Abiotic stresses, including low temperature, drought and salinity, can cause a significant reduction in crop yield and quality. Therefore, improving resistance to such adverse environments is extremely important for agriculture. New genes associated with stress resistance have been identified, and transgenic plants carrying these genes exhibit greater tolerance to drought stress, salt stress, cold stress, heavy metals, paraquat and wounding stress .
Non-specific lipid transfer proteins (nsLTPs) are widely present in the plant kingdom and are involved in the binding and transport of various lipids. All known plant nsLTP precursors include anN-terminal signal peptide, indicating that nsLTPs are secreted. Mature nsLTPs are small proteins characterized by an eight-cysteine motif (8CM) with the consensus sequence C-Xn-C-Xn-CC-Xn-CXC-Xn-C-Xn-C[7, 8]. The eight cysteines are part of the four disulfide bonds that stabilize the three-dimensional structure of the hydrophobic cavity. This allows different lipids and hydrophobic compounds to bind[8–10]. Based on sequence similarity, ten types nsLTPs are divided from Arabidopsis, rice, Solanaceae and other plants .
Lipids play an important role in stress tolerance by mediating plant responses to environmental stimuli and cell signal transduction pathways, resulting in changes in energy storage and maintaining basic cellular functions. In these processes, lipid transformation proteins (LTPs) act as lipid transporters. Non-specific lipid transfer proteins (nsLTPs) can bind or transfer various hydrophobic molecules in vitro, such as fatty acids, fatty acyl coenzyme A, phospholipids, glycolipids and cutin monomers. GsLTP belongs to the nsLTP family members of which playing important roles in numerous physiological processes, such as cutin and wax biosynthesis, abiotic and biotic stresses, seed development and germination, sexual reproduction, cell wall growth, nodulation, calmodulin (CaM) binding and responses to plant allergens[9, 12, 13].
Functions of LTPs under multiple abiotic stresses have been studied in various plants including Saccharum hybrid complex, Triticum turgidum,Solarium tuberosum and Setaria italica. NsLTPs from sugarcane respond to abiotic stresses and the signalling molecules salicyclic acid and methyl jasmonate. The wheat proteinTdLTP4promotes tolerance to abiotic and biotic stresses in Arabidopsis thaliana. Transgenic potato lines overexpressing StnsLTP1 display enhanced cell membrane integrity under stress. SiLTP enhances salt and drought stress tolerance in foxtail millet, and may be partially upregulated by SiARDP. Overexpression of a pepper nsLTP gene (CALTP1) in transgenic Arabidopsis increased the resistance of Arabidopsis against infection by Pseudomonas syringae pv. tomato and Botrytis cinerea. The tolerance of Arabidopsis to NaCl and drought stress at all stages of vegetative growth was improved .
Plant nsLTPs are involved in a varity of physiological functions, such as sebum transport, cutin synthesis, cell wall extension, pollen development, pollen tube growth and guidance, stigma and pollen adhesion, plant signalling, biological stresses, abiotic stresses and seed maturation[15, 19, 20]. Evidence suggests that nsLTPs are expressed in a wide range of plant cells, and they are involved in various physiological and biochemical processes, including cuticle synthesis and embryo development, adaption to a variety of stress environments, and resistance to microorganisms. However, the exact biological functions of these proteins have not been elucidated. At low temperatures, and under drought and NaCl treatment, high expression of nsLTPs has been detected in plant stems of tomato, barley and sunflower. These results indicate that nsLTPs help plants to adapt to adverse environmental conditions.
Glycine soja (Glycine soja Sieb. et Zucc), grown widely in Jilin Province in Northeast China, has a high tolerance to environmental challenges.It is a model species for molecular mechanisms of low temperature and salt stresses. The ancestor of cultivated soybean is a highly adaptable plant species that grows well in desert conditions[8, 25, 26]. It is also an excellent genetic germplasm for mining abiotic resistance genes for agricultural crop breeding. In one of our earlier studies in which Glycine soja was grown under 150 mM NaCl and salinity responsive ESTs were identified. The expression of GsLTP was found to be induced to high salinity condition. In this study, we investigated the functions of GsLTP under salt or osmotic stress treatment. Our results showed that GsLTP enhanced the salt tolerance and drought resistance of plants by regulating the synthesis of protective compounds. Thus, this study enhanced our understanding of the functions of LTP genes and the mechanisms by which plants tolerate multiple stresses. Moreover, it provides theoretical basis and practical genetic resources for crop breeding.