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Salicylic acid improves acclimation to salt stress by stimulating abscisic aldehyde oxidase activity and abscisic acid accumulation, and increases Na+ content in leaves without toxicity symptoms in Solanum lycopersicum L.

Pre-treatment with 10(-4)M salicylic acid (SA) in hydroponic culture medium provided protection against salinity stress in tomato plants (Solanum lycopersicum L. cv. Rio Fuego). The effect of 10(-7) or 10(-4)M SA on the water status of plants was examined in relation to the biosynthesis and accumulation of abscisic acid (ABA) in order to reveal the role of SA in the subsequent response to salt stress. Both pre-treatments inhibited the K+(86Rb+) uptake of plants, reduced the K+ content of leaves, and caused a decrease in leaf water potential (psi(w)). Due to the changes in the cellular water status, SA triggered the accumulation of ABA. Since the decrease in psi(w) proved to be transient, the effect of SA on ABA synthesis may also develop via other mechanisms. In spite of osmotic adaptation, the application of 10(-4)M, but not 10(-7)M SA, led to prolonged ABA accumulation and to enhanced activity of aldehyde oxidase (AO1, EC.1.2.3.1.), an enzyme responsible for the conversion of ABA-aldehyde to ABA, both in root and leaf tissues. AO2-AO4 isoforms from the root extracts also exhibited increased activities. The fact that the activities of AO are significantly enhanced both in the leaves and roots of plants exposed to 10(-4)M SA, may indicate a positive feedback regulation of ABA synthesis by ABA in this system. Moreover, during a 100mM NaCl treatment, higher levels of free putrescine or spermine were found in these leaves or roots, respectively, than in the salt-stressed controls, suggesting that polyamines may be implicated in the protection response of the cells. As a result, Na+ could be transported to the leaf mesophyll cells without known symptoms of salt toxicity.

Cross-talk between salicylic acid and NaCl-generated reactive oxygen species and nitric oxide in tomato during acclimation to high salinity

Hydrogen peroxide (H₂O₂) and nitric oxide (NO) generated by salicylic acid (SA) are considered to be functional links of cross-tolerance to various stressors. SA-stimulated pre-adaptation state was beneficial in the acclimation to subsequent salt stress in tomato (Solanum lycopersicum cv. Rio Fuego). At the whole-plant level, SA-induced massive H₂O₂ accumulation only at high concentrations (10⁻³-10⁻² M), which later caused the death of plants. The excess accumulation of H₂O₂ as compared with plants exposed to 100 mM NaCl was not associated with salt stress response after SA pre-treatments. In the root tips, 10⁻³-10⁻² M SA triggered the production of reactive oxygen species (ROS) and NO with a concomitant decline in the cell viability. Sublethal concentrations of SA, however, decreased the effect of salt stress on ROS and NO production in the root apex. The attenuation of oxidative stress because of high salinity occurred not only in pre-adapted plants but also at cell level. When protoplasts prepared from control leaves were exposed to SA in the presence of 100 mM NaCl, the production of NO and ROS was much lower and the viability of the cells was higher than in salt-treated samples. This suggests that, the cross-talk of signalling pathways induced by SA and high salinity may occur at the level of ROS and NO production. Abscisic acid (ABA), polyamines and 1-aminocyclopropane-1-carboxylic acid, the compounds accumulating in pre-treated plants, enhanced the diphenylene iodonium-sensitive ROS and NO levels, but, in contrast to others, ABA and putrescine preserved the viability of protoplasts.

Plant glutathione peroxidases: Emerging role of the antioxidant enzymes in plant development and stress responses

The plant glutathione peroxidase (GPX) family consists of multiple isoenzymes with distinct subcellular locations which exhibit different tissue-specific expression patterns and environmental stress responses. Contrary to most of their counterparts in animal cells, plant GPXs contain cysteine instead of selenocysteine in their active site and while some of them have both glutathione peroxidase and thioredoxin peroxidase functions, the thioredoxin regenerating system is much more efficient in vitro than the glutathione system. At present, the function of these enzymes in plants is not completely understood. The occurrence of thiol-dependent activities of plant GPX isoenzymes suggests that - besides detoxification of H2O2 and organic hydroperoxides - they may be involved in regulation of the cellular redox homeostasis by maintaining the thiol/disulfide or NADPH/NADP(+) balance. GPXs may represent a link existing between the glutathione- and the thioredoxin-based system. The various thiol buffers, including Trx, can affect a number of redox reactions in the cells most probably via modulation of thiol status. It is still required to identify the in vivo reductant for particular GPX isoenzymes and partners that GPXs interact with specifically. Recent evidence suggests that plant GPXs does not only protect cells from stress induced oxidative damage but they can be implicated in plant growth and development. Following a more general introduction, this study summarizes present knowledge on plant GPXs, highlighting the results on gene expression analysis, regulation and signaling of Arabidopsis thaliana GPXs and also suggests some perspectives for future research.

