Physiological significance of proline and glycinebetaine: Maintaining photosynthesis during NaCl stress in wheat

Experiments on the physiological significance of accumulation of proline and glycinebetaine (GB) in sustaining photosynthesis during salt stress in wheat in vivo showed that pre-treatment with GB, but not proline, alleviated NaCl-induced stomatal and non-stomatal inhibition of photosynthesis completely. A permeating and non-dissociating osmoticum, 3-orthomethyl-glucopyranose, also alleviated NaCl-induced perturbations of photosynthesis, suggesting that GB may work by maintaining chloroplast volume and not by specific effects on photosynthetic processes.     

Physiological significance of proline and glycinebetaine: Maintaining photosynthesis during NaCl stress in wheat

Experiments on the physiological significance of accumulation of proline and glycinebetaine (GB) in sustaining photosynthesis during salt stress in wheat in vivo showed that pre-treatment with GB, but not proline, alleviated NaCl-induced stomatal and non-stomatal inhibition of photosynthesis completely. A permeating and non-dissociating osmoticum, 3-orthomethyl-glucopyranose, also alleviated NaCl-induced perturbations of photosynthesis, suggesting that GB may work by maintaining chloroplast volume and not by specific effects on photosynthetic processes.     

Roles of glycine betaine and proline in improving plant abiotic stress resistance

Glycine betaine (GB) and proline are two major organic osmolytes that accumulate in a variety of plant species in response to environmental stresses such as drought, salinity, extreme temperatures, UV radiation and heavy metals. Although their actual roles in plant osmotolerance remain controversial, both compounds are thought to have positive effects on enzyme and membrane integrity along with adaptive roles in mediating osmotic adjustment in plants grown under stress conditions. While many studies have indicated a positive relationship between accumulation of GB and proline and plant stress tolerance, some have argued that the increase in their concentrations under stress is a product of, and not an adaptive response to stress. In this article, we review and discuss the evidence supporting each of these arguments. As not all plant species are capable of natural production or accumulation of these compounds in response to stress, extensive research has been conducted examining various approaches to introduce them into plants. Genetically-engineered plants containing transgenes for production of GB or proline have thus far faced with the limitation of being unable to produce sufficient amounts of these compounds to ameliorate stress effects. An alternative “shot-gun” approach of exogenous application of GB or proline to plants under stress conditions, however, has gained some attention. A review of the literature indicates that in many, but not all, plant species such applications lead to significant increases in growth and final crop yield under environmental stresses. In this review article, numerous examples of successful application of these compounds to improve plant stress tolerance are presented. However, to streamline useful and economic applications of these compounds, further investigations are needed to determine the most effective concentrations and number of applications as well as the most responsive growth stage(s) of the plant. All these factors may vary from species to species. Furthermore, a better understanding of the mechanisms of action of exogenously applied GB and proline is expected to aid their effective utilization in crop production in stress environments.     

Role of ROS and auxin in plant response to metal-mediated stress

Being unable to move away from their places of germination, in order to avoid excess metal-induced damages, plants have to evolve different strategies and complex regulatory mechanisms to survive harsh conditions. While both ROS and auxin are documented to be important in plant response to metal stress, the mechanisms underlying the crosstalk between ROS and auxin in metal stress are poorly understood. In this review, we provide an update on the regulation of plant responses to metal-stress by ROS and auxin signaling pathways, primarily, with a focus on the copper, aluminum and cadmium stress. We aim at surveying the mechanisms underlying how metal stress modulates the changes in auxin distribution and the network of ROS and auxin in plant response to metal stress based on recent studies.     

Overaccumulation of glycine betaine alleviates the negative effects of salt stress in wheat

To investigate the physiological mechanisms of glycinebetaine (GB) involved in the improvement of salt tolerance of wheat, three transgenic wheat (Triticum aestivum L.) lines-T1, T4, and T6-and the wild-type (WT) line Shi4185 were used. The transgenic lines were generated by introducing the BADH gene encoding betaine aldehyde dehydrogenase, which was cloned from Atriplex hortensis L. The BADH gene induced overexpression of GB in transgenic lines. Salt stress was induced by adding 200 mM NaCl, and the osmotic adjustment (OA), ion homeostasis, and antioxidant characteristics of wheat plants were observed. Under salt stress, the OA in the transgenic wheat lines was significantly higher than that in WT; this may be attributed to GB itself and/or the GB-induced overaccumulation of other osmolytes, such as free proline, soluble protein, and soluble sugar. Moreover, the transgenic lines could maintain the lower Na⁺ and Cl⁻ concentrations in their leaves by accumulating these ions in the sheaths in order to protect the leaves from ion toxicity; however, these lines maintained a higher K⁺ concentration in the leaves since K⁺ functions as an osmolyte and maintains ion homeostasis in the leaf cells. Furthermore, the in vivo overaccumulated GB could enhance or stabilize the activity of antioxidant enzymes that can scavenge reactive oxygen species (ROS) and mitigate oxidative damage of biomembranes. The experimental results suggest that GB overexpression can enhance the salt tolerance of transgenic plants by regulating ion homeostasis, enhancing OA, and scavenging ROS. Published in Russian in Fiziologiya Rastenii, 2009, vol. 56, No. 3, pp. 410–417. This text was submitted by the authors in English.     

