INTRODUCTION:

Salicylic acid (SA) is a phenolic compound, containing an aromatic ring with a hydroxyl group or its derivatives which is broadly distributed in plants, with basal levels differing widely among species. Salicylic acid acts as a potential non enzymatic antioxidant which plays an important role in various plant physiological processes (Fariduddin et al., 2003). It influences the taxol production in addition to increase in biomass. Exogenously supplied Salicylic acid was shown to affect large variety of processes in plants like stomatal closure, seed germination, fruit yield and glycolysis. Salicylic acid is water soluble antioxidant which can also regulate plant growth. It also has a role in abiotic stress tolerance such as drought tolerance. Salicylic acid applied on the plant stimulates growth and oil yield by enhancing photosynthesis and nutrient uptake.

It is also involved in activation of the stress induced antioxidant system stimulates flowering in many plants, increases flower life, control ion uptake by roots and stomatal conductivity in monocotyledonous and dicotyledonous plants, including rice, barley, crabgrass and soybean. The amount of SA in the leaves and reproductive organs of angiosperm plants have been found to be approximately 1mg g^-1 fresh weight and SA was detected in pathogen-infected necrotizing inflorescences.

SA has been regulatory signal mediating plant response to various abiotic stresses such as drought, chilling treatment, heavy metal tolerance and heat (Chini et al., 2004). It regulates the synthesis and signalling of other hormones such as jasmonic acid, ethylene and auxin. In addition, SA contributes to maintain cellular redox homeostasis through the regulation of antioxidant enzyme activity (Slaymaker et al., 2002) and induction of the alternative respiratory pathway (Moore et al., 2002).

Fig 1: Structure of Salicylic acid (modified after Vlot 2008)

Biosynthesis of Salicylic acid:

Salicylic acid is a seven carbon containing naturally occuring phenolic compound and act as a signalling molecule which is endogeneously synthesized in the plant. The shikimic acid pathway and malonic acid pathway are the two main pathways which are involved synthesis of SA (Fig. 2). The Shikimic acid pathway is involved in the biosynthesis of most of the phenolic compound. It simply converts simple carbohydrate which is obtained from glycolysis and pentose phosphate pathway into aromatic amino acids including Salicylic precursors, phenyalanine (Hermann et al., 1999). The most common pathway which is used in the biosynthesis of SA is phenyalanine pathway but it is completed by isochorismate pathway (Mustafa et al., 2009). Salicylic acid is produced after a number of reactions with the help of series of enzymes. The hydroxylation of benzoic acid catalyzed by enzymes benzoic acid hydroxylase which synthesizes SA. Benzoic acid is synthesized by cinnamic acid either through β-oxidation of fatty acids or a non oxidative pathway (Mustafa et al., 2009). Cinnamate 4- hydroxylase (CAH) catalyzes the second step in the pathway during the conversion of cinnamic acid to coumaric acid. Cinnmaic acid is actually produced from phenyalanine with the help of enzyme phenylalanine ammonialyase (PAL). Cinnamic acid is hydroxylated to form coumaric acid followed by the oxidation of side chain and further hydroxylated to form Salicylic acid.

Different abiotic stress factors stimulate the production of different enzymes which are involved in SA biosynthesis. Overproduction of the SA through the activity of different enzymes which are involved in its biosynthesis that help the plant to cope up against various envioronmental stresses. These enzymes are key regulators for the proper functioning of SA and are to be activated by different abiotic and biotic stress. In Arabidopsis, isochorismate synthase (ICS) have been found which are involved in the biosynthesis of SA during plants defense process (Wildermuth et al., 2001). It can positively regulate the ICS1 and improve tolerance of the plant towards drought in Arabidopsis (Hunter et al, 2013).

During salt stress, high expression of ICS and C4H enzymes can be correlated to the level of applied SA concentration. The exogeneous application of 1mM SA results in the higher production of ICS and C4H enzymes than 0.1mM SA in Carthamus tinctorius (Dehghan et al., 2014).

Fig 2: Biosynthesis of Salicylic acid (Modified after Catinot 2008).

Salicylic acid regulated physiological functions:

Seed Germination and Crop improvement:

Effect of different factors and interactions between the plant hormones like abscisic acid, jasmonic acid, gibberlins, ethylene and cytokinins regulate the seed germination. The role of SA in seed germination have been controversial as their are certain reports which suggest that it inhibits the seed germination. In Arabidopsis thaliana, SA concentrations greater than 1mM delay or inhibit germination (Rajjou et al., 2006). In barley doses greater than 0.250 mM inhibit seed germination (Xie et al., 2007).

