Role of Compatible Solutes in Alleviating Effect of Abiotic Stress in Plants Role of Compatible Solutes in Alleviating Effect of Abiotic Stress in Plants

: Plants face assorted of abiotic stresses such as, salinity, drought and heavy metals which produce ROS, and finally inhibit normal growth plant production. To stop cellular destruction due to oxidative stress, these abiotic stresses increase complex reactions in plants to avoid damage and boost their sustainability under severe stress situations. Plants produce several organic solutes known as osmoprotectant such as, polyamines, sugars, proline and glycine betaine (GB), to adjust the cellular mechanism and stable the membrane structure and proteins towards environmental stress. As well, they also defend the plant cells from oxidative stress by stopping the accumulation of damaging effect of ROS. In this review, we have deliberated the mechanisms of organic solutes as well as several functions in plants under abiotic stress situations. The organic solutes that are also known as osmolytes/osmoprotectants comprise soluble sugars, proline and glycine betaine.


1.
Introduction Plants face a variation of environmental stresses including, drought, heavy metal, heat salinity, light, cold and pesticide that impedes their cellular and physiological functioning . Environmental stresses are deliberated as the main hazard to worldwide agronomy systems (Jabeen et al., 2019;Khan et al., 2015). Additionally, activities of anthropogenic also controlled to degradation of agricultural environment and plant yield caused by increased metal, ozone and drought stress (Sharma et al., 2019). These environmental strains cause enormous crop damage by decreasing yield by maximum 50% in various plant crops recognized to the increased reactive oxygen species (ROS) that induce oxidative stress in plants (Shafi et al., 2009). Environmental stresses decrease plant growth and development by affecting various physio organic mechanisms for example, hormonal indicating, antioxidant systems and photosynthesis (Jabeen et al., 2019; Saed-Moucheshi; 2019). These abiotic stresses induce

Role of organic solutes to confer abiotic stress resistance
Organic solutes are osmolytes to maintain cell integrity and increase cell potential without hampering the regular uptake. The major purpose of osmoprotectant is to adjust the osmotic potential. These compatible solutes aid the plants to tolerate severe stress throughout their life cycle (Sing et al., 2015). These organic metabolites alleviate the osmotic changes involving cells surrounds and cytoplasm (Ozturk et al., 2021). As well, they keep plants from injury through stopping the accumulation of ROS (Laxa et al., 2019). Compatible solutes are neutral compounds that protection the membranes and also proteins against several diseases and stress aspects on cellular function (Ozturk et al., 2021). These organic solutes comprise glycinebetain, proline, alanine betaine, sucrose, trehalose and polyols. These osmolytes protect the metabolism of the plant and is the central approach approved by vegetation to response strains. Several studies have proposed the influence of organic solutes in salinity Wang et al., 2004), heavy metal (Sharma & Dietz, 2006)

Drought stress
The shrinkage in water pressure of plants is very important signs of water scarcity in crop plant. It disturbs the plant physio-logical mechanism such as, photosynthesis and roots plant loss (Laxa et al., 2019). Din et al. (2011) detected a severe decline in chlorophyll content in plants upon response to water pressure, which aspects to the influence on enzymes activity connected to photosynthetic content (Ashraf & Karim, 1991). Plants must establish several mechanisms to lessen water pressure, one of them being production of compatible solutes or osmolytes in the plants in reaction to scarcity. A variety of dynamic organic solutes like pyrols, proline, sugar, glycinebetain, and organic metabolites get stored to stable the water potential in the shortage of water. Because of accumulation of organic molecules in tissue

Role of Compatible Solutes in Alleviating Effect of Abiotic Stress in Plants
[ 143 ] under drought pressure, the cell osmotic pressure becomes very harmful, which sources water endos-mosis into the cell tissue and stable the cell water potential. The osmotic adjustment is probable caused by the production of organic solutes. Likewise, proline osmolytes is the most significant under abiotic stresses included drought, salinity . Alexieva et al. (2001) detected increased the level of proline in pea plant caused by scarcity. Also, Yamada et al. (2005) stated that production of proline in variety of Petunia hybrid under severe condition of stress. Various studies have proposed the part of proline in tolerance in case of water stress (Borgo et al., 2015). More accumulation of glycinebetain has been noticed in cotton plants improved to water stress Iqbal et al., 2005). Plants can endure to water stress mostly by osmotic change and stabilizing the antioxidant defense system that supports in molecules scavenging reactive oxygen species and provided that strength to the cell membranes organelles.

