Magda Pál

Exogenous 4-hydroxybenzoic acid and salicylic acid modulate the effect of short-term drought and freezing stress on wheat plants

Exogenous salicylic acid has been shown to confer tolerance against biotic and abiotic stresses. In the present work the ability of its analogue, 4-hydroxybenzoic acid to increase abiotic stress tolerance was demonstrated: it improved the drought tolerance of the winter wheat (Triticum aestivum L.) cv. Cheyenne and the freezing tolerance of the spring wheat cv. Chinese Spring. Salicylic acid, however, reduced the freezing tolerance of Cheyenne and the drought tolerance of Chinese Spring, in spite of an increase in the guaiacol peroxidase and ascorbate peroxidase activity. The induction of cross tolerance between drought and freezing stress was observed: drought acclimation increased the freezing tolerance of Cheyenne plants and cold acclimation enhanced the drought tolerance. The induction of drought tolerance in Cheyenne was correlated with an increase in catalase activity.

Speculation: Polyamines are important in abiotic stress signaling

The main role of polyamines was originally assumed to be as direct protective compounds important under stress conditions. Although in some cases a correlation was found between the endogenous polyamine content and stress tolerance, this relationship cannot be generalized. Polyamines should no longer be considered simply as protective molecules, but rather as compounds that are involved in a complex signaling system and have a key role in the regulation of stress tolerance. The major links in polyamine signaling may be H2O2 and NO, which are not only produced in the course of the polyamine metabolism, but also transmit signals that influence gene expression via an increase in the cytoplasmic Ca(2+) level. Polyamines can also influence Ca(2+) influx independently of the H2O2- and/or NO-mediated pathways. Furthermore, these pathways may converge. In addition, several protein kinases have been shown to be influenced at the transcriptional or post-translational level by polyamines. Individual polyamines can be converted into each other in the polyamine cycle. In addition, their metabolism is linked with other hormones or signaling molecules. However, as individual polyamines trigger different transcriptional responses, other mechanisms and the existence of polyamine-responsive elements and the corresponding transacting protein factors are also involved in polyamine-related signaling pathways.

Salicylic acid and photosynthesis: signalling and effects

Salicylic acid (SA) is a well-known signalling molecule playing a role in local and systemic acquired resistance against pathogens as well as in acclimation to certain abiotic stressors. As a stress-related signalling compound, it may directly or indirectly affect various physiological processes, including photosynthesis. The effects of exogenously applied SA on plant physiological processes under optimal environmental conditions are controversial. Several studies suggest that SA may have a positive effect on germination or plant growth in various plant species. However, SA may also act as a stress factor, having a negative influence on various physiological processes. Its mode of action depends greatly on several factors, such as the plant species, the environmental conditions (light, temperature, etc.) and the concentration. Exogenous SA may also alleviate the damaging effects of various stress factors, and this protection may also be manifested as higher photosynthetic capacity. Unfavourable environmental conditions have also been shown to increase the endogenous SA level in plants. Recent results strongly suggest that controlled SA levels are important in plants for optimal photosynthetic performance and for acclimation to changing environmental stimuli. The present review discusses the effects of exogenous and endogenous SA on the photosynthetic processes under optimal and stress conditions.

Synthesis and role of salicylic acid in wheat varieties with different levels of cadmium tolerance

Wheat genotypes with different endogenous SA contents were investigated, in order to reveal how cadmium influences salicylic acid (SA) synthesis, and to find possible relationships between SA and certain protective compounds (members of the antioxidants and the heavy metal detoxification system) and between the SA content and the level of cadmium tolerance. Cadmium exposure induced SA synthesis, especially in the leaves, and it is suggested that the phenyl-propanoid synthesis pathway is responsible for the accumulation of SA observed after cadmium stress. Cadmium influenced the synthesis and activation of protective compounds to varying extents in wheat genotypes with different levels of tolerance; the roots and leaves also responded differently to cadmium stress. Although a direct relationship was not found between the initial SA levels and the degree of cadmium tolerance, the results suggest that the increase in the root SA level during cadmium stress in the Mv varieties could be related with the enhancement of the internal glutathione cycle, thus inducing the antioxidant and metal detoxification systems, which promote Cd stress tolerance in wheat seedlings. The positive correlation between certain SA-related compounds and protective compounds suggests that SA-related signalling may also play a role in the acclimation to heavy metal stress.