However when the low doses of SA was applied to the plant exogeneously, it significantly improved the seed germination and seed establishment in Arabidopsis under different abiotic stress conditions (Rajjou et al., 2006). Under salt stress only 50% of the seeds of Arabidopsis thaliana germinate, however in the pressence of SA 80% of the seeds germinate. Exogeneous application of SA partially combats the inhibitory effect of heat stress and other abiotic stress in plants ( Alonso et al., 2009). Farridudin et al., (2003) reported that dry matter accumulation was significantly increased in Brassica juncea, when lower concentrations of SA was applied. However higher concentration of SA has an inhibitory effect. Researchers observed significant increase in growth, pigments and photosynthetic rate in maize, sprayed with Salicylic acid.

Photosynthesis:

Salicylic acid is an important regulator of photosynthesis because it affects leaf and chloroplast structure (Uzunova et al., 2000), stomatal closure, chlorophyll and carotenoid contents and the activity of enzymes such as RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase). It was observed in experiments that the exogeneous application of SA on photosynthetic parameters differs, depending on the concentration and the particular plant species tested. Higher SA concentrations (1-5 mM) caused reduction in the photosynthetic rate and RuBisCO activity in barley plants (Pancheva et al., 1996) and reduced chlorophyll contents in cowpea, wheat, and Arabidopsis (Rao et al., 1997). The decline of RuBisCO was attributed to a 50% reduction in protein content as compared to non treated barley plants and there was decrease in total soluble protein content to a 50%. Exogeneous application of SA affects leaf anatomy which includes reduced area of adaxial and abaxial epidermis and of the mesophyll tissue.

However on the application of the lower concentration of the SA (10µM) improves the photosynthetic net CO₂ assimlation in mustard seedlings. The positive effect of SA on the photosynthesis is the protection of the barley seedlings against oxidative stress induced by cadmium (Ananieva et al., 2002). The role of SA in photosynthetic parameters and short term accilmation to high light was reduced from the phenotypes shown by Arabidopsis thaliana plants with contrasting endogeneous SA levels.

Growth:

The effect of exogenous application of SA on growth depends upon the plant species, developmental stages of the plant and the SA concentrations. Growth stimulating effect of SA was reported in many species like Soyabean (Coronada et al., 1996), wheat, maize and chamomile (Gunes et al., 2007. There was increase in the root and shoot growth of plant, when they were treated with different concentrations of SA. Wheat seedlings treated with 50µM SA stimulates enhanced cell division in addition to increase in its height.

In Arabidopsis thaliana, exogenous application of SA has a negative effect on the trichome development, but its application reduces the trichome density and number (Traw et al., 2003).

Flowering:

Salicylic acid has been well known for its ability to induce flowering for long time in plants. The exogenous application of SA promotes flower bud formation from tobacco callus (Lee and Skoog, 1965). Salicylic acid (3-10µM) enhances flowering in long day, short day and photo insensitive plants of Lemnaceae genera. The inflorescense of thermogenic plants have high endogeneous SA levels (Rakin et al., 1990) and in non thermogenic plants like tobacco and Arabidopsis. The level of SA increases 5 and 2 fold in leaves at the initiation of or during the transition of flowering in tobacco and Arabidopsis (Yalpani et al., 1993).

The role of SA and flowering was recently observed in Sunflower. The transcription factor HAHB10 belongs to the family HD-ZIP and when it is constitutively expressed in the plant, induces flowering by upregulating the specific flower transition genes and repressing the biotic stress genes. However the exogenous application of SA did not induce flowering under non stress conditions, proving that SA may be necessary but not sufficient to induce flowering. Different plant species including Ornamental plants like Sinningia speciosa flowered much earlier as compared to untreated one on exogenous application of Salicylic acid (Martin et al., 2003).

