Mechanism Of Aminobutyric Acid(GABA) Regulating Maize Seed Germination And Seedling Growth Under Salt Stress

Maize is an important food and forage crop in our country, and it plays an important role in the national economy. Salinized soil is a widely distributed low yield soil, which has come to one of the main environmental factors causing worldwide crop production cuts. Salt Stress can affect normal growth and development of crops, while maize is moderately salt-sensitive crops, salt stress will seriously affect the physiological metabolism of maize, which directly affect the quality of growth, development and yield formation of maize. To explore the mechanism of γ-aminobutyric acid(GABA) to improve salt tolerance of maize seedling, this paper choose two different salt tolerance of maize as materials, to study the effects of GABA on seed germination, seedling growth, antioxidant enzyme system, nitrogen metabolism, endogenous hormones content, photosynthetic systems, and differential gene expression of maize seedlings under salt stress, aimed to clarify the physiological pathways of GABA to improve the salt tolerance of maize and its related functions induced differential expression of genes and the relations to salt tolerance.This experiment was conducted in the greenhouse of Northeast Agricultural University during 2014 and 2015. Zhengdan958(salt tolerant)、DongNong 253(salt sensitive) and Chang7-2 were chosen as experimental materials. According to the results of the preliminary screening test, we choose 100 mmol/L and 150 mmol/L as the salt concentration at germination and seedling stages respectively. The administration concentration of GABA were 0, 0.25, 0.5, 1 and 2 mmol/L. Seed germination experiment using the corresponding concentration of the solution according to experimental design as the substrate, and seedlings experiment select 1/2 Hoagland nutrient solution as hydroponic media. The maize seedlings were processed, when grow up to three-leaf. The main findings are as follows:(1) The application of different concentrations of γ-aminobutyric acid(GABA) could effectively relieve Na Cl stress caused germination inhibition. T he germination rate, germination potential, vigor index and storage material transport rates of Zhengdan 958 and Dongnong 253 treated with 0.5 mmol/L GABA under salt stress was higher than these maize seedlings that treated with salt stress purely. The 0.5mmol/L GABA has the best effect to alleviate the growth inhibition of radicle and coleoptile of maize seeds caused by salt stress, the length of radicle and coleoptile of Zhengdan958 and Dongnong 253 increased by 46.76%, 49.69% and 58.52%, 45.14% respectively compared with NaCl stress alone. GABA could significantly improve α-amylase activity, thereby promoting seed germination, wherein 0.5 mmol/L GABA has the best effect.(2) Under NaCl stress, different concentrations of GABA could alleviate the stress on seedling growth, and this was conducive to seedling morphogenesis. The plant height and fresh and dry weight of shoot of maize seedlings treated with GABA were higher than Na Cl treatment alone, in which the treatment effect of 0.5 mmol/L concentration of GABA was better than other concentrations. The alleviation of GABA on growth inhibition of Dongnong was better than Zheng 958. GABA could increase the fresh and dry weight of roots, total root length, root surface, root volume and the number of tips; reduce the average diameter of the root system of maize seedlings under Na Cl stress. The root scanning image displayed that, 0.5 mmol/L GABA in favor of the generation of root hairs, promoted the absorption of water and nutrients, which guaranteed maize seedling grow well under NaCl stress.(3) Different concentrations of GABA could effectively reduce content of O2·- and MDA in maize seedlings leavese under NaCl stress. Na Cl stress significantly improved superoxide dismutase(SOD), peroxidase(POD) and catalase(CAT) activity in order to clear MDA and O2·-, wherein 0.5mmol/L GABA has the best effect.(4) Applying exogenous GABA could increase endogenous GABA content in leaves and roots of seedlings. Under normal growth and NaCl stress, GABA could increase the soluble protein content and reduce pyruvate content in leaves and roots. Exogenous GABA would produce feedback regulation, decrease GAD activity. The drop of GAD activity would lead to the rise of glutamate levels, which has a feedback regulation on GS, GOGAT and GDH activity, and decrease these enzymes activity. The feedback regulation could make the conversion between the various substances during nitrogen metabolism, thus making the reduction of soluble protein hydrolysis and the metabolic intermediate pyruvate.(5) Under Na Cl stress, ABA content within leaves and roots significantly increased, which was conducive to improve plant resistance. Since ABA is a kind of growth inhibitory hormone, and it has antagonism with growth promoting hormone, so the content of IAA, GA and ZR in the leaves and roots were declined, and from the point of view phenotype was manifested in growth decline. The addition of exogenous GABA could improve a variety of metabolism of plants, such as increase antioxidant enzyme systems, thereby reduce the damage caused by stress, so the ABA content decline, IAA, GA and ZR content increased under the treatment with GABA and the plant recovery growth from the phenotype look.(6) The addition of exogenous GABA could relieve the damage on maize leaves caused by Na Cl stress, reduce the decomposition of chlorophyll, thereby enhancing the value of SPAD, photosynthetic rate, stomatal conductance and transpiration rate. T he improvement of plant photosynthesis would ameliorate the utilization of CO2, it would make intercellular carbon dioxide concentration decline or keep stable trend.(7) By analyzing the result of transcriptome sequencing, we selected the gene differentially expressed under salt stress induced by GABA, and we found the differentially expressed genes associated with physiological parameters by comparative gene function. There were genes regulating peroxidase enzyme on protein biogenesis, genes regulating auxin response factors, genes regulating chlorophyll a, b of LHCII type bind to protein in the upregulating genes. There were genes regulating ABA response in the down-regulated expression genes and genes regulating glutamate synthetase. The up or down-regulated expression of these genes could cause physiological indicators corresponding changes, thus revealing the function and ro le of GABA under Na Cl stress at the molecular level.