Salinity Tolerance Mechanism of Osmotin and Osmotin-like Proteins: A Promising Candidate for Enhancing Plant Salt Tolerance

Introduction: Salt stress is one of the most important abiotic stress factors which severely affect agricultural production. Osmotins and OLPs (osmotin like proteins) are kinds of proteins which were produced during plant adapting to the environmental stress. Objective: These proteins were closely related to osmotic regulation and resistance stress. They are widely distributed in plants. Their expression for these genes was induced by salt stress, which played important roles in plants responding to salt stress. Conclusion: During salt stress, osmotin can help accumulate proline, and quench reactive oxygen species and free radicals.


INTRODUCTION
Salt stress is an important factor limiting plant growth and development, and it is also a major abiotic stress factor affecting crop yield. Salt damage is one of the most important adverse circumstances in agricultural production, which seriously restricts the sustainable development of grain production and agriculture [1]. There are more than 20% cultivated land with salinization in the whole world, and the high concentration of salt in the soil causes osmotic stress and ion toxicity, which affects the growth of crops.
Plants under salt and other stresses will produce many stress-responding protein, and osmotin is one of the important. Their expression is induced by salt, and is closely related to salt tolerance. Osmotin in salt stress as osmotic stress protective agent to protect cells from seepage pressure regulating the impact, so as to maintain osmotic balance [2], but its detailed mechanism of salt tolerance is unclear. Singh et al. (1985) analyzed protein composition of tobacco cell treated by different concentrations of NaCl, 26 KD Osmotin increased significantly, 12.3% of total protein of cell can reach, its content and cell resistance was positively rising [3]. The Osmotin protein can be accumulated in the cells without stress and stress [4]. When no stress induced, Osmotin is housekeeping genes involved in cell basic *Address correspondence to this author at the Salt-soil Agricultural Center, Institute of Agricultural Resources and Environment Jiangsu Academy of Agricultural Sciences, Nanjing 210014, China; Tel/Fax: +86-25-54391921; E-mail: shaohongbochu@126.com metabolism. In salt adapted tobacco cells, the osmotic proteins were present in two forms, soluble in water soluble I and II soluble in detergent, and their ratio was about 2:3, and the isoelectric point was 8.2. Osmotin shows significant sequence homology at amino acid level with the sweet-tasting protein thaumatin and shares several similar characteristic features like molecular weight, lack of sulfhydral residues, basic pI, disulfide bonds, and high proline content. Osmotin is not sweet in taste and the probable reason may be due to lack of lysine residues that are present [5]. All the groove of osmotin protein contains Glu, Asp, Asp, Asp4 strong acidic amino acid residues, so that it is acidic because of negative charge. But its homologous protein osmotin like proteins-OLPs is neutral.

POSSIBLE MECHANISMS OF OSMOTIN UNDER SALT STRESS
The acting mechanism of osmotin is not well understood, however, osmotin protects the cells from osmotic shock especially during abiotic stresses by compartmentalization of solutes or by structural or metabolic alterations [28]. Osmotin with the help of cell wall components is involved in permeabilization of membrane and plasmolysed cells become insensitive to osmotin, suggesting that the cell wall components are needed for its activity [29].
Salt stress in plants is mainly caused by Na + , high Na + concentration can cause osmotic stress, ion toxicity and K + / Na + ratio imbalance, resulting in plant metabolic abnormalities to damage to plant cells [1][2][3]30]. Osmotin plays important roles in salt stress tolerance by sequestering Na + ions and compartmentalizing them into vacuoles and intercellular spaces. The association of tobacco osmotin protein with tonoplast [3] and the OLP identified from Mesembryanthemum crystallinum suggests the role of osmotin in the intracellular compartmentation of Na + ions [30]. But, whether osmotin upregulates sodium-proton antiporter1 (NHX1) gene to sequester Na + Ions or how it is able to perform this functions is not yet clear. It is also not clear if osmotin has a direct role to play or it stimulates other proteins that are downstream. Transgenics overexpressing osmotin gene exhibited salt tolerance in potato [20], tobacco [28], Triticum aestivum cv. Marvdasht [31], strawberry [23], tomatoes [24], mulberry [25], chili pepper [26], and soybean [27] by retaining chlorophyll, preventing the accumulation of ROS, with an increase in relative water content, proline accumulation, increase in root length, shoot length, plant height, leaf ex- Current Genomics, 2017, Vol. 18, No. 6 555 pansion, and improved root growth than controls. Overexpression of OLP lacking short C terminal cDNA also showed such an enhanced salt tolerance in potato [20]. Transgenic mulberry expressing osmotin driven by CaMV35S promoter displayed better tolerance to salt stress than the transgenics containing osmotin under the influence of rd29A promoter, though the rd29A promoter is responsive to dehydration while the CaMV35S promoter is constitutive [25]. Rice transgenic plants expressing OPBP1 showed salt tolerance with enhanced root length and root growth than the untransformed controls [32]. Currently, it is known that there are two main mechanisms for the osmotic regulation proteins.

OSMOTIN WAS TOLERANT TO SALT STRESS BY THROUGH ANTIOXIDATION
Under the stress condition, the antioxidant system of plant is damaged due to the increase of Reactive Oxygen Species (ROS). Under salt stress, the content of hydrogen peroxide in transgenic plants overexpressing osmotin was much lower than that in the control group, which showed that osmotin was helpful to control the accumulation of hydrogen peroxide. Pepper overexpressing osmotin under salt stress, Ascorbate Peroxidase (APX) and Superoxide Enzyme (SOD) increased, release of H 2 O 2 toxicity, but osmotin how to regulate the activity of APX and SOD is not clear. MDA is one of the most important products of membrane lipid peroxidation, which can indirectly determine the degree of damage to the membrane system. In plants, overexpressing osmotin, the decrease of MDA increased the salt stress tolerance [26]. Therefore, plants can neutralize the active oxygen produced by the increase of the ion and the expression of the antioxidant enzyme, thereby increasing the salt tolerance ability of plants.

OSMOTIN WAS TOLERANT TO SALT STRESS BY ACCUMULATION OF PROLINE
Under abiotic stress conditions, plants can accumulate a certain amount of proline, which can reach more times in some plants compared with control. As a free radical scavenger, it can help the plant to resist salt [33]. Proline can induce cell osmotic adjustment, which does not show any adverse effects on the accumulation in the cytoplasm, but it can eliminate the toxicity of reactive oxygen species and the formation of free radicals [34]. The osmotin can promote the accumulation of osmotic adjustment substances, such as proline and betaine [35], which affect the expression of proline and regulated by both constitutive and inducible promoters. In the potato [20], tobacco [22], tomato [24], mulberry [25], and pepper [26], overexpression of osmotin can lead to increased proline, while in some plants, there was no significant change in the degree of stress [36]. The change of osmotin was closely related to the level of osmotic adjustment protein and the proline content in transgenic plants. Therefore, the plants which overexpressed osmotin protein can enhance the salt tolerance ability through the accumulation of proline.

CONCLUSION
Osmotin, a multifaceted plant protein confers tolerance to abiotic stresses especially salt stress. Transgenic and native expression of osmotin and OLPs was observed in various plants when treated with salt stress. Osmotin was tolerant to salt stress through antioxidation, and accumulation of proline (Fig. 1). Due to the multiple activities of osmotin, it has potential functions in biotic and abiotic stress tolerance in crop plants.

CONSENT FOR PUBLICATION
Not applicable.

CONFLICT OF INTEREST
The authors declare no conflict of interest, financial or otherwise.