxidase, GSH peroxidase, and GSH-S-transferase, were observed under NaCl stress conditions (124). Chen et al. reported that the addition of proline in the growth medium quenched ROS as efficiently as other ROS scavengers, such as N-acetyl cysteine in the fungal pathogen Colletotrichum trifolii (17). The decrease in ROS was shown to be due to an increase in catalase activity by proline treatment (17). Altogether, different groups have reported that increased proline levels enhance antioxidant enzyme activity. Studies comparing the ability of different biological osmolytes to stabilize proteins have provided insights into the chaperone properties of proline. Proline stabilizes protein structures by driving burial of the peptide backbone and protein folding (7, 130, 143). This is different than protein folding in the absence of osmolytes, which is driven by fa- vorable burying of nonpolar side chains (7, 130, 143). Relative to other osmolytes, proline is categorized as a weak stabilizer of protein folding and ranks lower in ability to induce protein folding (7, 11). Thus, although proline helps stabilize proteins, besides facilitating protein folding, additional mechanisms likely contribute to the protective effect of proline during stress. Another mechanism by which proline protects cells against stress has been suggested to involve the chelation of metals. High proline content in metal-tolerant plants is not unusual (113). One of the major toxicities of heavy metals is pertur- bation of cellular redox balance by ROS production (114). A potent oxidizing agent of biological macromolecules in the cell is the hydroxyl radical (OH ), which is formed by the reduction of H 2 O 2 by transition metal ions such as Cu + and Fe 2 + (114). The function of proline as a metal chelator was suggested by Sharma et al. , who reported that proline can protect enzymes from zinc- and cadmium-induced inhibition by forming proline-metal complexes (115). A copper–proline complex was also reported in the copper-tolerant Armeria maritima (34). The ability of proline to directly react with ROS has been investigated by numerous laboratories (58, 114). Previous studies have shown that free and polypeptide-bound proline can react with H 2 O 2 and OH (pH 7–8) to form stable free radical adducts of proline and hydroxyproline derivatives as shown in Figure 3 ( e.g ., 4-hydroxyproline and 3-hydro- xyproline) (38, 58, 102, 106, 127). Although Floyd and Nagy (38) observed that nitroxyl radicals accumulate during the incubation of proline with H 2 O 2 , the reaction is very slow relative to that of proline and OH (5.4 · 10 8 M - 1 s - 1 ) (3). Recently, the ability of proline to scavenge H 2 O 2 was com- pared with pyruvate, a well-established scavenger of H 2 O 2 . At 30 min, H 2 O 2 levels were diminished by > 90% in cell medium supplemented with 1 m M pyruvate, whereas no significant decrease was observed with proline (5 m M ) (70). This observation further indicates that a direct reaction between H O and proline does not significantly contribute to the scavenging of cellular H 2 O 2 (48). Proline has also been shown not to directly scavenge O 2 - (58). An ROS-scavenging mechanism that is important to proline stress protection is the facile reaction of proline with singlet oxygen ( 1 O 2 ). In cultured skin fibroblasts, exogenously added proline has been shown to diminish 1 O 2 levels (141). Proline has been shown to protect human skin cells from photo-induced apoptosis, suggesting that proline suppresses photo-oxidative stress and skin carcinogenesis (141). In plants, Alia et al. reported that during strong illumination, the production of 1 O 2 in the thylakoids from the cotyledons of Brassica juncea was dramatically suppressed by proline (5). The five-membered ring of proline, pyrrolidine, has a low ionization potential that effectively quenches 1 O 2 most likely through a charge transfer mechanism in which molecular oxygen returns to the ground triplet state ( 3 O 2 ) (Fig. 3) (20, 81, 152). Alia and coworkers used irradiation of various photo- sensitizers to produce 1 O 2 , which is detected by measuring the formation of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) by EPR (4). TEMPO formation was completely inhibited by adding 20 m M proline to the reaction, indicating that proline directly scavenged or quenched 1 O 2 (4). Quenching of 1 O 2 is also documented for other secondary amine compounds as well, such as spermine (63). Due to its action