Antioxidative response mechanisms in halophytes: Their role in stress defence

Normal growth and development of plants is greatly dependent on the capacity to overcome environmental stresses. Environmental stress conditions like high salinity, drought, high incident light and low or high temperature cause major crop losses worldwide. A common denominator in all these adverse conditions is the production of reactive oxygen species (ROS) within different cellular compartments of the plant cell. Plants have developed robust mechanisms including enzymatic or nonenzymatic scavenging pathways to counter the deleterious effects of ROS production. There are a number of general reviews on oxidative stress in plants and few on the role of ROS scavengers during stress conditions. Here we review the regulation of antioxidant enzymes during salt stress in halophytes, especially mangroves. We conclude that (i) antioxidant enzymes protect halophytes from deleterious ROS production during salt stress, and (ii) genetic information from mangroves and other halophytes would be helpful in defining the roles of individual isoforms. This information would be critical in using the appropriate genes for oxidative stress defence for genetic engineering of enhanced stress tolerance in crop systems.     

Protection against oxygen radicals: an important defence mechanism studied in transgenic plants

Free radicals and other active derivatives of oxygen are inevitable by-products of biological redox reactions. Reduced oxygen species, such as hydrogen peroxide, the superoxide radical anion and hydroxyl radicals, inactivate enzymes and damage important cellular components. In addition, singlet oxygen, produced via formation of triplet state chlorophyll, is highly destructive. This oxygen species initiates lipid peroxidation, and produces lipid peroxy radicals and lipid hydroperoxides that are also very reactive. The increased production of toxic oxygen derivatives is considered to be a universal or common feature of stress conditions. Plants and other organisms have evolved a wide range of mechanisms to contend with this problem. The antioxidant defence system of the plant comprises a variety of antioxidant molecules and enzymes. Considerable interest has been focused on the ascorbate-glutathione cycle because it has a central role in protecting the chloroplasts and other cellular compartments from oxidative damage. It is clear that the capacity and activity of the antioxidative defence systems are important in limiting photo-oxidative damage and in destroying active oxygen species that are produced in excess of those normally required for signal transduction or metabolism. In our studies on this system, we became aware that the answers to many unresolved questions concerning the nature and regulation of the antioxidative defence system could not be obtained easily by either a purely physiological or purely biochemical approach. Transgenic plants offered us a means by which to achieve a more complete understanding of the roles of the enzymes involved in protection against stress of many types: environmental and man-made. The ability to engineer plants which express introduced genes at high levels provides an opportunity to manipulate the levels of these enzymes, and hence metabolism in vivo. Studies on transformed plants expressing increased activities of single enzymes of the antioxidative defence system indicate that it is possible to confer a degree of tolerence to stress by this means. However, attempts to increase stress resistance by simply increasing the activity of one of the antioxidant enzymes have not always been successful presumably because of the need for a balanced interaction of protective enzymes. The study of these transformed plants has allowed a more complete understanding of the roles of individual enzymes in metabolism. Protection against oxidative stress has become a feasible objective through the application of molecular genetic techniques in conjunction with a biochemical and physiological approach.     

Hydrogen Peroxide in Plants： a Versatile Molecule of the Reactive Oxygen Species Network

Plants often face the challenge of severe environmental conditions, which include various biotic and abiotic stresses that exert adverse effects on plant growth and development. During evolution, plants have evolved complex regulatory mechanisms to adapt to various environmental stressors. One of the consequences of stress is an increase in the cellular concentration of reactive oxygen species （ROS）, which are subsequently converted to hydrogen peroxide （H2O2）. Even under normal conditions, higher plants produce ROS during metabolic processes. Excess concentrations of ROS result in oxidative damage to or the apoptotic death of cells. Development of an antioxidant defense system in plants protects them against oxidative stress damage. These ROS and, more particularly, H2O2, play versatile roles in normal plant physiological processes and in resistance to stresses. Recently, H2O2 has been regarded as a signaling molecule and regulator of the expression of some genes in cells. This review describes various aspects of H2O2 function, generation and scavenging, gene regulation and cross-links with other physiological molecules during plant growth, development and resistance responses.     