Salicylic Acid-Mediated Abiotic Stress Tolerance

Plants are exposed to many environmental stresses, which are further aggravated by the effects of global climate change. So investigations on compounds capable of reducing the stress sensitivity of plants are of great importance. Salicylic acid is a phenolic compound produced to varying extents by a wide range of plant species. Its usefulness in human medicine was recognized much earlier than its role in plants. This endogenous plant growth regulator participates in many physiological and metabolic reactions. It was first demonstrated to play a role in responses to biotic stress. Soon afterwards; however, it became increasingly clear that salicylic acid also plays a role during the plant response to abiotic stresses such as heavy metal toxicity, heat, chilling, drought, UV-light and osmotic stress. Two kinds of evidence have accumulated to support this. First, endogenous salicylic acid levels rise in several species when they are exposed to abiotic stress conditions. Secondly, the application of salicylic acid at suitable concentrations induces stress tolerance in various plant species. The use of mutants and transgenic plants in which the synthesis, accumulation or translocation of salicylic acid is modified could help to clarify its molecular modes of action in physiological processes. Crosstalk with other hormones such as jasmonic acid, ethylene, abscisic acid, gibberellic acid and cytokinin is important part of a finely tuned immune response network. It can be seen that SA exerts an effect at several levels and its effect also depends on several factors, such as the mode of application, the concentration, environmental conditions, plant species and organs, etc. In the present chapter a summary will be given of the relationship between SA and various abiotic stress factors in relation to biotic stress and other plant hormones, followed by a summary of the known physiological and biochemical effects of SA that may explain the change in stress tolerance.

Cadmium-induced changes in the membrane lipid composition of maize plants

The effect of 10, 25 and 50 μM Cd(NO 3 ) 2 on the fatty acid composition was investigated in young maize seedlings ( Zea mays L., hybrid Norma). After 7 days’ exposure to cadmium slight changes were observed in the fatty acid composition, which were more pronounced in the roots than in the leaves. In the leaves cadmium did not affect the lipid composition of the monogalactosyldiacylglycerol (MGDG) or digalactosyldiacylglycerol (DGDG) fractions, while in the phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) fractions there was a decrease in the proportion of hexadecanoic acid (16:0) and an increase in the level of linoleic acid (18:2) and linolenic acid (18:3). The proportion of trans -Δ3-hexadecanoic acid in leaf PG also decreased. In the roots significant changes were observed in all the fractions examined after Cd stress. In the MGDG the level of stearic acid (18:0) and oleic acid (18:1) decreased, but that of 18:2 and 18:3 increased. In the case of PE the amount of 16:0 decreased, while that ...

Changes induced by powdery mildew in the salicylic acid and polyamine contents and the antioxidant enzyme activities of wheat lines

Investigations were made on four wheat (Triticum aestivum L.) lines under greenhouse conditions, in order to reveal the role of stress-protective materials, namely salicylic acid, polyamines and antioxidant enzymes in the level of tolerance to powdery mildew infection caused by Blumeria graminis (DC.) Speer f.sp. tritici Ém. Marchal. The four lines showed different levels of tolerance, assessed on the Saari-Prescott scoring scale: TC26 and TC33 proved to be susceptible and TC9 and TC19 resistant. In most of the lines, infection caused changes in the activities of antioxidant enzymes, especially in the case of guaiacol peroxidase. Four peroxidase isoenzymes, which responded differently to powdery mildew infection could be detected by gel electrophoresis. Infection had only a slight effect on the levels of salicylic acid (free and bound forms) in inoculated plants; while the levels of polyamines, especially spermidine and spermine increased after infection. Correlation analysis was also performed to examine how close a relationship exists between the parameters investigated. It was concluded that salicylic acid, polyamines and antioxidant enzymes have an important role in plant responses and defence mechanisms during this biotic stress and that in some cases there were significant relationships between them. However the levels of these compounds either initially or after pathogen inoculation, could not explain the degree of tolerance to powdery mildew in the four wheat lines investigated.

