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Salicylic acid (SA) is a phenolic derivative, distributed in a wide range of plant species. It is a natural product of phenylpropanoid metabolism. Decarboxylation of transcinnamic acid to benzoic acid and its subsequent 2-hydroxylation results to SA. It undergoes metabolism by conjugating with glucose to SA glucoside and an ester. SA has direct involvement in plant growth, thermogenesis, flower induction and uptake of ions. It affects ethylene biosynthesis, stomatal movement and also reverses the effects of ABA on leaf abscission. Enhancement of the level of chlorophyll and carotenoid pigments, photosynthetic rate and modifying the activity of some of the important enzymes are other roles assigned to SA. This chapter gives a comprehensive coverage to all the above aspects.

Salicylic acid is a plant growth regulator that increases plant bioproductivity. Experiments carried out with ornamental or horticultural plants in greenhouse conditions or in the open have clearly demonstrated that they respond to this compound. Moreover, lower quantities of SA are needed to establish positive responses in the plants. The effect on ornamental plants is expressed as the increase in plant size, the number of flowers, leaf area and the early appearance of flowers. In horticultural species, the effect reported is the increase of yield without affecting the quality of the fruits. It is proposed that the increase in bioproductivity is mainly due to the positive effect of SA on root length and its density.

The SA action on the membrane transport is its least studied physiological property. The changes in compound fluxes between the cell and the environment are, however, one of the early responses to SA treatment. Even low concentrations of SA retard potassium influx and increase that of calcium and alters proton influxes. These ion transport changes are related to the plasmalemma depolarization resulting from the loss of membrane selectivity and the activity of electrogenic pump. The data arguing for the SA-induced intercellular transport changes are also reviewed. One reason for these changes may be the reduction of plasmodesmata conductance resulting from rapid and short-lived callose deposition around the neck regions, the narrowest point of plasmodesmata. The possibility of SA influencing the callose synthase and the β-1,3-glucanase activities is discussed. The loss of plasmodesmata conductance may influence the messengers transport or the pathogens spread. The isolation of an infected cell, brought about by callose deposition is one of the earliest plant defense reaction followed by the initiation of some other defense mechanisms.

Salicylic acid (SA) is an endogenous plant growth regulator. When applied to wheat plants in concentration similar to that used in case of exogenous hormones (0.05 mM), SA causes growth promoting and protective effects against an abiotic stresses. SA was shown to cause changes in hormonal system associated with transitory parallel accumulation of IAA and ABA with no change in cytokinins, which took place in case of treatment of seeds before sowing as well as seedling treatment. SA-induced accumulation of ABA lead to no detrimental effects, evidenced by clearcut stimulation of growth of root cells both by division and expansion, accumulation of raw and dry mass of seedlings and productivity of wheat treated with SA. This indicated an important role to IAA in the expression of growth stimulating action of SA. ABA is likely to be intermediator in manifestation of antistress action of SA. This is evidenced by the data showing that SA-induced accumulation of ABA was followed by enhanced expression of genes of dehydrins and accumulation of proline, i.e. substances having a relation with osmoprotection of cells. Moreover, SA causes activation of superoxide dismutase and peroxidase, including anionic peroxidase, phenylalanin-ammonia-lyase, favouring accelerated lignification of cell walls of seedlings roots. This is likely to contribute to a decline in the extent of injurious effects of salinity and water deficit on plants, pretreated with SA, evidenced by a decline in the level of lipid peroxidation and leakage of electrolytes from plant tissues as well as by more intensive growth processes as compared to control plants. It is important to underline that pretreatment with SA prevents a sharp decline in IAA and cytokinin content observed under stress and maintains a high level of ABA. Such a character of SA effect on the state of hormonal system may well contribute to protective reactions of plants and acceleration of reparative processes during a post-stress period.

Investigations on compounds capable of reducing the stress sensitivity of crops are of great importance from both the theoretical and the practical point of view. In terms of stress physiology, salicylic acid was first demonstrated to play a role in responses to biotic stress. However, it was gradually found to have more and more effects that could be of importance for other stress factors, and a great deal of evidence has accumulated in recent years suggesting that salicylic acid also plays a role in responses to abiotic stress effects (such as low and high temperature, UV-B irradiation, ozone, heavy metals, etc.). Most papers, on this subject, have reported on the protective effect of exogenous salicylic acid against abiotic stress. When applied in satisfactory concentrations salicylic acid may cause a temporary low level of oxidative stress in plants, which acts as a hardening process, improving the antioxidative capacity of the plants and helping to induce the synthesis of protective compounds such as polyamines. Numerous mutant or transgenic plants are now available in which the salicylic acid metabolism has been modified in some way. These allow us to obtain a more accurate picture of the endogenous effect and role of salicylic acid. Evidence now suggests the existence of a regulatory defence mechanism in which salicylic acid plays an important role, but which is not stress-specific, apparently functioning against many different stress factors. This chapter provides a review of the effects exerted by salicylic acid and related compounds in relation to abiotic stress tolerance.

