Reactive oxygen species (ROS) are produced as a normal product of plant cellular metabolism. Various environmental stresses lead to excessive production of ROS causing progressive oxidative damage and ultimately cell death. Despite their destructive activity, they are well-described second messengers in a variety of cellular processes, including conferment of tolerance to various environmental stresses. Whether ROS would serve as signaling molecules or could cause oxidative damage to the tissues depends on the delicate equilibrium between ROS production, and their scavenging. Efficient scavenging of ROS produced during various environmental stresses requires the action of several nonenzymatic as well as enzymatic antioxidants present in the tissues. In this paper, we describe the generation, sites of production and role of ROS as messenger molecules as well as inducers of oxidative damage. Further, the antioxidative defense mechanisms operating in the cells for scavenging of ROS overproduced under various stressful conditions of the environment have been discussed in detail.

https://downloads.hindawi.com/archive/2012/217037.pdf

Reactive Oxygen Species, Oxidative Damage, and Antioxidative Defense Mechanism in Plants under Stressful Conditions

Salinity tolerance comes from genes that limit the rate of salt uptake from the soil and the transport of salt throughout the plant, adjust the ionic and osmotic balance of cells in roots and shoots, and regulate leaf development and the onset of senescence. This review lists some candidate genes for salinity tolerance, and draws together hypotheses about the functions of these genes and the specific tissues in which they might operate. Little has been revealed by gene expression studies so far, perhaps because the studies are not tissue-specific, and because the treatments are often traumatic and unnatural. Suggestions are made to increase the value of molecular studies in identifying genes that are important for salinity tolerance.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2005.01487.x

Genes and salt tolerance: bringing them together.

The reaction centers of PSI and PSII in chloroplast thylakoids are the major generation site of reactive oxygen species (ROS). Photoreduction of oxygen to hydrogen peroxide (H2O2) in PSI was discovered over 50 years ago by [Mehler (1951)][1]. Subsequently, the primary reduced product was identified

Production and scavenging of reactive oxygen species in chloroplasts and their functions.

Salinity is a major abiotic stress limiting growth and productivity of plants in many areas of the world due to increasing use of poor quality of water for irrigation and soil salinization. Plant adaptation or tolerance to salinity stress involves complex physiological traits, metabolic pathways, and molecular or gene networks. A comprehensive understanding on how plants respond to salinity stress at different levels and an integrated approach of combining molecular tools with physiological and biochemical techniques are imperative for the development of salt-tolerant varieties of plants in salt-affected areas. Recent research has identified various adaptive responses to salinity stress at molecular, cellular, metabolic, and physiological levels, although mechanisms underlying salinity tolerance are far from being completely understood. This paper provides a comprehensive review of major research advances on biochemical, physiological, and molecular mechanisms regulating plant adaptation and tolerance to salinity stress.

/pdf/mechanism-of-salinity-tolerance-in-plants-physiological-3ixvqhezre.pdf

Mechanism of Salinity Tolerance in Plants: Physiological, Biochemical, and Molecular Characterization

Stressful environments such as salinity, drought, and high temperature (heat) cause alterations in a wide range of physiological, biochemical, and molecular processes in plants. Photosynthesis, the most fundamental and intricate physiological process in all green plants, is also severely affected in all its phases by such stresses. Since the mechanism of photosynthesis involves various components, including photosynthetic pigments and photosystems, the electron transport system, and CO2 reduction pathways, any damage at any level caused by a stress may reduce the overall photosynthetic capacity of a green plant. Details of the stress-induced damage and adverse effects on different types of pigments, photosystems, components of electron transport system, alterations in the activities of enzymes involved in the mechanism of photosynthesis, and changes in various gas exchange characteristics, particularly of agricultural plants, are considered in this review. In addition, we discussed also progress made during the last two decades in producing transgenic lines of different C3 crops with enhanced photosynthetic performance, which was reached by either the overexpression of C3 enzymes or transcription factors or the incorporation of genes encoding C4 enzymes into C3 plants. We also discussed critically a current, worldwide effort to identify signaling components, such as transcription factors and protein kinases, particularly mitogen-activated protein kinases (MAPKs) involved in stress adaptation in agricultural plants.