Respiration:

Salicylic acid is involved in the regulation of the AOX (Alternative oxidase) pathway in thermogenic and non thermogenic plants by inducing its gene expression (Kapulnik et al., 1992). In tobacco cell suspension culture, addition of 2-20µM SA caused increased cyanide resistance and oxygen uptake capacity within 2 hours. AOX couples with the ubiquinol oxidation followed by the reduction of the molecular oxygen to yield water that is insensitive to the inhibitors of the cytochrome oxidase pathway. AOX reduces the production of ROS (reactive oxygen species) in mitochondria. In cultured tobacco cells, over expression of AOX results in 57% decrease in the ROS (Maxwell et al., 1999).

In addition to the initiation of the respiratory pathway in plants which is dependent on expression of the AOX gene, SA is also involved to control electron transport and oxidative phosphorylation in in plants mitochondria (Norman et al., 2004). Its concentrations less than 20µM inhibit both ATP synthesis and respiratory oxygen uptake within minutes of incubation of tobacco cell cultures. Treatment with 500µM of SA decreases ATP levels by 50% within 30 minutes of incubation.

Stress management:

Biotic and Abiotic stresses are among the most serious threats in agricultural system and affect the economic yield of the crop plants (Iqbal et al., 2014). These stresses cause an adverse effect on plants, affect their physiological and molecular processes and ultimately lead to the death of the plant. Phytohormones such as SA acts as a powerful tool in combating the stress caused by biotic and abiotic factors (Khan, 2013).

Table 1: Responses of Plants in the Presence of SA under Abiotic Stress

Plant Name.	Applied Salicylic acid Conc.	Stress studied	Response	Reference
Glycine max	0.5 mM	Salt stress.	Negative response towards Salicylic acid.	Ardebii et al., 2014.
Vigina radiate	0.5mM	Salt stress.	Positive response towards Salicylic acid.	Khan et al., 2014
Torreya grandis	0.5 mM	Salt stress	Negative response towards Salicylic acid.	Li et al., 2014.
Brassica juncea	0.5 mM	Cadmium stress	Positive response towards Salicylic acid.	Ahmed et al., 2011.
Cucumis melo	0.1 mM	Cadmium stress	Positive response towards Salicylic acid.	Zhang et al., 2015.
Punica granatum	1 and 2 mM	Chilling stress	Positive response towards Salicylic acid	Ghotbi et al.., 2014.
Citrus limon	2 mM	Chilling stress	Positive response towards Salicylic acid.	Siboza et al., 2014.
Musa acuminate	0.5 mM	Chilling stress.	Positive response towards Salicylic acid.	Kang et al., 2007.
Triticum aestivum	0.5 mM	Heat stress	Positive response towards Salicylic acid.	Khan et al., 2013.
Zea mays	0.001 mM	Drought stress	Positive response towards Salicylic acid.	Saruhan et al., 2012.

Table 2: Responses of Plants with the Application of SA under Biotic Stresses

Plant	Salicylic acid dosage	Disease name	Disease consequence	Post Salicylic acid changes	Reference
Oryza sativa	50 mg/kg of rice	Sheath blight (Fungal infection).	Yield loss	Protection from disease and increase in yield.	Sood et al., 2013.
Cucumis sativus	0.25%	Powdery mildew	Leaf loss and rotten fruit.	Significant disease reduction	Alkahtani et al., 2011
Vicia faba	100-1000µM	Rust	Retardation in growth and yield.	Attains rust resistance and increases in yield	Sillero et al., 2012
Prunus persica	0.05 mM	Blue mold	Damage in fruits	Resistance to disease	Chan et al., 2007.
Cicer arietinum	200µM	Wilt	Reduction in production.	Reduction in disease and increase in yield.	Saikia et al., 2003.
Solanum lycopersicum	0.109 µM	Bacterial spot	Yellow necrotic areas and spots, reduced growth	Reduction in disease to a greater extent.	Al – Saleh, 2011
Solanum tubersum	500µM	Potato virus	Loss in yield	Decrease in bacterial growth.	Anand et al., 2008
Brassica oleracea var. Capitata	200µM	Turnip mosaic virus	Regulates the endogeneous accumulation of Salicylic acid.	Resistance to virus.	Peng et al., 2013
Vigna mungo	100µM	Mosaic disease	Loss of yield	Increases chlorophyll, protein, carbohydrate content.	Kundua et al., 2011

Interaction of Salicylic acid with Mineral Nutrients:

Mineral nutrition is a basic requirement for proper growth and development of the plant under different environmental conditions. The mineral nutrient status plays an important role in combating the biotic stress in plants (Iqbal et al., 2011). Salicylic acid can help the plant in taking up the sufficient amount of nutrients which are required for metabolism, growth and development (Alpaslan et al., 2001). In addition the role of Salicylic acid in membrane integrity and nutrient uptake has been reported (Gunes et al., 2007). Salicylic acid is involved in the regulation and uptake of several important nutrients like Mn, Ca, Cu,Fe, and Zn (Wang et al., 2011). Calcium is known as one of the most important nutrient elements for various structural roles under optimum and stressful conditions (White, 2003). Interaction between SA and Ca²⁺ signalling might be involved in defence mechanism induced by stress.

