Exogenous glycine betaine and proline play a protective role in heat-stressed barley leaves (Hordeum vulgare L.): A chlorophyll a fluorescence study

Abstract

Effects of drought and exogenous glycine betaine and proline on Photosystem II (PSII) photochemistry were studied in barley leaves under heat stress induced by exposing them to 45°C for 10 min. Polyphasic fluorescence transient (OJIP) was used to evaluate PSII photochemistry in leaves treated with either glycine betaine or proline, combined or not with heat treatment. A distinct K step in the fluorescence transient OJIP appeared in control leaves, indicating an inactivation of the oxygen evolving complex (OEC). Drought stress and exogenous glycine betaine and proline modified the shape of the OJIP curve of leaves heated at 45°C and the K step was not as pronounced. Increased thermostability of PSII may be associated with the resistance of OEC and increased energy connectivity between PSII units. The thermostability of PSII was also reflected by a lower decrease in maximum quantum yield of primary photochemistry (?_Po = F _V/F _M) and performance index (PI). Exogenous application of glycine betaine or proline can play an important role in enhancing plant stress tolerance and may help reduce effects of environmental stresses.     

Different response of photosystem II to short and long‐term drought stress in Arabidopsis thaliana

Short‐ and long‐term drought stress on photosystem II (PSII) and oxidative stress were studied in Arabidopsis thaliana. Under drought stress, chlorophyll (Chl) content, Chl fluorescence, relative water content and oxygen evolution capacity gradually decreased, and the thylakoid structure was gradually damaged. Short‐term drought stress caused a rapid disassembly of the light‐harvesting complex II (LHCII). However, PSII dimers kept stable under the short‐term drought stress and significantly decreased only after 15 days of drought stress. Immunoblotting analysis of the thylakoid membrane proteins showed that most of the photosystem proteins decreased after the stress, especially for Lhcb5, Lhcb6 and PsbQ proteins. However, surprisingly, PsbS significantly increased after the long‐term drought stress, which is consistent with the substantially increased non‐photochemical quenching (NPQ) after the stress. Our results suggest that the PSII–LHCII supercomplexes and LHCII assemblies play an important role in preventing photo‐damages to PSII under drought stress.     

Heat stress results in loss of chloroplast Cu/Zn superoxide dismutase and increased damage to Photosystem II in combined drought‐heat stressed Lotus japonicus

Drought and heat stress have been studied extensively in plants, but most reports involve analysis of response to only one of these stresses. Studies in which both stresses were studied in combination have less commonly been reported. We report the combined effect of drought and heat stress on Photosystem II (PSII) of Lotus japonicus cv. Gifu plants. Photochemistry of PSII was not affected by drought or heat stress alone, but the two stresses together decreased PSII activity as determined by fluorescence emission. Heat stress alone resulted in degradation of D1 and CP47 proteins, and D2 protein was also degraded by combined drought–heat stress. None of these proteins were degraded by drought stress alone. Drought alone induced accumulation of hydrogen peroxide but the drought–heat combination led to an increase in superoxide levels and a decrease in hydrogen peroxide levels. Furthermore, combined drought–heat stress was correlated with an increase in oxidative damage as determined by increased levels of thiobarbituric acid reactive substances. Heat also induced degradation of chloroplast Cu/Zn superoxide dismutase (SOD: EC 1.15.1.1) as shown by reduced protein levels and isozyme‐specific SOD activity. Loss of Cu/Zn SOD and induction of catalase (CAT: EC 1.11.1.6) activity would explain the altered balance between hydrogen peroxide and superoxide in response to drought vs combined drought–heat stress. Degradation of PSII could thus be caused by the loss of components of chloroplast antioxidant defence systems and subsequent decreased function of PSII. A possible explanation for energy dissipation by L. japonicus under stress conditions is discussed.     

