Effect And Its Mechanism Of Oligochitosan With Certain Degrees Of Acetylation On Wheat Under Salt Stress

China has a vast area and various types of saline soil. According to statistics, there are about 36.93 million hectares modern saline soil, 44.87 million hectares residual saline soil and 17.33 million hectares potential saline soil in China. All kinds of saline soils have a total of about 100 million hectares, accounting for 13.3% of the available land which severely impedes the agricultural development of our country. With the rising population and food shortage, improving the output of crops in saline soil is the emphases and hotspot of the study on the reasonable development and utilization of saline soil and of importance in promoting food security, social and economic development.The current methods to improve crop yield in saline soil include breeding salt-tolerant crops, genetically engineered crops and the application of exogenous substances. The application of exogenous substances is undoubtedly the most effective and simple way to improve crop yield in saline soil. Chitosan and its derivatives can induce broad-spectrum resistance, enhance plant resistance, inhibit plant pathogenic microbial growth, and alleviate the damage of salt stress, low temperature stress and drought stress. The chitosan and its derivatives have a broad application prospects in agricultural.The molecular weight(Mw) and degrees of acetylation(DA) are the most important parameter influencing the chitosans’ various properties. The effectiveness of chitosan in various applications appears to be dependent on the Mw and DA. However, the current researches are not considering the influence of the Mw and DA on its activity, causing the inconsistent results which greatly hinder the application of chitosan and its derivatives in agricultural production. Hence, this paper studied the effect of chitosan with different Mw and DA on wheat under salt stress and studied the mechanism of induced resistance of wheat to salt stress. In this paper, the main results are as follows:1. Chitosan with different Mw could reduce the content of malondialdehyde(MDA) to protect the cell membranes from oxidative damage, improve the contents of soluble sugar and soluble protein in wheat to enhance the osmotic adjustment ability of plants, alleviate the degradation of chlorophyll to ensure the photosynthesis of plant.What’s more, chitosan with different Mw could enhance the antioxidant enzymes including superoxide dismutase(SOD), peroxidase(POD) and catalase(CAT) activities to scaveng the excess reactive oxygen species, reduce salt stress oxidative damage of cell membrane and organelles, thereby improve the salt resistance of wheat to protect wheat from oxidative damage and to improve the salt tolerance of wheat. And with the reduction of the Mw, the resistance activity of chitosan to salt stress increases. The oligochitosan with 1300 Da has the strongest activity of inducing resistance to salt stress.2. In determining the molecules, oligochitosans with different DAs were preparated. The results showed that treatment with exogenous oligochitosans with different DAs could decrease the concentration of MDA, increase the contents of soluble sugar, soluble protein and proline which protect cell membrane structure and improve the osmotic regulation ability. And the oligochitosans with different DAs could enhance the activities of SOD, POD and CAT to reduce the oxidative damage caused by salt stress on wheat seedlings. Under salt stress, application of oligochitosans with different DAs could increase chlorophyll content of wheat leaves, increase stomatal conductance(gs) to increase intake of CO2 in order to improve photosynthetic rate(Pn), improve the qP value and reduce the NPQ value to increase the absorption of light energy to the electronic transfer and reduce the heat dissipation, thus ensure the actual photochemical efficiency ФPSⅡ. The resistance to salt stress of oligochitosans with different DAs perform as DA50 % > DA68 % > DA29 % > DA2 % which suggest that DA has important influence on its resistance activity.3. The physiological mechanisms of exogenous oligochitosan with certain DA on wheat tolerance to salt stress were processed. And the results showed that:(1) Oligochitosan with DA 50% could adjust the ion transport in wheat cells and maintain a higher K+ content and lower Na+/K+ in plant cells to ensure the electron transfer, photosynthesis enzyme activity and alleviate the stomatal closure, promote the photosynthesis.(2) Oligochitosan with DA 50% could increase the antioxidant enzyme activity and the antioxidant content to alleviate the oxidative damage to membrane system including plant chloroplast thylakoid membrane and to ensure the integrity of plant photosynthetic apparatus, so as promote the photosynthesis and improve salt resistance of plant.4. The mRNA expression level examination of several salt stress response genes suggested that oligochitosan with DA 50% can effectively regulate the expression of SOS and NHX genes in wheat seedlings under salt stress to reduce the damage of salt ions with high concentration. Furthermore, oligochitosan with DA 50% can regulate the expression of SOD, POD, CAT, APX, DHAR and GR genes to enhance the ability of scavenging active oxygen, reduce oxidative damage to plants and improve salt resistance of plant.In this paper, the observed elicitor activity of oligochitosan with DA 50% and the other DAs may be explained by a mechanism dependent on specific receptors. The interaction between oligochitosan and plant is dependent on specific recognition of the N-acetyl moieties, which suggest that N-acetyl plays an important role in inducing its activation. Oligochitosans with different DAs may form various structures to interact with elicitor receptor on plant cell membrane because of their strict structural requirement. However, the mechanisms of recognition of elicitors such as COS and signal transduction of plant needed to be further studied.