as a 1 O 2 quencher, proline may help stabilize proteins, DNA, and membranes (81). Prolyl residues in proteins also provide protection against oxidative stress caused by 1 O 2 . For example, the epithelial small proline-rich protein, which is a precursor of the cornified envelope of the epidermal skin, is strongly induced after exposure of the skin to UV radiation (37, 133). Figure 3 summarizes the reactivity of proline with different ROS. In addition to the chemical properties of proline discussed earlier, changes in proline metabolic flux can also impact stress tolerance. The effects of proline metabolism on the intracellular redox state have been well studied by Phang and coworkers, who first proposed that the proline-P5C cycle can help maintain proper NADP + /NADPH levels in the cytosol and drive the oxidative pentose phosphate pathway (95). The cycling of proline and P5C via PRODH and P5CR results in the transfer of reducing equivalents from the cytosol into the mitochondria (84, 95). Electrons are passed into the mitochondrial ETC directly from PRODH via ubiquinone, leading to increases in oxidative phosphorylation and mitochondrial ROS production (84, 95). The proline-P5C cycle is, thus, thought to help maintain a proper NADP + /NADPH ratio (84, 95). The proline-P5C cycle may especially be important when increased PRODH activity is not balanced with P5CDH activity (84, 149). Currently, it is not known how P5C shuttles in/out of the mitochondria. An excellent review on the proline-P5C cycle and the wide ranging effects of proline metabolism was recently provided by Phang (97). Evidence for proline metabolic flux influencing the NADP + /NADPH ratio in plants has been reported by several groups and summarized in Figure 4 (44, 66, 112). Proline metabolic cycling was found to increase oxidation of NADPH in the soybean nodule, thereby enhancing the oxidative pentose phosphate pathway (66). Increased flux through the oxidative pentose phosphate pathway would support purine nucleotide biosynthesis during stress recovery (Fig. 4) (44, 66). Up-regulation of proline synthesis has also been proposed to maintain the NADP + /NADPH ratio at normal levels during photoinduced stress (44, 123). Significant decreases in the NADP + /NADPH ratio has been reported under different stress conditions due to decreased Calvin cycle activity (44, 123). Without sufficient levels of NADP + available for electron transfer, photosynthetic cells under stress conditions produce more 1 O 2 when exposed to high light (16, 123). Light exposure, however, promotes P5CS expression, leading to increased proline biosynthesis and NADP + levels, which ultimately diminishes 1 O 2 production in the chloroplasts (Fig. 4) (110, 124). These observations suggest a link between enhanced proline synthesis and photoinduced oxidative stress. Hare et al. suggest that the redox modulation accom- panying proline synthesis may be more important than proline accumulation (44). Manipulation of proline metabolic enzyme expression has also provided evidence for proline metabolism influencing NADP + levels in plants. A comparison of sense-orientated and antisense-orientated P5CR gene transgenic soybean plants showed that sense plants had higher NADP + levels and higher stress tolerance relative to antisense plants (25). Antisense knockdown of P5CR resulted in lower NADP + levels and higher sensitivity to stress (25). Recently, Sharma and coworkers reported that under low water stress, plants deficient in P5CS1 or PRODH1 exhibit a lower NADP + / NADPH ratio than wild-type plants (112). In addition, L- proline catabolism was suggested to be important for maintaining the NADP + /NADPH ratio, as the NADP + /NADPH ratio is significantly lower in the prodh mutant of Arabidopsis than wild-type Arabidopsis (112). Although PRODH1 activity apparently declines during stress, a low level of cycling between proline and P5C may be enough to support the main- tenance of proper NADP + /NADPH (44). Different studies have shown that proline addition to the cell medium and up-regulation of endogenous proline biosynthesis leads to increased total GSH and protection of intracellular reduced GSH (47, 118, 144). Guarding reduced GSH is especially important in heavy metal stress, as heavy metal ion toxicity is often associated with depletion of GSH (114). The ability of proline to protect GSH during metal ion stress was tested in Chlamydomonas reinhardtii in which transgenic algae expressing the ...