Interaction of Salicylic acid with other hormones:

Hormones are chemical substances that can influence the physiology of plants, even at low concentrations. Hormones play an important role in the growth and flowering of the plant (Aroca et al., 2013). Salicylic acid can regulate various processes of plant under stressful conditions and reacts with other hormones like auxin, cytokinin, gibberellins, abscisic acid (Khan et al., 2012). The reaction of Salicylic acid with other hormones can either be combined effort to combat the stress or it can work against combating the stress in plants. The expression of the PR1, a salicylic acid induced gene plays a very important role in signalling of auxin in stressed plants. Salicylic acid triggered the accumulation of ABA under both normal and salt stress, which in turn helped in the osmotic adaptation and improved the photosynthetic pigments and growth in Solanum lycopersicum (Szepesi et al., 2009). Abscisic acid may alter the SA-related abiotic stress response in plants.

The increased ethylene production under environmental stress condition is referred to as stress ethylene, which induces oxidative stress in the plants (Khan et al., 2015). During stressful conditions, the exogeneous application of Salicylic acid could inhibit the synthesis of ethylene by inhibiting the conversion of 1 aminocyclopropane carboxylic acid to ethylene (Lesli et al., 1986). Exogeneous Salicylic acid was reported to alleviate heat stress by restricting the synthesis of ethylene. Salinity stress induced cell death mainly by ethylene induced ROS-production. In contrast Ghania et al., 2014 suggested that there occurs a combined effort of both Salicylic acid and ethylene to combat the environmental stress, which can cause adverse damage to the plant.

Nitric oxide a major signalling molecule in the plants system plays a very significant role in a wide range of responses to environmental stress (Freschi, 2013). Salicylic acid is a major phytohormone, which interacts with NO and plays a role as a secondary messenger and controls stomatal moments in higher plants (Hao et al., 2010). Nitric oxide can be used to for signalling of Salicylic acid to reduce the oxidative damage in osmotic stressed condition in Triticum aestivum seedlings (Naser et al., 2014). There occurs an antagonistic interaction between Salicylic acid and Jasmonic acid at the level of MAPKs signalling and biosynthesis (Khan et al., 2012). The antagonistic interaction between Salicylic and Jasmonic acid can also influence the expression of PR protein genes, where induction and inhibition of PR genes can be possible with Salicylic acid and Jasmonic acid respectively (Thaler et al., 1999).

Interaction of Salicylic acid with Osmolytes:

To combat the adverse effect of abiotic stress produced ROS, plants have developed certain mechanisms facilitating their tolerance towards osmotic and ionic stress. To maintain the osmotic balance, plants have well developed protective mechanism called as osmoregulation. The process of osmoregulation is carried out by certain osmolytes such as glycine betaine, soluble sugars, amines etc. These compounds do not interfere in plants proper metabolic processes but contribute towards defense system of the plant during stress condition (Misra et al., 2009).

Glycine betaine is considered as one of the effective solute for protection against osmotic stress (Munns, 2005), salt stress (Khan et al., 2011), heat stress (Wang et al., 2010) and metals (Bharwana et al., 2014). The accumulation of GB in stressed plants adjusts cells osmotic balance, stabilizes membrane integrity, protects RuBisCo activity and also detoxifies toxic ions (Ashraf et al., 2007). Salicylic acid can induce GB-accumlation in the range of 0.5-2.5mM during the high levels of salt stress, drought and cold stresses (Ashraf et al., 2007). The induction of GB might have certain effects like activation of protein kinase which is activated during adverse stress condition (Hoyos et al., 2000). Salicylic acid increases the level of effective solute i.e; Glycine betaine, which can improve the plant growth. The increase in GB content increases the biomass of the Rauwolfia serpentia (Khan et al., 2014). Khan et al (2014) shown that the certain change in salinity inhibited photosynthesis and growth by Salicylic acid involves GB in V. radiata.