Systemic changes in photosynthesis and reactive oxygen species homeostasis in shoots of Arabidopsis thaliana infected with the beet cyst nematode Heterodera schachtii

Photosynthetic efficiency and redox homeostasis are important for plant physiological processes during regular development as well as defence responses. The second‐stage juveniles of Heterodera schachtii induce syncytial feeding sites in host roots. To ascertain whether the development of syncytia alters photosynthesis and the metabolism of reactive oxygen species (ROS), chlorophyll a fluorescence measurements and antioxidant responses were studied in Arabidopsis thaliana shoots on the day of inoculation and at 3, 7 and 15 days post‐inoculation (dpi). Nematode parasitism caused an accumulation of superoxide and hydrogen peroxide molecules in the shoots of infected plants at 3 dpi, probably as a result of the observed down‐regulation of antioxidant enzymes. These changes were accompanied by an increase in RNA and lipid oxidation markers. The activities of antioxidant enzymes were found to be enhanced on infection at 7 and 15 dpi, and the content of anthocyanins was elevated from 3 dpi. The fluorescence parameter R_fd, defining plant vitality and the photosynthetic capacity of leaves, decreased by 11% only at 7 dpi, and non‐photochemical quenching (NPQ), indicating the effectiveness of photoprotection mechanisms, was about 16% lower at 3 and 7 dpi. As a result of infection, the ultrastructure of chloroplasts was changed (large starch grains and plastoglobules), and more numerous and larger peroxisomes were observed in the mesophyll cells of leaves. We postulate that the joint action of antioxidant enzymes/molecules and photochemical mechanisms leading to the maintenance of photosynthetic efficiency promotes the fine‐tuning of the infected plants to oxidative stress induced by parasitic cyst nematodes.     

一氧化氮对植物重金属胁迫抗性的影响研究进展

一氧化氮(NO)作为一种重要的信号分子,在调节植物重金属胁迫抗性方面上起着非常重要的作用。综述了NO在植物体内的产生途径,重金属胁迫下植物体内内源NO含量的变化以及外源NO与内源NO对植物重金属胁迫抗性的影响。大量研究表明外源NO能够增强植物对重金属胁迫的抗性,一方面是通过增强植物细胞的抗氧化系统或直接清除活性氧,另一方面是通过影响植物对重金属的吸收以及重金属在植物细胞内的分布。然而内源NO在调节植物重金属胁迫抗性上的功能角色仍存在争议。有些研究表明内源NO是有益的,能够缓解重金属胁迫诱导的毒性;但是也有证据表明内源NO是有害的,能够通过促进植物对重金属的吸收以及对植物螯合素进行S-亚硝基化弱化其解毒功能,从而参与重金属诱导的毒害反应和细胞凋亡过程。     

Transgenic plants: An insight into oxidative stress tolerance mechanisms

Environmental stresses considerably limit plant productivity. At the molecular level the negative effect of stress is often mediated by reactive oxygen species-initiated oxidative damage. Hence, it was hypothesised that increased tolerance to several environmental constraints could be achieved through enhanced tolerance to oxidative stress. In recent years much effort has been undertaken to improve oxidative stress tolerance by transforming plants with native or bacterial genes coding either for reactive oxygen species-scavenging enzymes or for enzymes modulating the cellular antioxidant capacity. This review deals with data on transgenic plants with altered antioxidant capacity and focuses on the new insight into the antioxidant defence mechanism given by this type of experimental model.     

Antioxidant enzyme responses of plants to heavy metal stress

Heavy metal pollutions caused by natural processes or anthropological activities such as metal industries, mining, mineral fertilizers, pesticides and others pose serious environmental problems in present days. Evidently there is an urgent need of efficient remediation techniques that can tackle problems of such extent, especially in polluted soil and water resources. Phytoremediation is one such approach that devices effective and affordable ways of engaging suitable plants to cleanse the nature. Excessive accumulation of metal in plant tissues are known to cause oxidative stress. These, in turn differentially affect other plant processes that lead to loss of cellular homeostasis resulting in adverse affects on their growth and development apart from others. Plants have limited mechanisms of stress avoidance and require flexible means of adaptation to changing. A common feature to combat stress factors is synchronized function of antioxidant enzymes that helps alleviating cellular damage by limiting reactive oxygen species (ROS). Although, ROS are inevitable byproducts from essential aerobic metabolisms, these are needed under sub-lethal levels for normal plant growth. Understanding the interplay between oxidative stress in plants and role of antioxidant enzymes can result in developing plants that can overcome oxidative stress with the expression of antioxidant enzymes. These mechanisms have been proving to have immense potential for remediating these metals through the process of phytoremediation. The aim of this review is to assemble our current understandings of role of antioxidant enzymes of plants subjected to heavy metal stress.     