Photosynthetic pigments,ionic and antioxidative behaviour of hulled tetraploid wheat in response to NaCl

Salt stress causes extensive losses to agricultural crops, including wheat, throughout the world and has been the focus of wide research. Though, information is scarce on the potential of ancient wheat relatives in tackling this major limiting factor. Thus, six hulled tetraploid wheat genotypes (HW) were compared to a free-threshing durum wheat genotype (FTW) under different NaCl concentrations, ranging from 0 to 150 mM, at early growth stages in a sand culture experiment. Salt stress induced significant declines in the leaf chlorophyll (Chl) a, Chl b, total Chl, and carotentoid contents; the extent of the declines was greater in FTW compared to HW. Mean leaf proline (3.6-fold) and Na⁺ (1.58-fold) concentrations and Na⁺/K⁺ (2.48-fold) drastically increased with 150 mM of NaCl; the magnitude of the increases was greater in HW compared to FTW. While the carotenoids concentration decreased with progressive salinity both in HW and FTW, the activities of antioxidant enzymes, i.e., catalase, ascorbate peroxidase, and peroxidase were reduced in FTW, but remained unchanged in HW. The above responses to 150 mM NaCl were associated with a significant decrease in shoot dry mass of FTW and lack of significant changes in that of HW. Findings of the present study could help pave the way for further studies on physiological and molecular mechanisms of salt tolerance in these durum wheat relatives.     

高盐胁迫对罗布麻生长及离子平衡的影响

采用网室盆栽试验,研究了不同浓度NaCl（100～400 mmol·L^-1）胁迫30 d对罗布麻植株生物量积累、生长速率、根系活力、盐分和矿质离子吸收、分布等的影响.结果表明：100 mmol·L^-1 NaCl处理30 d,罗布麻植株鲜质量和生长速率显著下降,但对其干质量没有影响;随着盐度的增加,罗布麻植株干质量、鲜质量和生长速率均显著降低.100～200 mmol·L^-1 NaCl胁迫下,罗布麻根系活力明显高于对照;300～400 mmol·L^-1 NaCl盐胁迫下,其活力显著降低.随着盐浓度的增加,罗布麻根、茎和叶片Na⁺含量逐渐增加、K⁺含量缓慢降低;叶片Ca²⁺、Mg²⁺含量明显降低,茎部Ca²⁺和根部Mg²⁺含量有不同程度的增加.盐胁迫明显降低了罗布麻根、茎和叶片K⁺/Na⁺、Ca²⁺/Na⁺和Mg²⁺/Na⁺的比率,植株选择性吸收和运输K⁺、Ca²⁺的能力显著提高.罗布麻植株很强的拒盐能力,以及对K⁺、Ca²⁺的选择性吸收和运输是其具有高盐适应性的主要原因.     

盐胁迫下不同基因型冬小麦渗透及离子的毒害效应

以4种不同基因型冬小麦为试验材料,利用分根法研究了盐胁迫对小麦的渗透胁迫和离子毒害的效应。结果表明,在盐胁迫下,小麦既受渗透胁迫,也受盐离子胁迫。渗透胁迫效应比较快,大约在处理后1-2d内发生;离子毒害效应比较缓慢,大约需3-4d时间。在一半盐胁迫(200mmol/L NaCl)和一半非盐胁迫的分根条件下,小麦没有明显的渗透胁迫效应,小麦植株地上部Na⁺ 累积到毒性水平之前盐处理对小麦生长无抑制效应。小麦具有将Na⁺ 从盐胁迫一侧转移非盐一侧的能力,说明小麦吸收的Na⁺ 有一部分可以从地上部回流到根系中,回流率可达76%-89%。无水分胁迫(不加入PEG)的回流率大于水分胁迫(加入PEG)的回流率。不同基因型小麦在盐分吸收累积和回流,及渗透和离子胁迫的速度和程度等方面具有明显差异。NR 9405和小偃6号的Na⁺ 累积速度要少于陕229和RB 6;NR 9405根系排Na⁺ 能力强于陕229和RB 6。因此,NR 9405和小偃6号的耐盐性高于陕229和RB 6。     

