Orsolya Kinga Gondor

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.

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.

Salicylic Acid and Sodium Salicylate Alleviate Cadmium Toxicity to Different Extents in Maize (Zea mays L.).

The role of salicylic acid in Cd tolerance has attracted more attention recently but no information is available on the efficiency of different forms of salicylic acid. The aim was thus to investigate whether both the acid and salt forms of salicylic acid provide protection against Cd stress and to compare their mode of action. Young maize plants were grown under controlled environmental conditions. One group of 10-day-old seedlings were treated with 0.5 mM SA or NaSA for 1 day then half of the pants were treated with 0.5 mM Cd for 1 day. Another group of seedlings was treated with 0.5 mM CdSO4 for 1 day without pre-treatment with SA or NaSA, while a third group was treated simultaneously with Cd and either SA or NaSA. Both salicylic acid forms reduced the Cd accumulation in the roots. Treatment with the acidic form meliorated the Cd accumulation in the leaves, while Na-salicylate increased the phytochelatin level in the roots and the amount of salicylic acid in the leaves. Furthermore, increased antioxidant enzyme activity was mainly induced by the acid form, while glutathione-related redox changes were influenced mostly by the salt form. The acidic and salt forms of salicylic acid affected the two antioxidant systems in different ways, and the influence of these two forms on the distribution and detoxification of Cd also differed. The present results also draw attention to the fact that generalisations about the stress protective mechanisms induced by salicylic acid are misleading since different forms of SA may exert different effects on the plants via separate mechanisms.

Impact of UV-B on drought- or cadmium-induced changes in the fatty acid composition of membrane lipid fractions in wheat

UV-B radiation may have either a positive or negative impact under the same conditions in wheat, depending on the type of secondary abiotic stressor: Cd or drought. Supplemental UV-B prevented the wilting and leaf rolling induced by PEG treatment. In contrast, combined UV-B and Cd treatment resulted in pronounced oxidative stress. The opposite effect of UV-B radiation in the case of drought or cadmium stress may be related to the alteration induced in the fatty acid composition. UV-B caused changes in the unsaturation of leaf phosphatidylglycerol fractions, and the accumulation of flavonoid in the leaves may prevent the stress induced by subsequent drought treatment. However it resulted in pronounced injury despite the increased flavonoid content in roots exposed to Cd. This was manifested in a drastic decrease in the unsaturation of the leaf monogalactosyldiacylglycerol and the root phosphatidylglycerol and digalactosyldiacylglycerol fractions. Data on the flavonoid content and fatty acid composition showed that oxidative stress was induced by drought in the leaves, by Cd in the roots, and interestingly, by UV-B radiation in both the leaves and roots. The additive effect of the combined stresses was also detected in the roots. The results presented here suggest a relationship between the capacity of the plant to remodel the fatty acid composition and its resistance to various stress factors.

Janus-faced nature of light in the cold acclimation processes of maize

Exposure of plants to low temperature in the light may induce photoinhibitory stress symptoms, including oxidative damage. However, it is also known that light is a critical factor for the development of frost hardiness in cold tolerant plants. In the present work the effects of light during the cold acclimation period were studied in chilling-sensitive maize plants. Before exposure to chilling temperature at 5°C, plants were cold acclimated at non-lethal temperature (15°C) under different light conditions. Although exposure to relatively high light intensities during cold acclimation caused various stress symptoms, it also enhanced the effectiveness of acclimation processes to a subsequent severe cold stress. It seems that the photoinhibition induced by low temperature is a necessary evil for cold acclimation processes in plants. Greater accumulations of soluble sugars were also detected during hardening at relatively high light intensity. Certain stress responses were light-dependent not only in the leaves, but also in the roots. The comparison of the gene expression profiles based on a microarray study demonstrated that the light intensity is at least as important a factor as the temperature during the cold acclimation period. Differentially expressed genes were mainly involved in most of assimilation and metabolic pathways, namely photosynthetic light capture via the modification of chlorophyll biosynthesis and the dark reactions, carboxylic acid metabolism, cellular amino acid, porphyrin or glutathione metabolic processes, ribosome biogenesis and translation. Results revealed complex regulation mechanisms and interactions between cold and light signalling processes.