Differential responses of the activities of antioxidant enzymes to thermal stresses between two invasive Eupatorium species in China

The effect of thermal stress on the antioxidant system was Investigated in two invasive plants, Eupatorlum adenophorum Spreng. and E. odoratum L. The former is sensitive to high temperature, whereas the latter is sensitive to low temperature.Our aim was to explore the relationship between the response of antioxidant enzymes and temperature In the two Invasive weeds with different distribution patterns in China. Plants were transferred from glasshouse to growth chambers at a constant 25 ℃ for 1 week to acclimatize to the environment. For the heat treatments, temperature was Increased stepwise to 30, 35, 38 and finally to 42 ℃. For the cold treatments, temperature was decreased stepwise to 20, 15,10 and finally to 5 ℃.Plants were kept In the growth chambers for 24 h at each temperature step. In E. adenophorum, the coordinated Increase of the activities of antioxidant enzymes was effective In protecting the plant from the eccumulatlon of active oxygen species (AOS) at low temperature, but the activities of catalase (CAT), guaiacol peroxidase (POD), ascorbate peroxidase (APX),glutathione reductase (GR), and monodehydroascorbate reductase (MDAR) were not accompanied by the Increase of super-oxide dismutase (SOD) during the heat treatments. As a result, the level of lipid peroxidation in E. adenophorum was higher under heat stress than under cold stress. In E. odoratum, however, the lesser degree of membrane damage, as indicated by low monodehydroascorbate content, and the coordinated Increase of the oxygen. Dstoxlfying enzymes were observed in hest-treated plants, but the antioxidant enzymes were unable to operate in cold stress. This indicates that the plants have a higher capacity for scavenging oxygen radicals in heat stress than in cold stress. The different responses of antloxidant enzymes may be one of the possible mechanisms of the differences in temperature sensitivities of the two plant species.     

脱落酸与植物细胞的抗氧化防护

水分胁迫是一种影响植物生长发育、限制植物产量的重要胁迫因子.植物能够通过感知刺激、产生和传导信号、启动各种防护机制来响应与适应水分胁迫.植物激素脱落酸(ABA)作为一种胁迫信号,在调节植物对水分胁迫的反应中起着重要的作用.ABA不仅能诱导气孔关闭,而且能诱导编码耐脱水蛋白的基因表达.正在增加的证据显示,ABA增强水分胁迫的耐性与其诱导抗氧化防护系统有关.本文综述了ABA在诱导活性氧(ROS)产生、调节抗氧化酶基因表达以及增强抗氧化防护系统方面的作用,着重讨论了在ABA诱导的抗氧化防护过程中Ca2 、NADPH氧化酶与ROS之间的交谈机制.     

Methyl jasmonate regulated diploid and tetraploid black locust (<Emphasis Type="Italic">Robinia pseudoacacia</Emphasis> L.) tolerance to salt stress

Methyl jasmonate (MeJA) is an essential and promising plant growth regulation factor that can improve plant development and growth. Here, we explored the mechanism by which MeJA regulates the tolerance of black locust (Robinia pseudoacacia L.) to salt stress. In this study, diploid and tetraploid R. pseudoacacia were subjected to three treatments: 500 mM NaCl; 100 μM MeJA; and 500 mM NaCl and 100 μM MeJA, and the changes in plant growth, endogenous MeJA levels and the anti-oxidative metabolism of leaves were investigated. The results showed that salt stress significantly inhibited plant growth and induced the accumulation of Na⁺ and Cl^? ions, malondialdehyde (MDA) content and reactive oxygen species. However, these adverse effects could be alleviated by applying MeJA, which was followed by a marked increase in the activities of antioxidant enzymes. In addition, some genes encoding several antioxidant enzymes were also up-regulated. Simultaneously, the endogenous MeJA content in MeJA-treated plants was lower than in salt-treated plants. It is noteworthy that tetraploids always possessed higher salt tolerance and obtained greater positive effects from MeJA than diploids. These results suggested that MeJA might play a protective role in defense responses, enabling diploid and tetraploid black locust, especially tetraploid, to better tolerate the adverse effects of salt stress.     

