Abstract: Salicylic Acid (SA) is an endogenous growth regulator of phenolic nature, which participates in the regulation of physiological processes in plants. It plays an important role in the plant response to adverse environmental conditions such as salinity. Meanwhile, soil salinity is a major constraint to food production because it limits crop yield and restricts use of land previously uncultivated. The SA plays an essential role in preventing oxidative damage in plants by detoxifying super oxide radicals, produced as a result of salinity. A review was highlighted the exogenous application of the lower concentrations of salicylic acid proved to be beneficial in enhancing the photosynthesis growth and various other physiological and biochemical characteristics of plants. Based on the morphology of the plants and parameters investigation it was concluded that SA tolerant plant, made quicker response to abiotic stresses. In the present review, we have focused on the effect of exogenous application of SA ant its protective effects on changing antioxidant enzyme activity for the salt tolerance in plants.
INTRODUCTION
Salicylic Acid (SA) is considered as a hormone-like substance, which plays an important role in regulating a number of physiological processes such as growth, photosynthesis, nitrate metabolism, ethylene production, heat production and flowering (Raskin, 1992; Hayat et al., 2007) and also provide protection against biotic and abiotic stresses such as salinity (Kaya et al., 2002) in plants. A high salinity induces serious metabolic perturbations in plants, as it generates ROS which disturb the cellular redox system in favour of oxidized forms thereby creating an oxidative stress that may damage DNA, inactivate enzymes and cause lipid peroxidation (Smirnoff, 1993). The ameliorative effects of SA have been well documented in inducing salt tolerance in many crops (Aldesuquy et al., 1998; El-Tayeb, 2005; Gunes et al., 2005; Stevens et al., 2006).Salinity is the most serious threats to agriculture and for more important globally (Sahi et al., 2006) water consisted minerals and salt materials are major harmful factors in arid and semi-arid region of worldwide (Joseph et al., 2010). During the last 20 years SA drew the attention of researchers due to its ability to induce Systemic Acquired Resistance (SAR) in plants to different pathogens, which is manifested in the appearance of pathogenesis Related Proteins (PR), while SA is considered to serve as a signal in the induction of expression of these genes (Sakhabutdinova et al., 2003).
| Table 1: | Role of salicylic acid in plants |
At the same time at present considerable interest has been aroused by the ability of SA to produce a protective effect on plants under the action of stress factors of different abiotic nature (Sakhabutdinova et al., 2003). Thus convincing data have been obtained concerning the SA induced increase in the resistance of plants to salinity. Table 1 summarizes that the role of salicylic acid on plants. Still date number of literature indicates through various researches that exogenous application of salicylic acid to the stressed plants can potentially alleviate the toxic effects, generated by salinity. In the present review, we have focused on the effect of salicylic acid on salt tolerance associated with antioxidant enzymes in plants.
Role of Salicylic Acid in Plants under Salt Stress
The SA, an endogenous plant growth regulator has been found to generate
a wide range of metabolic and physiological responses in plants thereby affecting
their growth and development (Hayat et al., 2010).
The role of SA in defense mechanism to alleviate salt stress in plants was studied
(Afzal et al., 2006; Hussein
et al., 2007). The ameliorative effects of SA have been well documented
in inducing salt tolerance in many crops such as bean (Azooz,
2009; El-Tayeb, 2005; Gunes et
al., 2007; Stevens et al., 2006). Exogenous
application of salicylic acid enhanced the photosynthetic rate and also maintained
the stability of membranes, thereby improved the growth of salinity stressed
barley plants (El-Tayeb, 2005).
The damaging effects of salinity were also alleviated by exogenous application of SA in Arabidopsis seedlings (Borsani et al., 2001). Furthermore, foliar spray of SA significantly decreased lipid peroxidation caused by NaCl and improved the plant growth. This alleviation of NaCl toxicity by SA was related to decreases in Na contents, increases in K and Mg contents in shoots and roots and increases in the activities of superoxide dismutase (SOD), Catalase (CAT), Glutathione Peroxidase (GPX) and Dehydroascorbate Reductase (DHAR) and the contents of ascorbate and glutathione (He and Zhu, 2008).
In studies that focus on the application of SA on in vitro salt stressed Hibuscus species have the effect on shoot growth and multiplication, root formation, root elongation, plant survival rate and proline accumulation (Sakhanokho and Kelley, 2009). The SA treated thyme plants had greater shoot and root dry weights, photosynthetic rates, mesophyll efficiency and water use efficiency when exposed to salt stress (Najafian et al., 2009).