The overaccumulation of glycinebetaine alleviated damages to PSII of wheat flag leaves under drought and high temperature stress combination

To analyze the physiological mechanisms underlying the increased tolerance to drought and high temperature stress combination by overproduction of glycinebetaine (GB) in wheat, a transgenic wheat line T6 and its wild-type (WT) Shi4185 were used. The transgenic line was generated by introducing a gene encoding betaine aldehyde dehydrogenase (BADH) into a wheat line Shi4185. The gene was cloned from Garden Orache (Atriplex hortensis L.). Wheat plants were exposed to drought (withholding irrigation), high temperature stress (40 °C), and their combination at the flowering stage. Analyses of oxygen-evolving activity and photosystem II (PSII) photochemistry, modulated chlorophyll fluorescence, rapid fluorescence induction kinetics, and the polyphasic fluorescence transients (OJIP) were used to evaluate PSII photochemistry in wheat plants. The results suggest that the PSII in transgenic plants showed higher resistance than that in wild-type plants under the stresses studied here, this increased tolerance was associated with an improvement in stability of the oxygen-evolving complex and the reaction center of PSII; streptomycin treatment can impair the protective effect of overaccumulated GB on PSII. The overaccumulated GB may protect the PSII complex from damage through accelerating D1 protein turnover to alleviate photodamage. The results also suggest that the PSII under combined high temperature and drought stress shows higher tolerance than under high temperature stress alone in both transgenic and wild-type plants.     

Overexpression of sweet pepper glycerol-3-phosphate acyltransferase gene enhanced thermotolerance of photosynthetic apparatus in transgenic tobacco

In order to investigate the relationship between the lipid composition in thylakoid membrane and thermostability of pho-tosynthetic apparatus, tobacco transformed with sweet pepper sense glycerol-3-phosphate acyltransferase (GPA T) gene were used to analyze the lipid composition in thylakoid membrane, the net photosynthetic rate and chlorophyll fluorescence parameters under high temperature stress. The results showed that the saturated extent of monogalactosyldiacylglycerol (MGDG), suifoquinovosyldiacylglycerol, digalactosyldiacylglycerol and phosphatidylglycerol in thylakoid membrane of transgenic tobacco T1 lines increased generally. Particularly, the saturated extent in MGDG increased obviously by 16.2% and 12.0% in T1-2 and T1-1, respectively. With stress temperature elevating, the maximum efficiency of photosystem Ⅱ the two lines and wild type tobacco plants decreased gradually, but those parameters decreased much less in transgenic plants. Even though the recovery process appeared differently in the donor and acceptor side of PSII in transgenic tobacco compared with wild-type plants, the entire capability of PSII recovered faster in transgenic tobacco, which was shown in Increase in saturated extent of thylakoid membrane Iipids in transgenic plants enhanced the stability of photosynthetic apparatus under high temperature stress.     

Characterization of an NaCl-sensitive Staphylococcus aureus mutant and rescue of the NaCl-sensitive phenotype by glycine betaine but not by other compatible solutes.

To further study mechanisms of coping with osmotic stress-low water activity, mutants of Staphylococcus aureus with transposon Tn917-lacZ-induced NaCl sensitivity were selected for impaired ability to grow on solid defined medium containing 2 M NaCl. Southern hybridization experiments showed that NaCl-sensitive mutants had a single copy of the transposon inserted into a DNA fragment of the same size in each mutant. These NaCl-sensitive mutants had an extremely long lag phase (60 to 70 h) in defined medium containing 2.5 M NaCl. The osmoprotectants glycine betaine and choline (which is oxidized to glycine betaine) dramatically shortened the lag phase, whereas L-proline and proline betaine, which are effective osmoprotectants for the wild type, were ineffective. Electron microscopic observations of the NaCl-sensitive mutant under NaCl stress conditions revealed large, pseudomulticellular cells similar to those observed previously in the wild type under the same conditions. Glycine betaine, but not L-proline, corrected the morphological abnormalities. Studies of the uptake of L-[14C]proline and [14C]glycine betaine upon osmotic upshock revealed that the mutant was not defective in the uptake of either osmoprotectant. Comparison of pool K+, amino acid, and glycine betaine levels under NaCl stress conditions in the mutant and the wild type revealed no striking differences. Glycine betaine appears to have additional beneficial effects on NaCl-stressed cells beyond those of other osmoprotectants. The NaCl stress protein responses of the wild type and the NaCl-sensitive mutant were characterized and compared by labeling with L-[35 S]methionine and two-dimensional gel electrophoresis. The synthesis of 10 proteins increased in the wild type in response to NaCl stress, whereas the synthesis of these 10 proteins plus 2 others increased in response to NaCl stress in the NaCl-sensitive mutant. Five proteins, three of which were NaCl stress proteins, were produced in elevated amounts in the NaCl-sensitive mutant under unstressed conditions compared to the wild type. The presence of glycine betaine during NaCl stress decreased the production of three NaCl stress proteins in the mutant versus one in the wild type.     