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Citations
... The proline thus accumulated helps in minimizing osmotic potential in turn leaf water potential which renders the host plants to sustain the photosynthetic apparatus by retaining elevated organ hydration and turgor pressure maintenance (Ruiz-Lozano et al. 1995;Wang et al. 2004). Drought-resistant species tend to accumulate more proline (Charest and Ton Phan 1990;Liang et al. 2013). The results showed that under drought treatment, the proline content of zmpip2;4 mutant inoculated and uninoculated with AMF was significantly reduced by 36.8% and 27.9% compared with the wild-type, respectively. ...
Background and aims
Aquaporins and arbuscular mycorrhizal fungi (AMF) have been recognized to enhance plant tolerance to drought stress. However, the involvement of aquaporins in AMF-modulated mechanisms and the role of AMF-induced aquaporin genes in maize under drought stress remains unclear. The present study investigated the symbiotic relationship between maize and AMF in response to drought. Our aim was to elucidate the impact of AM-inducible aquaporin ZmPIP2;4 on mycorrhiza formation and drought tolerance.
Methods
The container experiment of the zmpip2;4 mutant verified the function of ZmPIP2;4 upon drought during AMF symbiosis and the Lotus japonicus (L. japonicus) overexpressing ZmPIP2;4 (ZmPIP2;4-OE) was also used to further investigate the drought-resistant function of ZmPIP2;4.
Results
ZmPIP2;4 was significantly induced upon drought treatment during AMF symbiosis. The biomass, colonization rate, relative water content (RWC), photosynthesis, POD and SOD activities, proline content, and the expression of other drought-related genes P5CS4, NECD1, and LEA3 were lower in the zmpip2;4 mutant than that in the wild-type after inoculation with the AMF under drought stress, indicating that ZmPIP2;4 promoted drought tolerance of maize during AMF symbiosis. The proline content, POD and SOD activities of ZmPIP2;4-OE L. japonicus plants were higher than those of wild-type MG20, suggesting that ZmPIP2;4 enhanced the drought tolerance of L. japonicus plants.
Conclusion
AMF symbiosis induced the expression of ZmPIP2;4 gene in the maize roots, and in return, ZmPIP2;4 further facilitated the AM symbiosis in roots. The results indicated that ZmPIP2;4 was influential in promoting maize’s tolerance to drought stress during AMF symbiosis.
... Therefore, the physiological consequences of these large differences between CS and SQ1 in the PAs they accumulated is yet to be determined. Although the signalling function of polyamines and free proline in plants subjected to drought stress is not well-documented, the accumulation of free proline is usually a good marker of stress development in plants [79]. Although free proline contents in CS and SQ1 were also measured in this experiment, results are not reported here, as free proline has been measured on several other occasions in CS and SQ1 under well-watered and droughted conditions, and results have varied considerably from experiment to experiment. ...
Water deficit affects the growth as well as physiological and biochemical processes in plants. The aim of this study was to determine differences in physiological and biochemical responses to drought stress in two wheat cultivars—Chinese Spring (CS) and SQ1 (which are parents of a mapping population of doubled haploid lines)—and to relate these responses to final yield and agronomic traits. Drought stress was induced by withholding water for 14 days, after which plants were re-watered and maintained until harvest. Instantaneous gas exchange parameters were evaluated on the 3rd, 5th, 10th, and 14th days of seedling growth under drought. After 14 days, water content and levels of chlorophyll a+b, carotenoids, malondialdehyde, soluble carbohydrates, phenolics, salicylic acid, abscisic acid (ABA), and polyamines were measured. At final maturity, yield components (grain number and weight), biomass, straw weight, and harvest index were evaluated. Physiological and biochemical parameters of CS responded more than those of SQ1 to the 14-day drought, reflected in a greater reduction in final biomass and yield in CS. Marked biochemical differences between responses of CS and SQ1 to the drought were found for soluble carbohydrates and polyamines. These would be good candidates for testing in the mapping population for the coincidence of the genetic control of these traits and final biomass and yield.