Salt stress
Salinity stress disturbs more than (1/3) of the land-living frame on World (Rajasheker et al., 2019). It is an ecological restraint that has two main elements: An ionic element and osmotic elements which are related to higher production of ions and decreased the outer osmotic potential of soil that positions hazard at greater absorption. Salinity stress reasons lipid breakdown, disturbs nutrient imbalance, water potential interrupts the activity of several enzymes, and produces reactive oxygen species that ultimately destruction of the photosynthetic machinery (Tang & Luo, 2018). Plants tolerate stress by storing low molecular weight organic solutes like polyamines, GB, and proline that helps to sustain the veracity of the membrane. These osmolytes boosts the germination rate, development, and growth thus inducing tolerance in reaction to salinity stress (Elhakem, 2020). Several studies proposed that salinity stress lessens the enzymes accountable in biosynthesis of glycinebetain and proline (Sumithra et al., 2006;Ahmed et al., 2019) and their higher accumulation is related with more tolerance of stress. Proline accumulation is an adaptive mechanism towards salt stress (Elhakem, 2020). Proline adjusts stress reactive proteins, causing in perfection of plant adjustment against abiotic stress. Moreover, peroxidase and catalase enzymes activities were showed to be greater in Pancratium maritimum in proline under salinity stress (Khedr et al., 2003). It improved the enzymes that play an important role in antioxidative potential as known in Nicotiana tobacum (Hoque et al., 2008). Glycinebeyaine is an organic molecule that defends the plants towards disease and abiotic stress . . It also protects the plant by alleviating proteins such as, RuBisCo and decreasing the production of ROS (Laxa et al., 2019). Lutts, (2000) detected that glycinebetain accumulation enhanced the endure capability and development of salted plants.

Temperature stress
Plants are exposed to an assortment of heat fluctuations in varying periods. Higher abiotic stresses effects in a reduced chlorophyll biosynthesis in crop cell (Reda, & Mandoura, 2011) which is a stress indicator in plastids (Li et al., 2010). Such as, temperature stress reduces the of 5-amino-levulinate-de-hydratase, an enzyme that protect in photosynthetic synthesis

Role of Compatible Solutes in Alleviating Effect of Abiotic Stress in Plants
[ 144 ] (Kumar et al., 1998). Also, chilling stress also disturbs crop production by damaging the biophysiological mechanism of plants. Chilling stress makes higher levels reactive oxygen species, mainly hydrogen-peroxide (H2O2), which might be regulator for harmfully disturbing germination and growth expansion (Xu et al., 2007). Plants have established several stratagems to stand temperature fluctuations. Some of the apparatuses suitable in plants under temperature stress is the organic molecules accumulation that have a defensive part in crop plant. Kishor et al. (2005) stated that improved in plants growth and showing to cold stress caused by higher proline. Foliar treatment of proline at low application improved tolerance against cold stress . Some works have described the role of glycinebetain in defending plants under disease temperature stress (Kawakami et al., 2008). Due to improvement in machinery, involved genes in biosynthesis of glycinebetaine are now moved in particular non-productive plants that aid plants endure under stresses. Such as, gene (cod-A) gives tolerance against chilling stress in Oryza sativa, which defends the plant from tissue injury, activity of ROS and enzymes (Sharma et al., 2019). The accumulation of ROS during the high temperature stress conditions destruct the photosynthetic appratus in chloroplast, but glycinebetaine sustained the action of amino acids and keeps the plant from abiotic environments.

Heavy metal stress
Contamination of heavy metals has occurred as a wide threat in adding to heat, salinity and drought strain (Kohli et al., 2018). Caused by rises in industrialization and urbanization, the treatment of heavy metals has improved at a disturbing level. The extreme production of reactive oxygen species in exposure to stress in plants (

Role of Compatible Solutes in Alleviating Effect of Abiotic Stress in Plants
[ 145 ] compounds, low molecular-weight that are generally non-toxic at more cell absorptions. Mostly, they defend plants from anxiety or diseases during diverse ways, comprising detoxification of ROS, involvement to cellular osmotic change, maintenance of membrane and protection of membrane stabality . Moreover, for several of these organic solutes also protect the plant structures from desiccation damage, they are usually discussed to as osmolytes/osmoprotectants. These organic solutes such as GB, proline, sucrose, pipecolate, trehalose alanine-betaine, (Ozturk et al., 2021).