Plastid-expressed choline monooxygenase gene improves salt and drought tolerance through accumulation of glycine betaine in tobacco

Glycine betaine (GlyBet), a quaternary ammonium compound, functions as an osmoprotectant in many organisms including plants. Previous research has shown that over-expression of enzymes for GlyBet biosynthesis in transgenic plants improved abiotic stress tolerance, but so far no study on the effects of plastid-expression of choline monooxygenase, the enzyme that catalyzes the conversion of choline into betaine aldehyde, has been reported. In the present study, tobacco (Nicotiana tabacum L. cv Wisconsin 38) plants were transformed with a gene for choline monooxygenase (BvCMO) from beet (Beta vulgaris) via plastid genetic engineering. Transplastomic plants constitutively expressing BvCMO under the control of the ribosomal RNA operon promoter and a synthetic T7 gene G10 leader were able to accumulate GlyBet in leaves, roots and seeds, and exhibited improved tolerance to toxic level of choline and to salt/drought stress when compared to wild type plants. Transplastomic plants also demonstrated higher net photosynthetic rate and apparent quantum yield of photosynthesis in the presence of 150 mM NaCl. Salt stress caused no significant change on the maximal efficiency of PSII photochemistry (Fv/Fm) in both wild type and transplastomic plants, but a decrease in the actual efficiency of PSII (PhiPSII) was observed, and such a decrease was much greater in wild type plants. Our results demonstrate the feasibility of improving salt and drought tolerance in plants through plastid transformation with BvCMO gene.     

Involvement of sulfoquinovosyl diacylglycerol in the structural integrity and heat-tolerance of photosystem II

To examine the role of sulfoquinovosyl diacylglycerol (SQDG) in thylakoid membranes, we compared the structural and functional properties of photosystem II (PSII) between a mutant of Chlamydomonas reinhardtii defective in SQDG ( hf-2) and the wild type. The PSII core complex of hf-2, as compared with that of the wild type, showed structural fragility when solubilized with a detergent, dodecyl beta- d-maltoside, suggesting that the physical properties of the PSII complex were altered by the loss of SQDG. On the other hand, exposure of the cells to 41 degrees C for 120 min in the dark decreased the PSII activity to 70% and 50% of the initial levels in the wild type and hf-2, respectively, which implies that the PSII activity, in the absence of SQDG, becomes less stable under heat-stress conditions. PSII inactivated to 60% of the initial level by dark incubation at 41 degrees C was reactivated by following illumination even at 41 degrees C to more than 90% in the wild type, but only to 70% in hf-2. These results suggest that PSII inactivated by heat recovers through some mechanism dependent on light, and that SQDG participates in functioning of the mechanism. The conformational disorder of PSII caused by the defect in SQDG might be correlated with the increased susceptibility of its activity to heat-stress.     

Engineering of enhanced glycine betaine synthesis improves drought tolerance in maize

Glycine betaine plays an important role in some plants, including maize, in conditions of abiotic stress, but different maize varieties vary in their capacity to accumulate glycine betaine. An elite maize inbred line DH4866 was transformed with the betA gene from Escherichia coli encoding choline dehydrogenase (EC 1.1.99.1), a key enzyme in the biosynthesis of glycine betaine from choline. The transgenic maize plants accumulated higher levels of glycine betaine and were more tolerant to drought stress than wild-type plants (non-transgenic) at germination and the young seedling stage. Most importantly, the grain yield of transgenic plants was significantly higher than that of wild-type plants after drought treatment. The enhanced glycine betaine accumulation in transgenic maize provides greater protection of the integrity of the cell membrane and greater activity of enzymes compared with wild-type plants in conditions of drought stress.     

Ascorbate peroxidase 1 plays a key role in the response of Arabidopsis thaliana to stress combination