Photosynthesis under stressful environments: An overview

A cDNA clone was isolated from cotton (Gossypium hirsatam) cDNA library and characterized with regard to its sequence, regulation in response to salt stress and functions in yeast mutants and transgenic tobacco plants. The clone, designated as GhNHXI, contains 2,485 nucleotides with an open reading frame of 1,629 nucleotides, and the deduced amino acid sequence showed high identities with other plant vacuolar-type Na + /H + antiporters. Northern blot analysis indicated that the mRNA accumulation of GhNHXI was strongly induced by salt stress and abscisic acid in cotton seedlings. The expression of GhNHXI in yeast Na + /H + antiporter mutant showed function complementation. The transgenic tobacco plants overexpressing GhNHXI also had higher salt tolerance than the wild-type plants. The salt-induced mRNA level of GhNHXI was 3 and 7 times higher in the salt-tolerant cotton cultivar ZM3 than those in the salt-sensitive cotton cultivars ZMS17 and ZMS12, respectively. Together, these results suggest that the products of the novel gene, GhNHXI, function as a tonoplast Na + /H + antiporter and play an important role in salt tolerance of cotton.

The cotton GhNHX1 gene encoding a novel putative tonoplast Na(+)/H(+) antiporter plays an important role in salt stress.

Fruit maturation and ripening are genetically regulated processes that involve a complex interplay of plant hormones, growth regulators and multiple biological and environmental factors. Tomato (Solanum lycopersicum) has been used as a model of biological and genetic studies on the regulation of specific ripening pathways, including ethylene, carotenoid and cell wall metabolism. This model has also been used to investigate the functions of upstream signalling and transcriptional regulators. Thus far, many ripening-associated transcription factors that influence fruit development and ripening have been reported. NAC transcription factors are plant specific and play important roles in many stages of plant growth and development, such as lateral root formation, secondary cell wall synthesis, and embryo, floral organ, vegetative organ and fruit development. Tissue-specific analysis by quantitative real-time PCR showed that SlNAC1 was highly accumulated in immature green fruits; the expression of SlNAC1 increased with fruit ripening till to the highest level at 7 d after the breaker stage. The overexpression of SlNAC1 resulted in reduced carotenoids by altering carotenoid pathway flux and decreasing ethylene synthesis mediated mainly by the reduced expression of ethylene biosynthetic genes of system-2, thus led to yellow or orange mature fruits. The results of yeast one-hybrid experiment demonstrated that SlNAC1 can interact with the regulatory regions of genes related lycopene and ethylene synthesis. These results also indicated that SlNAC1 inhibited fruit ripening by affecting ethylene synthesis and carotenoid accumulation in SlNAC1 overexpression lines. In addition, the overexpression of SlNAC1 reduced the firmness of the fruits and the thickness of the pericarp and produced more abscisic acid, resulting in the early softening of fruits. Hence, in SlNAC1 overexpression lines, both ethylene-dependent and abscisic acid-dependent pathways are regulated by SlNAC1 in fruit ripening regulatory network. SlNAC1 had a broad influence on tomato fruit ripening and regulated SlNAC1 overexpression tomato fruit ripening through both ethylene-dependent and abscisic acid-dependent pathways. Thus, this study provided new insights into the current model of tomato fruit ripening regulatory network.

/pdf/overexpression-of-tomato-slnac1-transcription-factor-alters-1ahux7ytsj.pdf

Overexpression of tomato SlNAC1 transcription factor alters fruit pigmentation and softening

A tomato (Lycopersicon esculentum Mill.) monodehydroascorbate reductase gene (LeMDAR) was isolated. The LeMDAR-green fluorescence protein (GFP) fusion protein was targeted to chloroplast in Arabidopsis mesophyll protoplast. RNA and protein gel blot analyses confirmed that the sense- and antisense- LeMDAR were integrated into the tomato genome. The MDAR activities and the levels of reduced ascorbate (AsA) were markedly increased in sense transgenic lines and decreased in antisense transgenic lines compared with wild-type (WT) plants. Under low and high temperature stresses, the sense transgenic plants showed lower level of hydrogen peroxide (H(2)O(2)), lower thiobarbituric acid reactive substance (TBARS) content, higher net photosynthetic rate (P(n)), higher maximal photochemical efficiency of PSII (F(v)/F(m)) and fresh weight compared with WT plants. The oxidizable P700 decreased more obviously in WT and antisense plants than that in sense plants at chilling temperature under low irradiance. Furthermore, the sense transgenic plants exhibited significantly lower H(2)O(2) level, higher ascorbate peroxidase (APX) activity, greater P(n) and F(v)/F(m) under methyl viologen (MV)-mediated oxidative stresses. These results indicated that overexpression of chloroplastic MDAR played an important role in alleviating photoinhibition of PSI and PSII and enhancing the tolerance to various abiotic stresses by elevating AsA level.