Stress proteins in plants

Summary

Several environmental stresses elicit specific plant genomic responses. These include temperature extremes, oxidative stress, water stress, anaerobiosis as well as pathogen attack. Molecular biological approaches are now yielding insights into the mechanisms whereby plant cells perceive the stress of temperature extremes and activate their defences in response at the gene level. These responses appear to be interconnected with responses to oxidative stress in plants and an outcome is a greater appreciation of the role, and the genetic regulation, of two important groups of ‘stress proteins’, namely the heat shock proteins and the antioxidant enzymes.     

Role of Triacontanol in Counteracting the Ill Effects of Salinity in Plants: A Review

Soil salinity is one among the common environmental threats to agriculture. It adversely affects the physio-biochemical processes of plants that eventually lead to the reduction in growth, development and crop productivity. To cope with such adverse conditions, plants develop certain internal mechanisms, but under severe conditions these mechanisms fail to tolerate the salt stress. To overcome this problem, various strategies have been employed that help plants to mitigate salinity effects. Among the various strategies, the application of plant growth regulators (PGRs) has gained significant attention to induce salt tolerance in plants. A number of PGRs have been used so far. Among these, triacontanol (TRIA), a new PGR is gaining a lot of importance to enhance the plant growth, productivity and salinity tolerance in different crops. The utility of TRIA is dependent on its applied concentration. Its lower concentrations generally alleviate the salinity effects. However, the knowledge of its biosynthesis, signalling and its role particularly to mitigate salinity effect remains scanty. In the present article, the focus has been given on the role of exogenous applications of TRIA in the regulation of physio-biochemical characteristics especially plant growth, photosynthesis, nutrient acquisition, oxidative stress, antioxidant systems, compatible solutes, yield attributes and its mode of action in plants under salinity conditions. The salient features of the review may provide new insights on the role of TRIA in countering the ill effect of salinity in different crop plants.

     

Fine and coarse regulation of reactive oxygen species in the salt tolerant mutants of barnyard grass and their wild-type parents under salt stress

The growth of the wild-type and three salt tolerant mutants of barnyard grass ( Echinochloa crusgalli L.) under salt stress was investigated in relation to oxidative stress and activities of the antioxidant enzymes superoxide dismutase (SOD: EC 1.15.1.1), catalase (CAT: EC 1.11.1.6), phenol peroxidase (POD: EC 1.11.1.7), glutathione reductase (GR: EC 1.8.1.7) and ascorbate peroxidase (APX: EC 1.11.1.1). The three mutants ( fows B17, B19 and B21) grew significantly better than the wild-type under salt stress (200 m M NaCl) but some salt sensitive individuals were still detectable in the populations of the mutants though in smaller numbers compared with the wild-type. The salt sensitive plants had slower growth rates, higher rates of lipid peroxidation and higher levels of reactive oxygen species (ROS) in their leaves compared with the more tolerant plants from the same genotype. These sensitivity responses were maximized when the plants were grown under high light intensity suggesting that the chloroplast could be a main source of ROS under salt stress. However, the salt sensitivity did not correlate with reduced K ⁺/Na ⁺ ratios or enhanced Na ⁺ uptake indicating that the sensitivity responses may be mainly because of accumulation of ROS rather than ion toxicity. SOD activities did not correlate to salt tolerance. Salt stress resulted in up to 10-fold increase in CAT activity in the sensitive plants but lower activities were found in the tolerant ones. In contrast, the activities of POD, APX and GR were down regulated in the sensitive plants compared with the tolerant ones. A correlation between plant growth, accumulation of ROS and differential modulation of antioxidant enzymes is discussed. We conclude that loss of activities of POD, APX and GR causes loss of fine regulation of ROS levels and hence the plants experience oxidative stress although they have high CAT activities.     