... Los aminoácidos también poseen un carácter funcional. La prolina, por ejemplo, es capaz de beneficiar la estabilización de proteínas y enzimas antioxidantes como resultado de la incidencia de factores adversos, que afectan el equilibrio redox intracelular en plantas (Kaur y Asthir, 2015;Liang et al., 2013), en consecuencia los aminoácidos también pueden ser caracterizados como agentes protectores de cultivos. ...
... • Apoyando los mecanismos de inactivación de las especies reactivas de oxígeno (ROS): (Barbosa et al., 2014), (Ronsein et al., 2006), (Ozden et al., 2009). • En el balance energético NADP/ NADPH: (Liang et al., 2013), (Sharma et al., 2011), (Szepesi y Szollosi, 2018), beneficiando las fases fotoquímica y bioquímica de la fotosíntesis y la fijación de carbono, que será destinado para la producción de frutos, lo cual beneficia la productividad de la actividad agrícola. ...
... La sobreproducción de ROS es responsable del daño al aparato fotosintético de los cultivos (Bhatla y A. Lal, 2018;Chaves y Oliveira, 2004;Mathur et al., 2014), y a biomoléculas esenciales de extrema importancia como la clorofila y diferentes componentes celulares como membranas, proteínas y ADN (Apel y Hirt, 2004;Liang et al., 2013), lo que en consecuencia puede mermar o reducir la productividad de un cultivo. ...
El manejo de cultivos es una actividad cambiante y dinámica, que involucra desde el uso de cultivares adaptados, hasta el uso de productos estimulantes que ayuden a la planta a tolerar condiciones estresantes. Factores bióticos en los agroecosistemas pueden generar desafíos y afectar significativamente la productividad de los cultivos; y el uso de aminoácidos puede ayudar a minimizar estos efectos negativos. Los aminoácidos potencializan procesos biológicos de interés agronómico, dando a los cultivos diferentes niveles de resiliencia ante condiciones estresantes, como las altas temperaturas, deficiencia hídrica, alta radiación solar, y salinidad. Sin embargo, aún existe escasez de información y falta de difusión objetiva con fundamento científico sobre los efectos, beneficios y eficiencia de estos productos. La aplicación de aminoácidos en cultivos puede ser foliar o en el riego por goteo, esta práctica economiza energía en la planta, que puede ser translocada hacia los sumideros, beneficiando la productividad. Además, favorece la síntesis de compuestos o enzimas de extrema importancia que confieren plasticidad ante altas temperaturas. Los efectos de los aminoácidos son amplios, por lo que, en esta revisión se sintetiza la información y se discute desde una perspectiva agronómica, con soporte fisiológico y del metabolismo de cultivos. Se espera que esta revisión pueda contribuir a desvendar el papel de los aminoácidos con sentido práctico, como referencia para profesionales en el manejo de sistemas agrícolas y para el área de la investigación agronómica.
... Some authors reported a decrease in proline content in olive trees grown under salt stress [21,53,54,82,89]. It has been suggested that proline may also act as a metabolic substrate helping in the maintenance of cellular energy and contributing to other metabolic pathways under stress conditions [90][91][92]. Thus, by acting as a reservoir to enhance plant growth and induce a stress response, it would decrease its levels in the cell [93]. ...
... In situations of nitrogen limitation, proline functions as an alternative metabolic substrate under conditions of stress, therefore helping the maintenance of cellular energy and the balance between NADP + and NADPH. This multifaceted role may be extended to its contribution to many pathways, including the tricarboxylic acid cycle and glutathione biosynthesis, which further emphasizes its dynamic nature as an organic reserve capable of supporting plant growth and meeting stress [90][91][92][93]194). Ben Ahmed et al. [138,195] documented that proline supply improved photosynthetic activity and antioxidant defense enzyme activities in stressed plants, and its application, both in the presence and absence of salinity, significantly influenced salt ion distribution in the leaves and roots of 'Chemlali' olive trees. ...