Within their natural habitat plants are subjected to a combination of different abiotic stresses, each with the potential to exacerbate the damage caused by the others. One of the most devastating stress combinations for crop productivity, which frequently occurs in the field, is drought and heat stress. In this study we conducted proteomic and metabolic analysis of Arabidopsis thaliana plants subjected to a combination of drought and heat stress. We identified 45 different proteins that specifically accumulated in Arabidopsis in response to the stress combination. These included enzymes involved in reactive oxygen detoxification, malate metabolism, and the Calvin cycle. The accumulation of malic enzyme during the combined stress corresponded with enhanced malic enzyme activity, a decrease in malic acid, and lower amounts of oxaloacetate, suggesting that malate metabolism plays an important role in the response of Arabidopsis to the stress combination. Cytosolic ascorbate peroxidase 1 (APX1) protein and mRNA accumulated during the stress combination. When exposed to heat stress combined with drought, an APX1-deficient mutant (apx1) accumulated more hydrogen peroxide and was significantly more sensitive to the stress combination than wild type. In contrast, mutants deficient in thylakoid or stromal/mitochondrial APXs were not more sensitive to the stress combination than apx1 or wild type. Our findings suggest that cytosolic APX1 plays a key role in the acclimation of plants to a combination of drought and heat stress.     

Drought-induced changes in chlorophyll fluorescence,photosynthetic pigments,and thylakoid membrane proteins of Vigna radiata

The effects of drought on chlorophyll fluorescence characteristics of PSII, photosynthetic pigments, thylakoid membrane protein (D1), and proline content in different varieties of mung bean plants were studied. Drought stress inhibits PSII activity and induces alterations in D1 protein. We observed a greater decline in the effective quantum yield of PSII, electron transport rate, and saturating photosynthetically active photon flux density (PPFD_sat) under drought stress in var. Anand than var. K-851 and var. RMG 268. This may possibly be due to either downregulation of photosynthesis or photoinhibition process. Withholding irrigation resulted in gradual diminution in total Chl content at Day 4 of stress. HPLC analysis revealed that the quantity of β-carotene in stressed plants was always higher reaching maxima at Day 4. Photoinactivation of PSII in var. Anand includes the loss of the D1 protein, probably from greater photosynthetic damage caused by drought stress than the other two varieties.     

Reversible changes in structure and function of photosynthetic apparatus of pea (Pisum sativum) leaves under drought stress

The effects of drought on photosynthesis have been extensively studied, whereas those on thylakoid organization are limited. We observed a significant decline in gas exchange parameters of pea (Pisum sativum) leaves under progressive drought stress. Chl a fluorescence kinetics revealed the reduction of photochemical efficiency of photosystem (PS)II and PSI. The non-photochemical quenching (NPQ) and the levels of PSII subunit PSBS increased. Furthermore, the light-harvesting complexes (LHCs) and some of the PSI and PSII core proteins were disassembled in drought conditions, whereas these complexes were reassociated during recovery. By contrast, the abundance of supercomplexes of PSII-LHCII and PSII dimer were reduced, whereas LHCII monomers increased following the change in the macro-organization of thylakoids. The stacks of thylakoids were loosely arranged in drought-affected plants, which could be attributed to changes in the supercomplexes of thylakoids. Severe drought stress caused a reduction of both LHCI and LHCII and a few reaction center proteins of PSI and PSII, indicating significant disorganization of the photosynthetic machinery. After 7 days of rewatering, plants recovered well, with restored chloroplast thylakoid structure and photosynthetic efficiency. The correlation of structural changes with leaf reactive oxygen species levels indicated that these changes were associated with the production of reactive oxygen species.     

Overaccumulation of glycine betaine enhances tolerance to drought and heat stress in wheat leaves in the protection of photosynthesis

We investigated the different responses of wheat (Triticum aestivum L.) plants to drought- (DS) and heat stress (HS), and analyzed the physiological mechanisms of glycine betaine (GB) involved in the improvement of wheat tolerance to the combination of these stresses. The transgenic wheat T6 line was generated by introducing a gene encoding betaine aldehyde dehydrogenase (BADH) into the wild-type (WT) Shi4185 line. The gene was cloned from the Garden Orache plant (Atriplex hortensis L.). Wheat seedlings were subjected to drought stress (30%, PEG-6000), heat stress (40°C), and their combination. Photosynthetic gas exchange, water status and lipid peroxidation of wheat leaves were examined under different stresses. When subjected to a combination of drought and heat, the inhibition of photosynthesis was significantly increased compared to that under DS or HS alone. The increased inhibition of photosynthesis by the combined stresses was not simply the additive stress effect of separate heat- and drought treatments; different responses in plant physiology to DS and HS were also found. HS decreased the chlorophyll (Chl) content, net photosynthetic rate (P _N), carboxylation efficiency (CE) and apparent quantum yield (AQY) more than DS but DS decreased the transpiration rate (E), stomata conductance (g _s) and intercellular CO₂ concentration (C _i) more than HS. GB over-accumulation led to increased photosynthesis not only under individual DS or HS but also under their combination. The enhancement of antioxidant activity and the improvement of water status may be the mechanisms underlying the improvement of photosynthesis by GB in wheat plants.     