Rapid evolution of antioxidant defence in a natural population of Daphnia magna

Natural populations can cope with rapid changes in stressors by relying on sets of physiological defence mechanisms. Little is known onto what extent these physiological responses reflect plasticity and/or genetic adaptation, evolve in the same direction and result in an increased defence ability. Using resurrection ecology, we studied how a natural Daphnia magna population adjusted its antioxidant defence to ultraviolet radiation (UVR) during a period with increasing incident UVR reaching the water surface. We demonstrate a rapid evolution of the induction patterns of key antioxidant enzymes under UVR exposure in the laboratory. Notably, evolutionary changes strongly differed among enzymes and mainly involved the evolution of UV‐induced plasticity. Whereas D. magna evolved a strong plastic up‐regulation of glutathione peroxidase under UVR, it evolved a lower plastic up‐regulation of glutathione S‐transferase and superoxide dismutase and a plastic down‐regulation of catalase. The differentially evolved antioxidant strategies were collectively equally effective in dealing with oxidative stress because they resulted in the same high levels of oxidative damage (to lipids, proteins and DNA) and lowered fitness (intrinsic growth rate) under UVR exposure. The lack of better protection against UVR may suggest that the UVR exposure did not increase between both periods. Predator‐induced evolution to migrate to lower depths that occurred during the same period may have contributed to the evolved defence strategy. Our results highlight the need for a multiple trait approach when focusing on the evolution of defence mechanisms.     

Induction of phytochelatins and antioxidant defence system in <Emphasis Type="Italic">Brassica juncea</Emphasis> and <Emphasis Type="Italic">Vigna radiata</Emphasis> in response to chromium treatments

Growth performance, chromium (Cr) accumulation potential and induction of antioxidative defence system and phytochelatins (PCs) were studied in hydroponically grown Brassica juncea (Indian mustard) and Vigna radiata (mungbean) at various levels of Cr treatments (0, 50, 100, 200 μM Cr). B. juncea accumulated twofolds and threefolds higher Cr in root and shoot, respectively than in V. radiata. Compared to B. juncea, V. radiata was found to be particularly sensitive to Cr as observed by the severity and development of Cr toxicity symptoms and decreased growth. Induction of PC and enzymes of antioxidant defence system were monitored as plant’s primary and secondary metal detoxifying responses, respectively. There was induction of PC and enzymes of antioxidant defence system in both the plants. PCs were induced significantly in roots and shoot of both the plants at all the levels of Cr treatments. Significantly higher activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) and glutathione reductase (GR) were observed in shoot of B. juncea than V. radiata at all the levels of Cr treatments. Induction of PCs along with antioxidant defence system in response to Cr stress suggests the cumulative role of PCs and antioxidants in conferring tolerance against accumulated Cr in B. juncea, and thereby signifies the suitability of this plant as one of the potential remediators of Cr.     

Rethinking the role of many plant phenolics – protection from photodamage not herbivores?

Phenolics have been considered classic defence compounds for protecting plants from herbivores, ever since plant secondary metabolites were suggested to have evolved for that reason. The resource availability and carbon-nutrient balance hypotheses proposed that variation in phenolic levels between and within plant species reflects environmental availability of nutrients and light, and represents a trade-off in allocation by plants to growth and defence against herbivores. In contrast to these concepts, we suggest that (1) the main role of many plant phenolics may be to protect leaves from photodamage, not herbivores; (2) they can achieve this by acting as antioxidants; and (3) their levels may vary with environmental conditions in order to counteract this potential photodamage. We therefore suggest that patterns in phenolic levels, often used to support the concept of trade-off between growth and herbivore defence in relation to resource availability, may actually reflect different risks of photodamage. We suggest that the level of many phenolics is low under some environmental conditions, not because resources to produce them are limited, but simply because the risk of photodamage is low and they are not required. If our photodamage hypothesis is correct, a reassessment of the ecological and evolutionary role of many phenolics in plant defence theory is required.