The olive tree (Olea europaea L.) is an evergreen tree that occupies 19% of the woody crop area and is cultivated in 67 countries on five continents. The largest olive production is concentrated in the Mediterranean basin, where the olive tree has had an enormous economic, cultural and environmental impact since the 7th century BC. In the Mediterranean region, salinity stands out as one of the main abiotic stress factors significantly affecting agricultural production. Moreover, climate change is expected to lead to increased salinisation in this region, threatening olive productivity. Salt stress causes combined damage by osmotic stress and ionic toxicity, restricting olive growth and interfering with multiple metabolic processes. A large variability in salinity tolerance among olive cultivars has been described. This paper aims to synthesize information from the published literature on olive adaptations to salt stress and its importance in salinity tolerance. The morphological, physiological, biochemical, and molecular mechanisms of olive tolerance to salt stress are reviewed.
... In addition to providing FADH 2 , a higher accumulation of P5C in mitochondria benefits the generation of glutamate and NADH by P5C dehydrogenase (P5CDH). NADH and FADH 2 helped maintain cellular energy status through the mETS (Alvarez et al., 2021;Liang et al., 2013). In our study, lower TOR expression concomitant with higher SnRK1 expression in rose flower during petal senescence could be responsible for lower cellular ATP accumulation along with higher cellular H 2 O 2 accumulation by suppressing H + -ATPase, Ca 2+ -ATPase, SDH, and CCO activities. ...
During bud opening of rose flowers, higher TOR along with lower SnRK1 expression was concomitant with sufficient cellular ATP provision, while during petal senescence of rose flowers, lower TOR along with higher SnRK1 expression was concomitant with insufficient cellular ATP supplying. During bud opening, higher TOR expression might ensure sufficient cellular ATP provision by promoting H+-ATPase, Ca2+-ATPase, SDH, and CCO activities. During petal senescence, higher SnRK1 expression might ensure for insufficient cellular ATP supplying which was associated with higher AOX, UCP1, ProDH and IVDH expression. By PSKα application, triggering TOR along with suppressing SnRK1 expression could be responsible for sufficient cellular ATP provision by promoting H+-ATPase, Ca2+-ATPase, SDH, and CCO activities accompanied by suppressing ProDH and IVDH expression. In addition, higher AOX and UCP1 expression might ensure lower H2O2 accumulation in cut rose flowers by PSKα application. By PSKα application, retarding senescence and extending vase life of cut rose flowers was associated with maintaining membrane integrity indicated by lower electrolyte leakage and MDA accumulation. Our results suggest the potential of TOR/SnRK1 signaling pathways as an effective endogenous anti-senescence molecular mechanism for extending the vase life of cut rose flowers.
... The decrease in L-Pro could be also an outcome of the inhibition of Na + and Cl − uptake by Si, an inhibition reported in previous studies (Shi et al., 2013;Zhu and Gong, 2014;Al Murad and Muneer, 2022). L-Pro, an essential amino acid, is involved in various signaling pathways under stress conditions (Liang et al., 2013), in addition to its osmoprotection function. In our case, alginate and SiNPs induced a decrease of L-Pro in mung beans 4 days after germination, when the treatments were applied before germination. ...
Seed coating ensures the targeted delivery of various compounds from the early stages of development to increase crop quality and yield. Silicon and alginate are known to have plant biostimulant effects. Rice husk (RH) is a significant source of biosilica. In this study, we coated mung bean seeds with an alginate–glycerol–sorbitol (AGS) film with embedded biogenic nanosilica (SiNPs) from RH, with significant plant biostimulant activity. After dilute acid hydrolysis of ground RH in a temperature-controlled hermetic reactor, the resulting RH substrate was neutralized and calcined at 650°C. The structural and compositional characteristics of the native RH, the intermediate substrate, and SiNPs, as well as the release of soluble Si from SiNPs, were investigated. The film for seed coating was optimized using a mixture design with three factors. The physiological properties were assessed in the absence and the presence of 50 mM salt added from the beginning. The main parameters investigated were the growth, development, metabolic activity, reactive oxygen species (ROS) metabolism, and the Si content of seedlings. The results evidenced a homogeneous AGS film formation embedding 50-nm amorphous SiNPs having Si–O–Si and Si–OH bonds, 0.347 cm³/g CPV (cumulative pore volume), and 240 m²/g SSA (specific surface area). The coating film has remarkable properties of enhancing the metabolic, proton pump activities and ROS scavenging of mung seedlings under salt stress. The study shows that the RH biogenic SiNPs can be efficiently applied, together with the optimized, beneficial alginate-based film, as plant biostimulants that alleviate saline stress from the first stages of plant development.