耐盐和盐敏感型小麦品种对NaCl胁迫的生理响应及耐盐性差异

为探究小麦品种类型对盐胁迫的生理响应及敏感性差异,以耐盐型品种济麦22和盐敏感型品种河农6425为材料,对幼苗期植株施以不同浓度的NaCl胁迫处理,比较分析了两个品种幼苗在胁迫条件下的生长、生理生化和叶绿体荧光特征等方面的差异。结果表明,NaCl胁迫对幼苗地上和地下部分的生长表现浓度依赖性的抑制效应,对耐盐型品种的抑制程度较小。在生理响应方面,幼苗体内的Na⁺含量随NaCl 浓度的增加而上升,K⁺和Ca²⁺含量则表现相反的变化趋势,耐盐型品种幼苗在胁迫处理前、后均具有较高的K⁺/Na⁺比和Ca²⁺/Na⁺比;NaCl胁迫导致幼苗的光系统Ⅱ受损,表现在Fv/Fm、Fv/Fo、qP 和YⅡ 等叶绿体荧光参数数值下降,降幅在耐盐性品种上相对较小。在氧化胁迫和抗氧化系统方面,NaCl胁迫导致幼苗体内活性氧水平的上升和过氧化物酶（POD）、过氧化氢酶（CAT）等抗氧化酶的响应;POD活性在处理后的0-12 d范围内呈先下降后上升的趋势,CAT活性则呈先上升后下降的趋势。耐盐品种的POD活性在胁迫早期受抑制时间较短,随后的响应更迅速且上升幅度更高;耐盐品种的CAT活性上升幅度更高,且在胁迫后期对高浓度NaCl和长时间胁迫导致的酶活性抑制的耐受性更强。耐盐品种抗氧化酶的这一响应特征与其较低的活性氧上升幅度一致,也与其较低水平的代表膜损伤程度的丙二醛（MDA）积累一致。耐盐型品种根部的MDA积累经200 mmol/L NaCl处理1 d后达到峰值,而盐敏感品种根部的MDA积累经150 mmol/L NaCl处理1 d后即达到峰值。以上研究结果表明,耐盐型小麦品种济麦22可分别通过其较强的K⁺/Na⁺、Ca²⁺/Na⁺调节能力和抗氧化酶体系缓解盐胁迫所导致的渗透胁迫和活性氧伤害,从而表现出耐盐的特征。     

Overaccumulation of glycine betaine enhances tolerance of the photosynthetic apparatus to drought and heat stress in wheat

To investigate the role of glycine betaine in photosynthesis under stress, a transgenic wheat (Triticum aestivum L.) line T6 overaccumulating glycine betaine and its wild type Shi4185 were used. Seedlings were exposed to conditions of drought (30%, PEG-6000), heat (40°C) and their combination. The results revealed ultrastructural damage to the chloroplast and thylakoid lamellae with the withered phenotype by both drought and heat stress, and the damage was exacerbated by the combination of drought and heat. The appearance of a K step in the typical O-J-I-P curve and the decrease of Hill activity indicated a reduction of oxygen evolving complex function caused by stress. The greater damage was found in wild type than T6. Overaccumulation of glycine betaine in T6 could protect lipids in the thylakoid membrane from damage and stabilize the index of unsaturated fatty acids under stress. A lower ratio of monogalactosyl diacylglycerol/digalactosyl diacylglycerol and higher phosphatidylglycerol content in the thylakoid membrane of T6 were also observed under stress. These effects can promote stability of the thylakoid membrane. Otherwise, glycine betaine overaccumulation decreased photoinhibition of PSII under stress. The results also suggest that xanthophyll cycle-dependent non-radiative energy dissipation may be involved in the GB-mediated effects on PSII function under stress conditions.     

The stabilization of beta-lactoglobulin by glycine and NaCl

Effect of glycine and NaCl on the thermal denaturation of beta-lactoglobulin was examined. The results showed that the transition temperature of beta-lactoglobulin is increased by the addition of glycine and NaCl at 0.5 and 1M. This observed stabilization by glycine and NaCl was interpreted in terms of their favorable interactions with the native state of beta-lactoglobulin and unfavorable interactions with the denatured state.     