Wheat (Triticum aestivum)
Wheat grain is a staple food used world wide. Its production is greatly
affected by salinity. An enhanced tolerance against salinity stress was observed
in wheat seedlings raised from the grains soaked in salicylic acid (Hamada
and Al-Hakimi, 2001). The detrimental effects of high salts on the early
growth of wheat seedlings may be alleviated by treating seeds with the proper
concentration of a suitable hormone such as SA (Shakirova
and Bezrukova, 1997). Wheat seedlings accumulated large amounts of proline
under salinity stress which was further increased when salicylic acid was applied
exogenously, thereby alleviating the deleterious effects of salinity (Shakirova
et al., 2003). Further, the treatment also lowered the level of active
oxygen species and therefore the activities of SOD and Peroxidase (POX) were
also lowered in the roots of young wheat seedlings (Shakirova
et al., 2003). These findings indicate that the activities of these
antioxidant enzymes are directly or indirectly regulated by salicylic acid,
thereby providing protection against salinity stress (Sakhabutdinova
et al., 2003). In addition, SA increased the resistance of wheat
seedlings to salinity.
The soaking of wheat seeds in 0.05 mM SA reduced the damaging effects of salinity on seedlings growth and accelerated the growth processes (Shakirova et al., 2003). The pre-sowing soaking treatment of seeds with SA positively affected the osmotic potential, shoot and root dry mass, K+/Na+ ratio and contents of photosynthetic pigments in wheat seedlings, under both saline and non-saline conditions (Kaydan et al., 2007). In the past Deef (2007) considered that the influence of SA on salt stress tolerance almost needed during the seed germination of wheat. But the SA reduced the damaging effects of salinity on seedlings’ growth and accelerated the growth processes (Mutlu et al., 2009). The mitigation effect of SA to abiotic stresses was investigated through SA application either by seed soaking of wheat genotypes (Al-Hakimi, 2006) or through rooting medium of wheat (Arfan et al., 2007). While, many studies have been reported that SA induced increases in the resistance of wheat to salinity and oxidative stress, respectively (Shakirova and Bezrukova, 1997; Sakhabutdinova et al., 2003; Bhupinder and Usha, 2003; Arfan et al., 2007).
Tomato (Lycopersicon esculuntum)
The tomato is now grown worldwide for its edible fruits, with thousands
of cultivars having been selected with varying fruit types and for optimum growth
in differing growing conditions. Soil salinity affects plant production in many
parts of the world, particularly on irrigated land (Wang
et al., 2003). Taken together these two effects was found that SA
pre-treatment improved the acclimation of tomato to high salinity (Zahra
et al., 2010; Szepesi et al., 2009).
An enhanced tolerance against salinity stress was observed in tomato plants
raised from the seeds soaked in salicylic acid and was presumed to be due to
the enhanced activation of some enzymes viz. aldose reductase and ascorbate
peroxidase and to the accumulation of certain osmolytes such as proline (Szepesi
et al., 2005).
Maize (Zea mays L.)
Maize is widely cultivated throughout the world and a greater weight of
maize is produced each year than any other grain. Maize is considered as a moderately
salt-sensitive plant. The mitigation effect of SA to abiotic stresses was investigated
through SA application by foliar spray of maize (Khodary,
2004). Many studies supported that SA induced increases in the resistance
of maize to salinity and osmotic stress, respectively (Tuna
et al., 2007). Studies have been employed the effects of salicylic
acid on some physiological and biochemical characteristics of maize seedlings
under salt stress were derived (Gunes et al., 2007;
Gautam and Singh, 2009; Purcarea
and Cachita-Casma, 2010).
Strawberry (Fragaria ananassa)
Strawberry is considered as a salinity sensitive species (Saied
et al., 2005) and it has been shown to reduce leaf number, leaf area,
shoot dry weight and number of crowns and low yield under salt stress (Pirlak
and Esitken, 2004). Salt stress negatively affected the growth, chlorophyll
content and mineral uptake of strawberry plants. However, plants treated with
SA often had greater shoot fresh weight, shoot dry weight, root fresh weight
and root dry weight as well as higher chlorophyll content under salt stress
(Karlidag et al., 2009).
Role of Salicylic Acid in Drought and Salt Stressed Plants
Salt stress afflicts plant agriculture in many parts of the world, particularly
irrigated land (Epstein et al., 1980). Compared
to salt stress, the problem of drought is even more pervasive and economically
damaging (Boyer, 1982). Drought and soil salinization
in both dry land and irrigated agricultural settings are the most important
factors limiting modern agricultural production systems (Cushman
and Bohnert, 2000). From the statistical data, the drought and semi-drought
areas in the world amount to one-third continent acreage in the globe and a
half continent acreage in China. In addition, a salinity areas about 3.6 hundred
thousands hectares needs to be reclaimed and utilized in China. Therefore, it
is very important to develop new varieties of crops with strong tolerance to
drought and salt (Shiqing et al., 2005). Application
of exogenous SA enhanced the drought and salt stress resistance of plants (Tari
et al., 2002; Deef, 2007), but the results
were contradictory and depended on the developmental phase of plants (Borsani
et al., 2001) or on the experimental conditions (Nemeth
et al., 2002). The SA has been suggested to be physiologically important
in stress tolerance since exogenous SA brought about plants tolerance to various
abiotic stress including drought and salt (Stevens et
al., 2006).
