Role of PGR’s in Salinity Tolerance
Salinity is a major threat to Agriculture productivity in many parts around the world. FAO estimated that 19.5 % of the globally irrigated land is salt effected. Salinity is actually the overtime accumulation of salts in the soil due to low precipitation or by use of saline irrigation water. There are different types of salts present in the soil such as Sodium, Chlorine, Boron, Selenium etcetera affecting its productivity. Soils can be classified into sodic, saline or saline-sodic salts on the basis of the type and amount of the salts present (FAO, 2018). Salinity can affect many crops at any growth stage to cause a considerable loss in the yield.
This yield loss is induced by various physiological and biochemical malfunctions caused by the salt stress such as photosynthesis and carbohydrate metabolism (Pakar et al., 2016). Roots of the plants have cells with selectively permeable membrane which only allow water to pass through avoiding the salts but when there are high levels of salts, even water could not penetrate the membrane instead it lose water and salts enter the root due to osmotic pressure gradient. Salt leads to stunted growth and development of brown leaf tips and margins which is due to the accumulation of salts.
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Salinity causes poor germination of seeds and reduced seed vigor. Effect of salinity stress is dependent on specie of plant, crop duration and growth stage of the plant (Ali et al., 2004). Salts impose two types of restrictions on the plants namely hyper-osmotic and hyper-ionic. Hyper-osmotic is due to the lower soil water potential which leave the plant water deficit and the hyper-ionic stress is due to the various ions which cause toxicity and antagonism (Neumann, 1997). When plants are affected by salinity various reactive oxygen species are produced by cell metabolic reactions such as superoxide, hydroxyl radicles, hydrogen peroxide and singlet oxygen. These free radicles destroy the cell membranes by lipid peroxidation, does protein, photosynthesis and nucleic acids denaturation(Lin and Kao, 2000).
Role of PGR’s
Plants use different types of defense mechanisms to survive the salt stress such as accumulation of osmolytes such as proline, sugars etcetera and production of antioxidants such as catalase, peroxidase, superoxide dismutase and ascorbate peroxidase(Parida and Das, 2005). These antioxidants scavenge the reactive oxygen species and protect cell from their damage. Antioxidants can also be regulated by plant growth regulators. (Li et al., 1998) conducted a study on the effect of plant growth regulators on antioxidants in seedlings of maize cultivars under water stress.
They found that maize seedlings treated with three different PGR’s namely brassinolide, methyl jasmonale and uniconazale resulted in the increased antioxidant enzyme activities. They found that seedlings treated with PGR’s showed more resistance to water stress than the control. So, in the recent years use of exogenous plant growth regulators have been progressed for dealing with stress physiology to have quick results instead of long term inbreeding programs. Different plant growth regulators have been studied for their role in salinity tolerance such as Salicylic acid, jasmonates and polyamines etcetera.
ABA is a key hormone that help plant to cope with adverse environmental conditions such as drought or salt stress. When plant is under salt stress, levels of endogenous ABA increase rapidly due to activation of ABA biosynthesis genes such as ABA- aldehyde oxidase, zeaxanthin oxidase and 9-cis- epoxy carotenoid dioxygenase etcetera (Chinnusamy et al., 2006). ABA production in plant induce the stomata to close and lead to accumulation of proteins and osmoprotectants during osmotic stress created by salts. But when there is very high salinity inducing larger amounts of ABA production in the plant can also lead to growth defects.
Cytokinins have been studied extensively for their various roles in plant growth and development such as cell division, vascular differentiation, apical dominance and leaf senescence etcetera. Role of cytokinins in increasing abiotic stress tolerance such as temperature and salinity has also been reported (Javid et al., 2011). Cytokinins are produced in the roots and translocated to the shoots where they control plant growth and development. Salt stress effects the production of cytokinins in the roots and ultimately effects the growth and development of plant.
Exogenous application of cytokinins such as kinetin can help breaking the seed dormancy caused by salt stress in tomato, barley and cotton (Javid et al., 2011). Therefore, effect of kinetin denotes that it may be direct scavenger of free radicles or is involved in antioxidant mechanism to protect the breakdown of purines (Javid et al., 2011). But there is a cross talk that cytokinins are antagonist to the ABA because it delays stomatal closure and senescence which denotes its negative role in the stress tolerance. Therefore, more detailed studies are needed to be carried out to better understand the role of cytokinins on salt stress.