Glycine betaine
Several quaternary ammonium-compounds recognized in plants, glycinebetaine happen most in large amounts in reaction to drought stress (Aamer et al., 2018). Glycinebetaine is mostly abundant in chloroplast where it shows a important part in protection of thylakoid membrane, thus retaining photosynthetic efficacy . In plants, glycinebetaine is accumulated in chloroplast from serine to ethanolamine (Sharma et al., 2019). Choline is changed into betaine-aldehyde, with the presence of choline monooxygenase (CMO), which is then transfored into glycinebetaine by betaine-aldehydedehydrogenase (BADH). While other ways like, through N methylation glycine is also identified, the pathway from choline to glycine betaine has been known in all glycinebetaine producing plant species (Weretilnyk et al., 1989). Glycinebetaine is recognized to synthesized in reaction to stress in several plants, as well as spinach (Spinacia oleracea), sorghum (Sorghum bicolor) sugar beet (Beta vulgaris), barley (Hordeum vulgare) and wheat (Triticum aestivum) (Ashraf & Foolad, 2007). Foliar applied glycinebetaine during the chickpea bud stage exposed much superior enhancement in flowering and pollen growth, active pollen sustainability, larger receptivity in stigma, tremendous growth in pollen tube, and wide feasibility of ovule in chilling stress situations. Instead, during pod filling, application of glycinebetaine showed in a higher in the yield, and its number of seeds, pods and seed biomass were considerably better after application. Glycinebetaine significantly induced the chilling stress in plants over active

Role of Compatible Solutes in Alleviating Effect of Abiotic Stress in Plants
[ 146 ] enhancement in total chlorophyll and leaf water contents for now, it also decline the abscisic acid and ROS . Several plant species spinach (Spinacia olerecea L.) and barley (Hordeum vulgare L.) (Jagendorf & Takabe, 2001) produce supplementary glycinebetaine in several parts of chloroplasts than plants similar tobacco and rockcress (Arabidopsis) (Sulpice et al., 2003). Improving the activities of flower, pod formation and various yield attributes under the foliar application of glycinebetaine to chickpea plant, provides a moderately cold tolerance in the plants (Nayyar et al., 2005). When foliar applied glycinebetaine on plants, it proceeds successfully from the tomato leaves and formerly transfers it into several parts. Foliar applied glycinebetaine at the maximum level in plants plays a significant role towards cold high temperature. Maximum glycinebetaine is moved inside the cytosol. But, a small amount transfers to the chloroplasts and probably to several parts of cell. Consequently, photosynthesis is secure and cold tolerance is higher in glycinebetaine treated plants. It is needed to notice large quantities of glycinebetaine in the leaves like as the buds and apices in sprayed plants. Higher amount of glycinebetaine in these cells are supposed to be essential for plant protection and supporting growth rescue after cold stresses (Park et al., 2006). Then, the position of glycinebetaine production in cell parts of plants has the vital part on amount of its efficacy on the plant acceptance to pressures.

Proline
Proline is an amino-acids type that is extensively occurs in plants and regularly incorporated in excessive content in reaction to biotic and abiotic stresses. Proline is not only controlled in involvement as an organic solute, then moreover it alleviates several parts of cells including as membranes, hunts free radicals and defenses redox potential of cells towards environmental stresses (Kahlaoui et al., 2018;Ashraf & Foolad, 2007). Proline is the major source of energy that can involve to generation heat in the voodoo lily (Hare & Cress, 1997). Proline biosynthesis has been connected to ornithine or glutamate pathway and too particularly, linked to glutamate-glutamine metabolism and oxidative pentose phosphate pathway (Verslues & Sharma, 2010). Accumulation of proline in the cytoplasm by the glutamate pathway, glutamate changed into 1 pyrroline 5 carboxylate by 1 pyrroline 5 carboxylate synthetase, which is changed into proline by 1 pyrroline 5 carboxylate reductase. Instead, the ornithine pathway relates to proline accumulation in the mitochondrion from ornithine. Ornithine is converted into P5 C and glutamate-semialdehyde by ornithine-δ aminotransferase (OAT) and changed to proline. Also, ProDH, a fundamental enzyme in proline metabolism and catabolized proline into P5C (Trovato et al., 2008). The subsequent of bio-products are both proline synthetase, OAT and P5CS. This model confirms that the ProDH is the fundamental controller in the increasing proline production in plants (Szabados & Savoure, 2010).