一氧化氮在微波预处理提高小麦幼苗抗镉胁迫中的作用

为了探讨一氧化氮在微波预处理提高小麦抗镉胁迫中的作用,利用微波(126mW·cm-2)辐照小麦种子10 s,待其长至一叶一心时,用150 μmol·L-l CdCl2胁迫其幼苗,并通过添加一氧化氮专一清除剂cPTIO(0.5％,w/v),研究一氧化氮对微波预处理小麦幼苗脂质过氧化、抗氧化酶活性、光合色素含量及幼苗生长的影响.结果表明:微波预处理可使镉胁迫的小麦幼苗丙二醛含量显著降低(P＜0.05),超氧化物歧化酶、过氧化物酶、过氧化氢酶活性、一氧化氮、叶绿素a、叶绿素b、类胡萝卜素含量和株高、根长、地上部干重、根干重显著增加(P＜0.05);经过适当剂量微波辐照镉胁迫小麦幼苗再加外源cPTIO处理能有效逆转微波对镉胁迫小麦幼苗的防护作用.说明微波对镉胁迫损伤小麦幼苗起到一定的防护作用主要是通过NO起作用.     

The effect of 5-azacytidine on wheat seedlings responses to NaCl stress

The effect of 5-azacytidine (5-azaC) on the alleviation of damaging effects of NaCl treatment was studied in two wheat (Triticum aestivum L.) cultivars differing in salt tolerance (salt-tolerant Dekang-961 and sensitive Lumai-15). The plants were pre-treated or not with 50 μM 5-azaC and then subjected to salt stress induced by 100 or 150 mM NaCl. Salinity caused reduction in biomass accumulation and increase in malondialdehyde content in root tissues in both cultivars, but less in pre-treated seedlings. The activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) in the roots of both cultivars increased during salt stress, but the rate of increase was higher in Dekang-961. Plants treated with 5-azaC had higher root SOD, CAT and POD activities under salt stress than untreated plants. Content of 5-methylcytosine (5mC) decreased in both cultivars under salt stress, and the level of demethylation was higher in Dekang-961 than that in Lumai-15. Moreover, the degree of methylation was lower in both cultivars under salt stress after 5-azaC application compared to only salt-treated groups. These findings suggested that 5-azaC could protect plants from salt stress.     

A barley mutant with improved salt tolerance through ion homeostasis and ROS scavenging under salt stress

To investigate key regulatory components and genes with great impact on salt tolerance, near isogenic or mutant lines with distinct salinity tolerance are suitable genetic materials to simplify and dissect the complex genes networks. In this study, we evaluated responses of a barley mutant genotype (73-M4-30), in comparison with its wild-type background (Zarjou) under salt stress. Although the root growth of both genotypes was significantly decreased by exposure to sodium chloride (NaCl), the effect was greater in the wild type. The chlorophyll content decreased under salt stress for the wild type, but no change occurred in the mutant. The mutant maintained the steady-state level of [K⁺] and significantly lower [Na⁺] concentrations in roots and higher [K⁺]/[Na⁺] ratio in shoots under salt conditions. The catalase (CAT), peroxidase (POD) activity, and proline content were higher in the mutant than those in the wild type under controlled conditions. The soluble proline was higher after 24 h of salt stress in roots of the mutant but was higher after 96 h of salt stress in the wild type. The CAT and POD activity of the mutant increased under salt stress which was as a coincidence to lower levels of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) contents. The ratio of dry-to-fresh weight of the roots increased for the mutant under salt stress which was as a result of the higher phenylalanine ammonia-lyase (PAL) gene expression and peroxidase activity and involved in cell wall lignification. Consequently, it seems that ion homeostasis and increased peroxidase activity have led to salt tolerance in the mutant’s genotype.     

The protection of wheat plasma membrane under cold stress by glycine betaine overproduction

We aimed to study the protection of wheat plasma membrane (PM) under cold stress (0–2 °C) by the overaccumulation of glycine betaine (GB). For this, we used wild-type winter wheat (Triticum aestivum L.) cv. Shi 4185 (WT) and 3 transgenic lines (T1, T4, and T6) expressing the BADH gene isolated from Atriplex hortensis L. Under cold stress, the transgenic lines with higher GB content maintained better membrane integrity and higher plasma membrane H⁺-ATPase activity than WT. In these transgenic lines, ROS production and membrane lipid peroxidation were lower, while antioxidative enzyme activities and compatible solute contents were higher in comparison with WT. This may be attributable to their enhanced cold-stress tolerance mediated by GB overproduction.     

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.