Changes in Thermostability of Photosystem Ⅱ and Leaf Lipid Composition of Rice Mutant with Deficiency of Light-harvesting Chlorophyll a/b Protein Complexes

We studied the difference in thermostability of photosystem Ⅱ （PSII） and leaf lipid composition between a T-DNA insertion mutant rice （Oryza sativa L.） VG28 and its wild type Zhonghuau. Native green gel and SDS-PAGE electrophoreses revealed that the mutant VG28 lacked all light-harvesting chlorophyll a/b protein complexes. Both the mutant and wild type were sensitive to high temperatures, and the maximal efficiency of PSII photochemistry （FJ Fm） and oxygen-evolving activity of PSII in leaves significantly decreased with increasing temperature. However, the PSII activity of the mutant was markedly more sensitive to high temperatures than that of the wild type. Lipid composition analysis showed that the mutant had less phosphatidylglycerol and sulfoquinovosyl diacylglycerol compared with the wild type. Fatty acid analysis revealed that the mutant had an obvious decrease in the content of 16：1t and a marked increase in the content of 18：3 compared with the wild type. The effects of lipid composition and unsaturation of membrane lipids on the thermostability of PSII are discussed.     

High light induced disassembly of photosystem II supercomplexes in Arabidopsis requires STN7-dependent phosphorylation of CP29

Photosynthetic oxidation of water and production of oxygen by photosystem II (PSII) in thylakoid membranes of plant chloroplasts is highly affected by changes in light intensities. To minimize damage imposed by excessive sunlight and sustain the photosynthetic activity PSII, organized in supercomplexes with its light harvesting antenna, undergoes conformational changes, disassembly and repair via not clearly understood mechanisms. We characterized the phosphoproteome of the thylakoid membranes from Arabidopsis thaliana wild type, stn7, stn8 and stn7stn8 mutant plants exposed to high light. The high light treatment of the wild type and stn8 caused specific increase in phosphorylation of Lhcb4.1 and Lhcb4.2 isoforms of the PSII linker protein CP29 at five different threonine residues. Phosphorylation of CP29 at four of these residues was not found in stn7 and stn7stn8 plants lacking the STN7 protein kinase. Blue native gel electrophoresis followed by immunological and mass spectrometric analyses of the membrane protein complexes revealed that the high light treatment of the wild type caused redistribution of CP29 from PSII supercomplexes to PSII dimers and monomers. A similar high-light-induced disassembly of the PSII supercomplexes occurred in stn8, but not in stn7 and stn7stn8. Transfer of the high-light-treated wild type plants to normal light relocated CP29 back to PSII supercomplexes. We postulate that disassembly of PSII supercomplexes in plants exposed to high light involves STN7-kinase-dependent phosphorylation of the linker protein CP29. Disruption of this adaptive mechanism can explain dramatically retarded growth of the stn7 and stn7stn8 mutants under fluctuating normal/high light conditions, as previously reported.     

Stress metabolites and their role in coastal plants

The stress metabolites proline, glycine betaine and sorbitol were accumulated in the leaves of some angiosperms from sand dunes and shingle. Chloride, where it was measured, was not accumulated to high concentrations in leaves suggesting that these soils are not saline. Sand dunes and shingle soils have low water-holding capacity, so it is possible that solute accumulation was a response to drought which could be of adaptive significance. In sand dunes low water availability could be associated with increased leaf temperatures because of reduced transpiration rates and high soil temperatures. The role of stress metabolites in heat tolerance was considered. Proline, betaine, sorbitol and mannitol increased the heat stability of glutamine synthetase (GS) and glutamate: oxaloacetate aminotransferase from Ammophila arenaria. For GS the effect increased with solute concentration. The polyols were more effective at high temperatures. The heat stability of GS from the moss Tortula ruraliformis and the brown alga Fucus vesiculosus was increased by mannitol. The effect of the solutes was independent of plant species and type of enzyme. It is suggested that the accumulation of solutes may have ecological importance in protecting sand-dune plants from heat damage during periods of drought.     