Salicylic acid (SA) is an endogenous plant growth regulator. SA is involved in various physiological processes of plant growth and development and plays an active role in plant defense responses. SA also plays a major role during the early stages of -legume symbiosis. Nod factors produced by rhizobia, in response to legume produced flavonoids, affect SA content of the host plant during the early stages of nodulation. On the other hand, SA inhibits bacterial growth and the production of Nod factors by rhizobia. Exogenous application of SA delays nodule formation and decreases the number of nodules at the roots of the host plant. SA protects plants under abiotic stresses such as drought, salinity, low and high temperatures, and the damaging action of heavy metals. The ability of SA to protect plants exposed to abiotic stresses is due to the induction of a series of signal transduction cascades leading to the expression of genes responsible for the protection of plants from the stress.

Our own research has found a number of potentially useful effects of medium supplementation with salicylate on potato microplants. These useful effects are obtained taking advantage of the stress and antistress effects of salicylic acid on plants. Growth inhibition is a common stress effect of salicylic acid on plants. This stress effect can be directed to culture technology, including promotion of tuberization and growth retardation during germplasm preservation. Antistress effects of salicylates can also be used in a planned manner to improve culture technology and hardening in potato with different applications like induction of thermotolerance during thermotherapy for virus elimination, organogenesis for micropropagation, and induction of tolerance to freezing and heat in microplants after transplanting to soil, in glasshouse trials. Tolerance to late blight () in potato has also been observed in field. We have also induced some of these effects in microplants by treatment with HO which is consistent with evidence associating salicylate and HO as endogenous signaling molecules. Stress and antistress effects appeared to be mediated by some antioxidant enzymes especially catalase, and by HO accumulation. The use of salicylates would have agricultural relevance to culture technology and field crops.

Activation of salicylic acid (SA) biosynthesis in association with changes in redox homeostasis occurs in plants exposed to diverse biotic and abiotic stresses such as pathogens infection, excess of UV radiation, or increased levels of ozone (O). Under these conditions, reactive oxygen species (ROS) and SA are the crucial signals for triggering defense-related processes that are genetically controlled, e.g. programmed cell death (PCD) and the expression of genes that cause defense against stress. Increasing evidence in the yesteryears supports the idea that SA interplays with ROS in the genetic-controlled defense reactions. In this chapter we discuss this evidence, particularly focusing on the expression of stress defense genes. In the first section we are giving an overview about how the changes in SA levels and redox homeostasis occur in the establishment of the defense reaction against stressful conditions. In the second section we will review the information obtained from genetic and biochemical approaches about signaling proteins and promoter DNA elements, involved in the activation of defense genes by SA. Redox controlled transcriptional co-regulators, transcription factors and promoter DNA elements have been shown to mediate SA induced activation of these genes. In the third section we are going to analyze available transcriptome data obtained from Arabidopsis plants, either treated with SA or analogs or subjected to stress conditions. We have classified the up-regulated genes according to their known or putative functions. Interestingly, we found genes coding for proteins with antioxidant and detoxifying functions, together with other defense-related functions. Taking together, these evidences suggest that SA plays a role in controlling the cellular redox balance at the onset of the defense response.

There is increasing interest in the interactive role between salicylic acid (SA), reactive oxygen species (ROS) and other plant signalling molecules in regulating cell death in plants. Initial evidence suggested that SA was a potent inhibitor of heme-containing enzymes such as catalase and ascorbate peroxidase, thus capable of stimulating ROS accumulation during various biotic and abiotic stress conditions. However, others suggested that the mode of action of SA may in fact be related to its ability to prime the defense response, by increasing the levels of various defense compounds. SA was also proposed as both a potent inducer of the NADPH-oxidase and an inhibitor of the alternative oxidase, thus capable of indirect regulation of the redox status of plant cells. This role in regulating the redox status has been linked to the programmed cell death (PCD) typically observed during the hypersensitive response (HR) but also during development (leaf laces, tracheary elements, root cap, germinationl) and some abiotic stress responses (salt and heavy metal stress, anoxia). Today, an interplay between SA, ROS and other signalling molecules is proposed in the regulation of PCD in plants. The present chapter reviews the evidence that has accumulated on the interactive nature of the relationship between ROS and SA and addresses this love-hate relationship in view of cell death in plants.

Salicylic acid (SA) is a natural signaling molecule involved in plant defense response against pathogen infection. This chapter covers the recent progress in our understanding of the SA biology in plants, especially the signaling pathways and mechanisms by which SA performs its role as defense inducer are highlighted. The topics related to SA signal transduction covered here include (1) general biological roles played by SA; (2) biosynthesis, storage and translocation of SA; (3) oxidative SA metabolisms regulating the SA actions; (4) roles of reactive oxygen species and calcium ion in SA signaling paths; (5) the link between oxidative burst and other signaling paths; and (6) regulation of gene expression. Lastly, we illustrated the key signaling networks that coordinately lead to both early and late phases of SA-induced gene expression.