Overexpression of chloroplastic monodehydroascorbate reductase enhanced tolerance to temperature and methyl viologen-mediated oxidative stresses

DnaJ proteins act as essential molecular chaperones in protein homeostasis and protein complex stabilization under stress conditions. The roles of a tomato (Lycopersicon esculentum) chloroplast-targeted DnaJ protein (LeCDJ1), whose expression was upregulated by treatment at 4 and 42 °C, and with high light, NaCl, polyethylene glycol, and H2O2, were investigated here using sense and antisense transgenic tomatoes. The sense plants exhibited not only higher chlorophyll content, fresh weight and net photosynthetic rate, but also lower accumulation of reactive oxygen species and membrane damage under chilling stress. Moreover, the maximal photochemistry efficiency of photosystem II (PSII) (F v/F m) and D1 protein content were higher in the sense plants and lower in the antisense plants, and the photoinhibitory quenching was lower in the sense plants and higher in the antisense plants, suggesting that the inhibition of PSII was less severe in the sense plants and more severe in the antisense plants compared with the wild type. Furthermore, the PSII protein complexes were also more stable in the sense plants. Interestingly, the sense plants treated with streptomycin (SM), an inhibitor of organellar translation, still showed higher F v/F m, D1 protein content and PSII stability than the SM-untreated antisense plants. This finding suggested that the protective effect of LeCDJ1 on PSII was, at least partially, independent of D1 protein synthesis. Furthermore, chloroplast heat-shock protein 70 was identified as the partner of LeCDJ1. These results indicate that LeCDJ1 has essential functions in maintaining PSII under chilling stress.

/pdf/a-chloroplast-targeted-dnaj-protein-contributes-to-361ksydxiw.pdf

A chloroplast-targeted DnaJ protein contributes to maintenance of photosystem II under chilling stress

The purpose of this study was to explore how the mitochondrial AOX (alternative oxidase) pathway alleviates photoinhibition in Rumex K-1 leaves. Inhibition of the AOX pathway decreased the initial activity of NADP-malate dehydrogenase (EC 1.1.1.82, NADP-MDH) and the pool size of photosynthetic end electron acceptors, resulting in an over-reduction of the photosystem I (PSI) acceptor side. The over-reduction of the PSI acceptor side further inhibited electron transport from the photosystem II (PSII) reaction centers to the PSII acceptor side as indicated by an increase in V(J) (the relative variable fluorescence at J-step), causing an imbalance between photosynthetic light absorption and energy utilization per active reaction center (RC) under high light, which led to the over-excitation of the PSII reaction centers. The over-reduction of the PSI acceptor side and the over-excitation of the PSII reaction centers enhanced the accumulation of reactive oxygen species (ROS), which inhibited the repair of the photodamaged PSII. However, the inhibition of the AOX pathway did not change the level of photoinhibition under high light in the presence of the chloroplast D1 protein synthesis inhibitor chloramphenicol, indicating that the inhibition of the AOX pathway did not accelerate the photodamage to PSII directly. All these results suggest that the AOX pathway plays an important role in the protection of plants against photoinhibition by minimizing the inhibition of the repair of the photodamaged PSII through preventing the over-production of ROS.

Mitochondrial alternative oxidase pathway protects plants against photoinhibition by alleviating inhibition of the repair of photodamaged PSII through preventing formation of reactive oxygen species in Rumex K-1 leaves.

Qing-Wei Meng

Papers

The cotton GhNHX1 gene encoding a novel putative tonoplast Na(+)/H(+) antiporter plays an important role in salt stress.

Overexpression of tomato SlNAC1 transcription factor alters fruit pigmentation and softening

Overexpression of chloroplastic monodehydroascorbate reductase enhanced tolerance to temperature and methyl viologen-mediated oxidative stresses

A chloroplast-targeted DnaJ protein contributes to maintenance of photosystem II under chilling stress

Mitochondrial alternative oxidase pathway protects plants against photoinhibition by alleviating inhibition of the repair of photodamaged PSII through preventing formation of reactive oxygen species in Rumex K-1 leaves.