... Both proline and sugars, which often accumulate in plants as a response to abiotic and biotic stressors, are reliable biomarkers of stress conditions in plants (Jeandet et al. 2022;Liang et al. 2013;Sharma et al. 2019). Soluble sugars (SS) Fig. 2 Oxidative stress parameters in FDp-infected grapevines. ...
... Levels in M38-infected leaves were higher than the control only at the second time point. Proline acts as a protective molecule in plants during various stress conditions acting as an osmolyte, but also by being involved in the stabilization of proteins, direct scavenging of ROS and promoting downstream cellular signalling pathways related to plant defence mechanisms (Liang et al. 2013). Its relative content positively correlated (r = 0.85) with relative FDp DNA abundance (Fig. 3b, Online resource 2). ...
... Although it is a direct response mechanism and it is being affected by phytoplasma titre, the hypothesis is that this response is not a part of the PAMP-triggered immunity. Rather, our hypothesis is that proline accumulation is a response to FDp-induced symptoms in infected grapevine leaves, due to its general and fundamental role in stress response (Liang et al. 2013). A similar conclusion was discussed for FDp-infected grapevine cultivar 'Loureio' (Teixeira et al. 2023). ...
Phytoplasmas are phytopathogenic bacteria that cause serious damage to agriculture. A quarantine pathogen, the flavescence dorée phytoplasma (FDp), often associated with grapevine yellows disease, affects viticultural production across Europe. However, the mechanisms of FDp pathogenicity are not still elucidated. In this study, symptomatic and asymptomatic grapevine (Vitis vinifera L. var. ‘Pinot gris’) leaves were sampled. Two different FDp genotypes (M38 and M54) were identified, and genotype-dependent changes to grapevine physiological responses through the development of FDp infection were analysed. Correlation analyses established a potential link between measured physiological parameters and relative FDp DNA abundance. Increased malondialdehyde levels pointed to the oxidative stress in infected leaves, and highly correlated with the activation of L-ascorbic acid synthesis. Levels of hydrogen peroxide were reduced in infected leaves, possibly as an FDp mechanism to avoid plant-derived oxidative damage. Genotype M54 was associated with a lower accumulation of soluble sugars and lower damage to photosynthetic pigments while retaining a higher titre than M38. Therefore, pronounced phytoplasma genotype-dependent changes in grapevine physiology, potentially caused by the differences between M54 and M38 on the level of the efficiency of their effectors should be further investigated. Altogether, results provide data on certain targets of FDp in grapevine and could assist in the identification of potential specific effectors of this phytoplasma to aid the efforts of FDp management in European vineyards.
... Proline, a non-essential proteinogenic amino acid, plays a multifaceted role in protein synthesis, redox balance, cell fate regulation, brain development, and other cellular and physiological processes [39]. Numerous studies have linked proline metabolism with ROS [40]. Proline synthesis and degradation are both highly redox-active processes [41]. ...
The consumption of low-mineral water has been increasing worldwide. Drinking low-mineral water is associated with cardiovascular disease, osteopenia, and certain neurodegenerative diseases. However, the specific mechanism remains unclear. The liver metabolic alterations in rats induced by drinking purified water for 3 months were investigated with a metabolomics-based strategy. Compared with the tap water group, 74 metabolites were significantly changed in the purified water group (6 increased and 68 decreased), including 29 amino acids, 11 carbohydrates, 10 fatty acids, 7 short chain fatty acids (SCFAs), and 17 other biomolecules. Eight metabolic pathways were significantly changed, namely aminoacyl–tRNA biosynthesis; nitrogen metabolism; alanine, aspartate and glutamate metabolism; arginine and proline metabolism; histidine metabolism; biosynthesis of unsaturated fatty acids; butanoate metabolism; and glycine, serine and threonine metabolism. These changes suggested that consumption of purified water induced negative nitrogen balance, reduced expression of some polyunsaturated fatty acids and SCFAs, and disturbed energy metabolism in rats. These metabolic disturbances may contribute to low-mineral-water-associated health risks. The health risk of consuming low-mineral water requires attention.