Salicylic Acid Induction of Flavonoid Biosynthesis Pathways in Wheat Varies by Treatment

Salicylic acid is a promising compound for the reduction of stress sensitivity in plants. Although several biochemical and physiological changes have been described in plants treated with salicylic acid, the mode of action of the various treatments has not yet been clarified. The present work reports a detailed comparative study on the effects of different modes of salicylic acid application at the physiological, metabolomic, and transcriptomic levels. Seed soaking and hydroponic treatments were found to induce various changes in the protective mechanisms of wheat plants. The possible involvement of the flavonoid metabolism in salicylic acid-related stress signaling was also demonstrated. Different salicylic acid treatments were shown to induce different physiological and biochemical processes, with varying responses in the leaves and roots. Hydroponic treatment enhanced the level of oxidative stress, the expression of genes involved in the flavonoid metabolism and the amount of non-enzymatic antioxidant compounds, namely ortho-hydroxycinnamic acid and the flavonol quercetin in the leaves, while it decreased the ortho-hydroxycinnamic acid and flavonol contents and enhanced ascorbate peroxidase activity in the roots. In contrast, seed soaking only elevated the gene expression level of phenylalanine ammonia lyase in the roots and caused a slight increase in the amount of flavonols. These results draw attention to the fact that the effects of exogenous salicylic acid application cannot be generalized in different experimental systems and that the flavonoid metabolism may be an important part of the action mechanisms induced by salicylic acid.

Central role of the flowering repressor ZCCT2 in the redox control of freezing tolerance and the initial development of flower primordia in wheat

BackgroundAs both abiotic stress response and development are under redox control, it was hypothesised that the pharmacological modification of the redox environment would affect the initial development of flower primordia and freezing tolerance in wheat (Triticum aestivum L.).ResultsPharmacologically induced redox changes were monitored in winter (T. ae. ssp. aestivum cv. Cheyenne, Ch) and spring (T. ae. ssp. spelta; Tsp) wheat genotypes grown after germination at 20/17°C for 9 d (chemical treatment: last 3 d), then at 5°C for 21 d (chemical treatment: first 4 d) and subsequently at 20/17°C for 21 d (recovery period). Thiols and their disulphide forms were measured and based on these data reduction potentials were calculated. In the freezing-tolerant Ch the chemical treatments generally increased both the amount of thiol disulphides and the reduction potential after 3 days at 20/17°C. In the freezing-sensitive Tsp a similar effect of the chemicals on these parameters was only observed after the continuation of the treatments for 4 days at 5°C. The applied chemicals slightly decreased root fresh weight and increased freezing tolerance in Ch, whereas they increased shoot fresh weight in Tsp after 4 days at 5°C. As shown after the 3-week recovery at 20/17°C, the initial development of flower primordia was accelerated in Tsp, whereas it was not affected by the treatments in Ch. The chemicals differently affected the expression of ZCCT2 and that of several other genes related to freezing tolerance and initial development of flower primordia in Ch and Tsp after 4 d at 5°C.ConclusionsVarious redox-altering compounds and osmotica had differential effects on glutathione disulphide content and reduction potential, and consequently on the expression of the flowering repressor ZCCT2 in the winter wheat Ch and the spring wheat Tsp. We propose that the higher expression of ZCCT2 in Ch may be associated with activation of genes of cold acclimation and its lower expression in Tsp with the induction of genes accelerating initial development of flower primordia. In addition, ZCCT2 may be involved in the coordinated control of the two processes.

Effects of Freezing on Thermoluminescence in Various Plant Species

The aim of this study was to monitor the effect of sudden frost on the photosynthetic electron transport chain in the leaves of various plant species using the thermoluminescence (TL) technique. A short period of freezing caused a decrease in the afterglow (AG) band in young maize leaves, with a slight upshift in the maximum temperature. The B band induced by far‐red (FR) illumination started to decrease at a significantly lower temperature. The flash‐induced B band also showed a substantial decrease in intensity after short preliminary freezing. In contrast to other species, for which there was always a well‐detectable TL signal even after relatively drastic freezing, there was no TL singal at all in geranium below a threshould temperature. The behavior of the FR‐induced TL curve in cucumber plants was a mixture of that found in wheat or pea, on the one hand, and maize, on the other: the AG band gradually decreased with decreasing temperature and finally totally disappeared, as in maize. The FR‐induced B band showed an upshift after freezing. These results suggest that AG is a normal component of TL bands induced not only by FR, but also by single turnover flash.