Exogenous salicylic acid-triggered changes in the glutathione transferases and peroxidases are key factors in the successful salt stress acclimation of Arabidopsis thaliana

Salicylic acid (SA) applied exogenously is a potential priming agent during abiotic stress. In our experiments, the priming effect of SA was tested by exposing Arabidopsis thaliana (L.) Heynh. plants to 2-week-long 10-9-10-5 M SA pretreatments in a hydroponic medium, followed by 1 week of 100mM NaCl stress. The levels of reactive oxygen species and H2O2, changes in antioxidant enzyme activity and the expression of selected glutathione transferase (GST) genes were investigated. Although 10-9-10-7 M SA pretreatment insufficiently induced defence mechanisms during the subsequent salt stress, 2-week pretreatments with 10-6 and 10-5 M SA alleviated the salinity-induced H2O2 and malondialdehyde accumulation, and increased superoxide dismutase, guaiacol peroxidase, GST and glutathione peroxidase (GPOX) activity. Our results indicate that long-term 10-6 and 10-5 M SA treatment mitigated the salt stress injury in this model plant. Enhanced expression of AtGSTU19 and AtGSTU24 may be responsible for the induced GST and GPOX activity, which may play an important role in acclimation. Modified GST expression suggested altered signalling in SA-hardened plants during salt stress. The hydroponic system applied in our experiments proved to be a useful tool for studying the effects of sequential treatments in A. thaliana.

Physiological and molecular responses to heavy metal stresses suggest different detoxification mechanism of Populus deltoides and P. x canadensis

Plants have divergent defense mechanisms against the harmful effects of heavy metals present in excess in soils and groundwaters. Poplars (Populus spp.) are widely cultivated because of their rapid growth and high biomass production, and members of the genus are increasingly used as experimental model organisms of trees and for phytoremediation purposes. Our aim was to investigate the copper and zinc stress responses of three outstanding biomass producer bred poplar lines to identify such transcripts of genes involved in the detoxification mechanisms, which can play an important role in the protection against heavy metals. Poplar cuttings were grown hydroponically and subjected to short-term (one week) mild and sublethal copper and zinc stresses. We evaluated the effects of the applied heavy metals and the responses of plants by detecting the changes of multiple physiological and biochemical parameters. The most severe cellular oxidative damage was caused by 30μM copper treatment, while zinc was less harmful. Analysis of stress-related transcripts revealed genotype-specific differences that are likely related to alterations in heavy metal tolerance. P. deltoides clones B-229 and PE 19/66 clones were clearly more effective at inducing the expression of various genes implicated in the detoxification process, such as the glutathione transferases, metallothioneins, ABC transporters, (namely PtGSTU51, PxMT1, PdABCC2,3), while the P. canadensis line M-1 accumulated more metal, resulting in greater cellular oxidative damage. Our results show that all three poplar clones are efficient in stress acclimatization, but with different molecular bases.

Glutathione-Related Enzyme System: Glutathione Reductase (GR), Glutathione Transferases (GSTs) and Glutathione Peroxidases (GPXs)

The glutathione-related enzymes are usually considered to accompany the main non-enzymatic antioxidative compounds of the ascorbate–glutathione cycle. Taking into account that the redox processes are not spontaneous in cells, but the adequate reaction velocity and appropriate specificity are achieved by the catalyzing activity of enzymes, special attention has raised toward the glutathione-utilizing enzymes. Glutathione reductase (GR) is a NADPH-dependent oxidoreductase which catalyzes the conversion of oxidized glutathione (GSSG) to reduced glutathione (GSH). Some members of the diverse glutathione transferase (GST) enzyme family have GSH-dependent thiol transferase activity and participate in the recycling of antioxidants (ascorbate, flavonoids, quinones), while other isoenzymes, due to their S-transferase activity, are involved in the detoxification mechanisms using GSH as co-substrate. A significant portion of GST isoenzymes also has glutathione peroxidase activity and can convert lipid peroxides and other peroxides to less harmful compounds. The plant glutathione peroxidase enzymes (GPXs) may be involved in the detoxification of H2O2 and organic hydroperoxides and in the regulation of the cellular redox homeostasis by maintaining the thiol/disulfide balance. Most of plant GPXs prefer to use thioredoxin (TRX) instead of glutathione as a reducing agent, and it is thought that the GPXs may represent a link between the glutathione- and the thioredoxin-based system. The GR, GPX and some GST isoenzymes have Cys in their active center and thus are directly regulated by redox status. This chapter summarizes their roles in stress responses as antioxidant enzymes, in determining the redox status of cells, and emphasizes their connection to redox signaling mechanisms.