Salicylic acid acts a signaling molecule in the abiotic stresses and controls the defense mechanisms of the plant. SA have been found to reduce the salinity induced oxidative stress in Arabidopsis, tomato and mung bean. There are also certain compounds derived from salicylic acid such as S-methymethionine (SMM). SMM has an important role in plant metabolism because it is involved in methylation and methionine production.
Therefore, exogenous application of SMM enhance the production of protective compounds against the environmental stress (Poór et al., 2011). SA application can alter the activities of antioxidants enzymes per-oxidase and superoxide dismutase which help plant tolerate the environmental stresses (Anosheh et al., 2012). Salicylic acid has also shown a promising role of PGR’s in reducing the salt stress by improvement of growth rate in salt effected pistachio seedlings (Bastam et al., 2013).
There are different polyamines such as spermine, putrescine and spermidine. Production of Polyamines may have an integral part in response to salt stress(Das et al., 1995). Polyamines might save the plants from salinity stress by hunting free radicals, maintaining membrane and cellular stability, balancing anions and cations, passive ion channels and production of energy in the form of ATP controlling the active ion channels (Kamiab et al., 2014). Janicka-Russak et al. (2010) confirmed the role free polyamines in the osmotic adjustment and cell metabolic reactions in the tissues of salt stressed plants. There are also some studies which are focused on external application of polyamines to remove salt stress from plants (Kamiab et al., 2014).
Jamonates and its relative compounds are lipid-based plant hormones which have role in various plant functions ranging from growth to photosynthesis and reproductive systems. They are also important components of signaling pathways which activates the defense genes in response to salt stress (Tsonev et al., 1998). Levels of Jasmonic acid were reported to be increased in the barley when under osmotic stress. Barley plants receiving the treatment of Jasmonic acid before the salt stress actually showed less inhibitory effects on the growth (Tsonev et al., 1998).
CCC is another PGR called chlormequat chloride which is an organic chloride comprising equal chlormequat and chloride ions. Studies have shown that application of CCC has resulted in the improvement of plant performance. It functions as increase in root growth, water potential and stomatal resistance which actually improve plants water use efficiency. In some studies, CCC have resulted in reduced plant height and more number of tillers and grains per plant. It also improved the resistance to salinity, cold, insects and fungi (Pakar et al., 2016).
Ahmadi et al. (2018) conducted a study to see the effect of methyl jasmonate on physiology and biochemistry of rapeseed under different levels of salinity. Plants which received only salt treatments exhibited a decrease in photosynthesis, growth parameters and increase in respiration, sugar content, proline content and antioxidant activities. Then exogenous application of methyl jasmonate mitigated all the inhibitory effects discussed earlier at all salinity levels. It also increased the relative water content, photosynthesis rate and soluble sugar content which actually counteracted the inhibitory effects of Salts (Ryu and Cho, 2015).
PGR’s in trees (Pistachios)
Rahemi and Heidari (2002) conducted a study to see effects of Plant growth regulators on pistachios under salinity stress. They used three different rootstocks and exposed them to various levels of salinity and three different PGR’s were used namely gibberellic acid (GA3), benzyl adenine (BA) and cycocel. They found that GA3 increased growth and BA increased the shoot dry weight in all three rootstocks.
Cycocel had no effect on the growth but chemical composition of the rootstocks treated with cycocel were the same as in other rootstocks treated with GA3 and BA. Chemical composition here refers to the accumulation of antioxidants which avoid the photo damage such as flavonols in this study. Quercertin was the flavonol which was found to be increased in response to the exogenous application of PGR’s. Different PGR concentrations and the salinity levels affected the growth and chemical compositions of the rootstocks differently.
Plants have developed many mechanisms to adapt to the adverse environmental conditions. As, we discussed above plant hormones are the major pathways that have been adopted by plants to cope with various stresses. As we studied the physiology that plant hormones change the chemical composition of the plants in terms of eradicating the free radicles which may damage the photosystems when plant is under stress. This is a great tool to deal with environmental stress mainly salinity as we have discussed above there are wide range of plant growth regulators which have a role in salinity tolerance.
But still there are lot of molecular puzzles and signaling pathways which need to be studied further to have their better understanding and exploiting them to deal with the problem of salinity. There are several ways by which plant deals with salt stress such as exclusion of salts, accumulation of salts, secretion of salts and allocation of salts etcetera. But by use of Plant growth regulators we can only deal with few mechanisms with the present-day technology and research. So, I would also recommend exploiting the breeding and genetics techniques for incorporating salinity tolerance mechanisms in plants long with use of Plant growth regulators.