Localisation and processing of the precursor form of photosystem II protein D1 inSynechocystis 6803

Pure plasma membrane and thylakoid membrane fractions from Synechocystis 6803 were isolated to study the localisation and processing of the precursor form of the D1 protein (pD1) of photosystem II (PSII). PSII core proteins (D1, D2 and cytb559) were localised both to plasma and thylakoid membrane fractions, the majority in thylakoids. pD1 was found only in the thylakoid membrane where active PSII is known to function. Membrane fatty acid unsaturation was shown to be critical in processing of pD1 into mature D1 protein. This was concluded from pulse-labelling experiments at low temperature using wild type and a mutant Synechocystis 6803 with a low level of membrane fatty acid unsaturation. Further, pD1 was identified as two distinct bands, an indication of two cleavage sites in the precursor peptide or, alternatively, two different conformations of pD1. Our results provide evidence for thylakoid membranes being a primary synthesis site for D1 protein during its light-activated turnover. The existence of the PSII core proteins in the plasma membrane, on the other hand, may be related to the biosynthesis of new PSII complexes in these membranes.     

Comparative effect of water, heat and light stresses on photosynthetic reactions in Sorghum bicolor(L.) Moench

Five varieties of Sorghum bicolor (L.) Moench., differing in their drought tolerance under field conditions have been used to study the effect of individual components of drought stress, namely high light intensity stress, heat stress and water stress, on their photosynthetic performance. Chlorophyll content, chlorophyll fluorescence, ribulose-1,5-bisphosphate carboxylase (Rubisco, EC 4.1.1.39) content, phosphoenolpyruvate carboxylase (PEPcase, EC 4.1.1.31) activity and photo-synthetic oxygen evolution were used as key parameters to assess photosynthetic performance. The results indicated that photochemical efficiency of photosystem II (PSII) was severely reduced by all three stress components, whereas PEPcase activity was more specifically reduced by water stress. Degradation of Rubisco and chlorophyll loss occurred under high light and water stress conditions. Of the four drought-tolerant varieties, E 36-1 showed higher PEPcase activity, Rubisco content and photochemical efficiency of PSII, and was able to sustain a higher maximal rate of photosynthetic oxygen evolution under each stress condition as compared to the other varieties. A high stability to stress-induced damage, or acclimation of photosynthesis to the individual components of drought stress may contribute to the high yields of E 36-1 under drought conditions. In the E 36-1 variety markedly higher levels of the chloroplastic chaperonin 60 (cpn 60) were observed under all stress conditions than in the susceptible variety CSV 5.Key words: Chlorophyll fluorescence, drought stress, oxygen evolution, phosphoenopyruvate carboxylase, Sorghum.      

Increased glycine betaine synthesis and salinity tolerance in AhCMO transgenic cotton lines

Glycine betaine is an osmoprotectant that plays an important role and accumulates rapidly in many plants during salinity or drought stress. Choline monooxygenase (CMO) is a major catalyst in the synthesis of glycine betaine. In our previous study, a CMO gene (AhCMO) cloned from Atriplex hortensis was introduced into cotton (Gossypium hirsutum L.) via Agrobacterium mediation to enhance resistance to salinity stress. However, there is little or no knowledge of the salinity tolerance of the transgenic plants, particularly under saline-field conditions. In the present study, two transgenic AhCMO cotton lines of the T₃ generation were used to study the AhCMO gene expression, and to determine their salinity tolerance in both greenhouse and field under salinity stress. Molecular analysis confirmed that the transgenic plants expressed the AhCMO gene. Greenhouse study showed that on average, seedlings of the transgenic lines accumulated 26 and 131% more glycine betaine than those of non-transgenic plants (SM3) under normal and salt-stress (150 mmol l⁻¹ NaCl) conditions, respectively. The osmotic potential, electrolyte leakage and malondialdehyde (MDA) accumulation were significantly lower in leaves of the transgenic lines than in those of SM3 after salt stress. The net photosynthesis rate and Fv/Fm in transgenic cotton leaves were less affected by salinity than in non-transgenic cotton leaves. Therefore, transgenic cotton over-expressing AhCMO was more tolerant to salt stress due to elevated accumulation of glycine betaine, which provided greater protection of the cell membrane and photosynthetic capacity than in non-transgenic cotton. The seed cotton yield of the transgenic plants was lower under normal conditions, but was significantly higher than that of non-transgenic plants under salt-stressed field conditions. The results indicate that over-expression of AhCMO in cotton enhanced salt stress tolerance, which is of great value in cotton production in the saline fields.