Relationship of SA with Antioxidant System and its Impact on the Plants
Exposed to Stress
Abiotic stress conditions such as salinity and drought favor the accumulation
of Reactive Oxygen Species (ROS) such as superoxide radicals(O2●-),
hydroxyl radicals (OH-) hydrogen peroxide (H2O2)
and cause oxidative stress (Panda et al., 2003a,
b). The peak level of ROS leads in plants to cause oxidative
damage in biomolecules such as lipids, proteins and nucleic acids, thus alters
the redox homeostasis,(Smirnoff, 1993). The SA was found
to enhance the activities of antioxidant enzymes, CAT, peroxidase (POX) and
superoxide dismutase (SOD), when sprayed exogenously to the salinity stressed
plants of B. juncea (Yusuf et al., 2008).
When applied exogenously at suitable concentrations, SA was found to enhance
the efficiency of an antioxidant system in plants (He et
al., 2002).
Furthermore, the treatment with salicylic acid resulted in temporary reduction of catalase (CAT) activity and increased H2O2 level takes place in plants (Janda et al., 2003) which possibly played a key role of providing the SAR and tolerant capacity against the oxidative stress (Gechev et al., 2002) in plants. It was found that inhibition of catalase by SA plays an essential role in the generation of reactive oxygen species (Nemeth et al., 2002). By increasing H2O2 concentration of the tissues, moderate doses of SA may activate the antioxidative mechanisms. The activities of antioxidant enzymes such as SOD, POX increased by the application of SA but the CAT activity remain unchanged in salt stressed sunflower plants (Noreen et al., 2009). Moreover, SA was effective in increasing the activities of enzymes mainly involved in the antioxidant system in salt stressed tomato plants (Szepesi et al., 2008).
However, the application of bioregulators such as brassinolide (BR) and Salicylic Acid (SA) as seed soaking and foliar spraying alleviate the harmful effect of salt stress and oxidative stress on maize plants (El-Khallal et al., 2009a, b) and the application of SA enhanced the activities of antioxidant enzymes ascorbate peroxidase (APO) and SOD with a concomitant decline in the activity of CAT in maize plants (Krantev et al., 2008).
Salicylic Acid and its Role of Protective Effects in Plants
For the last two decades, SA has received much attention because of its
involvement in plant defense mechanisms under both biotic and abiotic stresses.
The protective effect of SA against abiotic stress factors such as toxic metals
(Strobel and Kuc, 1995) heat stress (Dat
et al., 1998), low temperature (Janda et al.,
1999; Mora-Herrera et al., 2005) oxidative
damage (Kusumi et al., 2006) and biotic stresses
have been demonstrated. Recently, Sakhanokho and Kelley
(2009) recorded that the SA typically showed the salt tolerance under in
vitro conditions in the two botanical medicinal Hibiscus species.
In order to that SA provided particular interest in positive effect of SA on
root formation and growth though some reports on in vitro salt tolerance
studies suggest that rooting and root growth are not only highly affected by
salt but also positively correlate with salt tolerance at the whole plant level
(Martinez et al., 1996; Cano
et al., 1998).
Conclusions and Future Perspectives
Hence, it may be resolved from the survey of literature cited above that
salicylic acid play’s diverse physiological roles in plants and potentially
alleviates the devastating effects generated by salt stress. However, this recently
introduced phytohormone still demands a lot of work to be carried out to elucidate
the exact pathways of its biosynthesis; wheather major or minor, key regulatory
points of biosynthesis, mechanism of action and other specific and collaborative
regulatory roles played by salicylic acid that have remained elusive till date.
The work is also needed on how this plant hormone interacts and being regulated
by the cross-talk in harmony with other established phytohormones and plant
growth regulators working at long range (auxins, cytokinins, gibberellins, ethylene
etc.), short range (NO, jasmonates, brassinosteroids etc.) and very short range
(ROS, H2O2). One could also argue how the regulated doses
of these short range phytohormones mostly produced in-vicinity to biotic infestation
and then transported systemically to play their role during broad range abiotic
stresses. It is also worthwhile to elucidate the role of aforesaid phytohormone
in tissue-specific differentiation and growth of plant parts during growth and
development. Biochemical inhibitors of key enzymes of pathways and mutant study
might incident some light on such aspects. Locating tissue-specific concentrations
during seedling development fusing with reporter genes or radioactive molecules
could pave the way in this concern.
In future, the exogenous application of this phytohormone might act as a powerful tool in enhancing the growth, productivity and also in combating the ill effects generated by various abiotic stresses in plants. The future applications of this plant hormone holds a great promise as a management tool for providing tolerance to our agricultural crops against the aforesaid constrains consequently aiding to accelerate potential crop yield in near future.
ACKNOWLEDGMENT
The authors gratefully acknowledged to our Malankara Catholic College, Mariagiri, Correspondent Fr. Prem Kumar (MSW) given encouragement and support for preparation of this research manuscript. We wish to express our sincere thanks to Mr. K. Suresh and the research scholars in the Interdisciplinary Research centre for updated article collection and all the efficient supports of this review preparation.