The accumulation of another major osmolyte Proline is one of the major adaptive mechanisms that plants operate during salt stress condition. Proline detoxifies excess ROS, adjusts different metabolic processes within the plant (Iqbal et al., 2014). Researchers have found that SA is involved in increasing the metabolism of plant during abiotic stress (Khan et al., 2013). Exogeneous application of Salicylic acid (at 0.5mM) increases the biosynthesis of certain enzymes such as pyrroline-5-carboxylate reductase and glutamyl kinase under salinity stress along with increased pro content. Salicylic acid treatment (0.5mM) modified the heat stress in Triticum aestivum by increasing pro production (Khan et al., 2013). Accumulation of soluble sugars and sugar alcohols mannitol are reported to combat the abiotic stress within the plant and acts as an osmoprotectants (Murakeozy et al., 2003). Improved plant length can be achieved with increased levels of polysaachrides and soluble sugars (Yuan et al., 2014).

Interaction of Salicylic acid with Antioxidants:

The generation and removal of Reactive Oxygen Species like O^2-, H₂O_2, and OH are normal during the aerobic conditions in the plant. Reactive Oxygen Species in minimal levels play an important role in signal transduction. However during the production of high level of ROS, may lead to the oxidative stress in the plants (Anjum et al., 2012). The abiotic stresses cause the imbalance between generation and removal of ROS and ultimately lead to the disturbed physiological conditions within the plant called as Oxidative stress. There occur a large number of adverse consequences during the oxidative stress within the plant like cell death, arrest of the plant growth and development (Gill et al., 2010). Apoplastic- ROS have been found as a regulator of cell death through their role in signal transduction including SA-mediated signaling pathway (Overmyer et al., 2003). Both endogeneous and exogeneous Salicylic acid play an important role in antioxidant metabolism and have tight control over ROS (Kang et al., 2014). Salicylic acid acts as a signal for the development of the acquired resistance (Shirasu et al., 1997) and induces the activation of a protein kinase. Arabidopsis thaliana were able to recognize the ROS-SA interaction through an antagonistic action of Salicylic acid (Brosche et al., 2014). Salicylic acid plays an important role in the regulation of defense response and also involved in the regulation of light acclimation process and thereby increasing photosynthesis (Mullineaux et al., 2002).

Involvement of Salicylic acid in the modulation of antioxidant metabolism has been widely reported to control plants tolerance to major abiotic stresses including ozone, UV-B, heat, and osmotic stress (Wang et al., 2010). Salicylic acid pre treatment was found to mitigate the adverse effects of salinity stress on photosynthesis and growth in V.radiata through increasing the activity of antioxidant enzymes including SOD, CAT, GPX, APX, and GR (Khan et al., 2014). Activities of H₂O₂ metabolizing enzymes such as CAT, POD, and APX and superoxide-dismutating enzymes like SOD were also inflated with Salicylic acid in plants exposed to drought (Saruhan et al., 2012). Salicylic acid application increased the activity of enzymes involved in AsA-GSH pathway increased the tolerance of Brassica juncea to salinity stress (Nazar et al., 2015). AsA and GSH act as redox active compounds which play an important role in protection against biotic and abiotic stresses (Anjum et al., 2010). In Cd exposed Triticum aestivum varieties, Salicylic acid signaling was correlated with GSH related mechanisms (Kovacs et al., 2014). Salicylic acid improves salt tolerance in Triticum aestivum by increasing the level of AsA and GSH (Li et al., 2013). Salicylic acid mediated induction in SOD and GSH- based H₂O₂metabolizing enzymes namely GPX and GST was found to improve the ozone tolerance in plants (Milla et al., 2003).

CONCLUSION:

Salicylic acid is a secondary plant product which performs an important function in growth and development. It is a potent signalling molecule in plants which is used to combat adverse biotic and abiotic stresses. Application of the SA has been shown to be beneficial for plants in normal and stress conditions. SA regulates various metabolic processes and also maintains plants nutrient status even during the stress.

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Received on 15.05.2018 Modified on 18.06.2018

Research J. Pharm. and Tech 2018; 11(7): 3171-3177.

DOI: 10.5958/0974-360X.2018.00583.8