Effects Of γ-aminobutyric Acid On Reactive Oxygen Species Metabolism And Key Enzymes Expression Of Melon Seedlings Under Hypoxia Stress

Hypoxia stress comes into appearance as a result of the low dissolved oxygen, the high consumption rate of hydroponic culture, and water mismanagement, root- cushion formation of media culture in soilless culture, which influences the growth and development of melon seedling. The hypoxia stress has become an important limiting factor on sustainable development of vegetable production in protected cultivation.γ- Aminobutyric acid （GABA） is a four-carbon non-protein free state amino acid, which can enhance plant resistance to acid, salt, low temperature, hypoxia and other abiotic stresses and pests and other organisms by regulating cytoplasmic pH value , being as the citric acid cycle slip, temporary nitrogen pool, inducing ethylene synthesis and the osmotic adjustment.This paper investigated the effects of exogenous y-aminobutyric acid （GABA） on reactive oxygen species （ROS） metabolism, antioxidant enzyme isozyme contents and the antioxidant enzyme gene expression of melon seedlings‘Xiyu 1’under hypoxia stress with hydroponics culture. Seedlings were cultured under normoxia+GABA (5mmol·L^-1), hypoxia stress, hypoxia stress+GABA (5 mmol·L^-1), with normoxia treatment as the control.The results showed that the productive rate of O₂^- and contents of H₂O₂ and MDA in roots and leaves were significantly increased when melon seedlings were grown in the nutrient solution under hypoxic stress, meanwhile the activities of SOD, POD, CAT, APX, GR and the contents of AsA, GSH were improved. Exogenous application GABA under hypoxia stress could decreased the productive rate of O₂^-, the contents of H₂O₂ and MDA of melon seedlings by increasing the activities of SOD, CAT, APX, GR and the contents of AsA, GSH. However, exogenous application GABA could significantly improve the activities of CAT, GR and the contents of AsA, GSH, significantly decrease the contents of H₂O₂ and MDA, but had little influence on other physiological indexes. The results suggested that exogenous GABA alleviated the harm of hypoxia stress in melon, most likely by decreasing ROS accumulation and stability of cell membrane structure through significantly increasing antioxidant enzymes activity and antioxidant contents in roots and leaves of melon seedlings.Compared with the control treatment, the expression of the antioxidant enzyme SOD, POD, CAT, and APX isoenzymes were significantly increased under hypoxic stress, which showing that one or more roads of the isoenzyme spectrum become more wide or more bright. Regardless of the conditions of normoxia or hypoxic stress, antioxidant enzymes SOD, CAT and APX isozyme levels were significantly higher with exogenous GABA application. However, the expression of antioxidant enzymes POD isozyme was lower with exogenous GABA application. The isoenzyme spectrum of melon seedling roots was particularly obvious under normoxia conditions with exogenous GABA treatment. The result showed that with exogenous GABA, the expression of antioxidant enzymes SOD, CAT, APX isozyme increased, enhance the melon plants resistant to hypoxia. The result is consistent with the determination of activity trends of the antioxidant enzymes SOD, POD, CAT, and APX.Under hypoxic stress conditions, the relative expression level of antioxidant enzymes in melon seedling roots of CmSOD, CmPOD, CmCAT, and CmAPX significantly higher than control with real-time qPCR. In the condition of exogenous application GABA under hypoxia stress, the relative expression levels of the antioxidant enzymes CmSOD, CmPOD, CmCAT, the CmAPX gene reaches its maximum at 12h²4h, the antioxidant enzyme gene expression rate priority hypoxia stress handling. Compared with normoxia, relative expression level of CmCAT and CmAPX genes were significantly higher in the conditions of normoxia with exogenous application GABA treatmen, but the antioxidant enzymes CmSOD, CmPOD gene relative expression level have no significant differences. The test results showed that, under hypoxic stress, the amount of antioxidant enzyme gene expression of melon seedlings can increase for removing excess reactive oxygen species. The antioxidant enzyme gene is fast, efficient expression with exogenous GABA. It investigated that, GABA plays a role of signaling molecules in the response to outside stress and takes in part of the defense response in plants involved in plant defense information transfer and regulation of key enzyme gene expression, to ease the plant body by the injury further.