... It is a compatible osmoprotectant and free radical scavenger of reactive oxygen species that protects against oxidative damage in plants during the process of stress and supplies energy for resumed growth after stress (Ramachandra et al. 2004;Verslues and Bray 2006;Hayat et al. 2012). The proline contents of plant cells are crucial enzymes Δ1-Pyrroline-5carboxylate synthetase (P5CS) in the glutamate pathway is the major route for proline synthesis during stress (Hu et al. 1992;Liang et al. 2013). Furthermore, it is predicted that increased proline accumulation under heat-stress conditions contributes to protein and membrane stability (Sung et al. 2003;Mirzaei et al. 2012). ...
pyrroline-5-carboxylate synthetase (P5CS) is one of the key regulatory enzymes involved in the proline biosynthetic pathway. Proline acts as an osmoprotectant, molecular chaperone, antioxidant, and regulator of redox homeostasis. The accumulation of proline during stress is believed to confer tolerance in plants. In this study, we cloned the complete CDS of the P5CS from pearl millet (Pennisetum glaucum (L.) R.Br. and transformed into tobacco. Three transgenic tobacco plants with single-copy insertion were analyzed for drought and heat stress tolerance. No difference was observed between transgenic and wild-type (WT) plants when both were grown in normal conditions. However, under heat and drought, transgenic plants have been found to have higher chlorophyll, relative water, and proline content, and lower malondialdehyde (MDA) levels than WT plants. The photosynthetic parameters (stomatal conductance, intracellular CO2 concentration, and transpiration rate) were also observed to be high in transgenic plants under abiotic stress conditions. qRT-PCR analysis revealed that the expression of the transgene in drought and heat conditions was 2–10 and 2–7.5 fold higher than in normal conditions, respectively. Surprisingly, only P5CS was increased under heat stress conditions, indicating the possibility of feedback inhibition. Our results demonstrate the positive role of PgP5CS in enhancing abiotic stress tolerance in tobacco, suggesting its possible use to increase abiotic stress-tolerance in crops for sustained yield under adverse climatic conditions.
... Similarly, the proline application also could not maintain the MDA content under PM and VOC, compared to ornithine. Proline plays a role as an osmolyte, metal chelator, and ROS scavenger (Hayat et al. 2012;Liang et al. 2013). However, the use of proline to scavenge ROS is relatively slow (Liang et al. 2013). ...
... Proline plays a role as an osmolyte, metal chelator, and ROS scavenger (Hayat et al. 2012;Liang et al. 2013). However, the use of proline to scavenge ROS is relatively slow (Liang et al. 2013). PM and VOC stress may be more complex than other forms of abiotic stress. ...
Phytoremediation has become famous for removing particulate matter (PM) and volatile organic compounds (VOC) in situ. Plants for removing PM and VOC were associated with botanical biofilters to attract pollution to the plant. On the other hand, persistent pollution exposure can lower plant health and phytoremediation effectiveness; therefore, improving plant tolerance against stress is necessary. Various elicitors can enhance plant tolerance to certain stressors. This study aims to investigate different elicitors to maintain plant health and improve the use of plants in phytoremediation for PM and VOC pollution. This experiment used Sansevieria trifasciata hort. ex Prain under PM and VOC stress. Exogenous elicitors, such as proline, ornithine, and a commercial product, were applied to the leaf parts before exposure to PM and VOC stress. The initial concentrations of PM1, PM2.5, and PM10 were 300–350, 350–450, and 400–500 µg m⁻³, respectively, while the VOC concentration was 2.5–3.0 mg m⁻³. The plant was stressed for 7 days. The result indicated that ornithine 10 mM is vital in improving plant tolerance and inducing antioxidant enzymes against PM and VOC, while proline 50 mM and a commercial product could not reduce plant stress. This study suggests that ornithine might be an important metabolite to improve plant tolerance to PM and VOC.