Foliar application of proline on plant
Proline is an osmolyte that stores in qualified levels in plants under drought stress (Junior et al., 2018). It performs many roles in plants, as well as signaling, protein stabilizing, stress tolerance, radical hunting and helps as a nutrient tank . Its foliar treatment is also active in the mitigation of drought stress. Some instructions have underlined its

Role of Compatible Solutes in Alleviating Effect of Abiotic Stress in Plants
[ 147 ] ameliorative effect in several abiotic stresses (Singh et al., 2018). Exogenously applied proline better the enzymes activity and the turgor potential of pea under nickel stress (Gajewska & Skłodowska, 2005). Likewise, foliar application of proline increased the higher biomass, antioxidant defense potential and photosynthetic activities of plants under Cd stress conditions (Hayat et al., 2021). Foliar application of proline improved activities enzyme and greater chlorophyll content, absorption of nutrient and all growth traits under Cd stress in pigeon pea plants. Germination is the main sensitive phase in the plant cycle (Hubbard et al., 2012;Muhammad et al., 2021) as it is actual complex to environment stresses. In certain, salt stress causes osmotic stress that ion toxicity and bounds absorption water caused by the higher production of Na + and Cl − (Farissi et al., 2011;Farissi et al., 2013). Treatment of proline improves the harmful effect of NaCl -, but at the rate of 100 mM proline did not showed a significant effect. Also, foliar applied proline at the level of 50 mM significantly enhanced seed germination of both cultivars of S. bicolor under salinity stress (Nawaz et al., 2010). Then foliar application of proline at appropriate amount may improve the adverse influence of salinity stress by osmotic adjustment, and then comprehensive studies behind these records are still necessary to well know the involvement of molecular mechanism. Proline treated improved 1000-grain weight and grain yield of salt-stressed T. aestivum (El Moukhtari et al., 2020; Rady et al., 2019). Exogenously applied proline improved the number of seeds, 100-grain weight and total grain weight under stressed Z. mays (Alam et al., 2016). Over-all, foliar treatment of proline improved plant growth and production under salt stress however the primary mechanisms, almost certainly related to several compounds regulations, still remain intangible. Proline can also save cell membranes and proteinsagainst salt stress and oxidative stress by increasing the several antioxidative activities (El Moukhtari et al., 2020). Such as tobacco growth, cells suspension under salinity stress was stimulated by foliar treatment of proline, which was suggested to be due to proline activity as a defender of membranes and enzymes (Okuma et al., 2004). In stressed soya-bean plant, a significantly improved in activities of POD and SOD under foliar treatment of proline (Hua & Guo, 2002). In barley under salt stress exogenouslly applied of proline caused in a reduction in NaCl production and an improved in growth (Lone et al., 1987). Such enhanced influences of proline were showed to be because of stabilization of membrane (Mansour, 1998).

Soluble sugars
Carbohydrates/sugars are very significant osmoprotectants to maintain plant metabolism and membrane structure during environmental stresses conditions . During abiotic stresses, soluble sugars have a role to stable the osmotic regulations and organize reactive oxygen species. Soluble carbohydrates are fundamentals in metabolic processes and protect some metabolic processes, as well as respiration, photosynthesis, and oxidative pathway from two scavenging structures and production of ROS. For example, mannitol to protect the chloroplastic organelle from poisonous harms due to free radicals during abiotic stress situations. Similarly, trehalose adjusts the mechanisms of ABA and carbon when plants are response to osmotic stress (Upadhyaya et al., 2013).

Role of Compatible Solutes in Alleviating Effect of Abiotic Stress in Plants
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Conclusion
Several plant species produce naturally organic solutes or osmolytes as a major proline, glycinebetaine and soluble sugars when response to environmental strains. These solutes are to show very important roles in facilitate to osmotic adjustment and defending cellular mechanisms in strained plants. But, not all plants produce organic solutes proline and glycinebetaine in abundant levels to support avoiding severe impact of environmental pressures. These organic solutes stable the redox potential of cells by increasing the productivity of reactive ROS hunting and result in lessening of oxidative destruction to yield. Therefore, many methodologies have been considered to raise the amount of these complexes in plants under stress situations to enhance their strain tolerance. First, it is the use of short-cut method; foliar applied proline is a positive approach in increasing the plant production under abiotic stresses. The application of proline, GB, commercially, artificially has been used. Many studies reported proline, GB osmolytes proved to overcome the adverse influence of environmental experiments on stressed plants. Other method has been to genetically engineer plants that are proficient of manufacturing appropriate concentrations of these compounds in exposure to abiotic pressures. While several developments has been made in leading genes for the accumulation of these compounds in naturally low production or non producing plant species, production level in transgenic plants have often been lacking or low to increase plant stress tolerance. I conclude that, the positive effect of osmolytes in plants under water stress. Foliar applied proline and GB increased the plant crop production under stressed plant.