Study Of The Regulating Mechanism Of Abscisic Acid And Antioxidative System On Aluminum Tolerance In Soybean (Glycine Max L.)

Aluminum (Al) toxicity is one of the most significant constaints limiting plant growth in acid soils. The primary symptom of Al toxicity is a rapid inhibition of root growth, resulting in a reduced and damaged root system and limited water and mineral nutrient uptake. Abscisic acid (ABA) considered as an important endogenous plant hormone, ABA accumulates to high concentrations in organs and tissues of dormancy plants. It appeared that ABA was a general endogenous inducer of tolerance to environment stresses and significantly increased drought,salt and chilling tolerance by activating the anti-adverse stress system. In the present study, with solution culture, Jiyu70 was selected as the Al-tolerant cultivars for the research of effect of ABA involved in Al tolerance in soybean. By the split-root experiment and ³H-ABA radioisotope technique to explore the transport and distribution of ABA in soybean, and from aspects of organic acid secretion, antioxidant systems and signaling molecule NO to approach how ABA regulated Al tolerance .The results were as follows:1. ABA was speculated to involve in modulating Al tolerance in soybeanThe major Al toxicity symptom observed in plants is inhibition of root elongation. The root elongation of soybean was inhibited under Al stress, with the increasing of Al concentration, the degree of inhibition enhanced; Al induced root elongation inhibition was respectively alleviated by exogenous ABA and aggravated by ABA synthesis inhibitor-fluridone. In addition, Al-treatment increased the endogenous level of abscisic acid (ABA) in soybean roots in a dose- and time-dependent manners, Al increased the ABA content in both roots and leaves. With similar chemical properities, La only increased the ABA content in roots. Split-root experiments (Part A and Part B)show that in the same soybean seedings, inhibition of root growth when the roots of Part A exposed to Al, the growth of roots of Part B without Al did not inhibited. Endogenous ABA in the roots of Part A elevated significantly compared with control, endogenous ABA in the roots of Part B reduced compared with control. We found that ABA inclined to redistribute in the root part under Al stress. By the whole-root and split-root experiment, binding with ³H-ABA radioisotope technique, ABA could be transported up or down in soybean seedling. Calculated transport rate was 3.2 cm·min^-1 or more. Al accelerated ABA transport and ABA inclined to distribute in the root part under Al stress. More ABA was accumulated in the roots with direct contact of Al.Above all, ABA is probably a signal molecule transmission between aboveground and underground under Al stress to involve in modulating Al tolerance in soybean2. ABA and Al induced citrate exudation of soybeanAl induced organic acid secretion is one of the most important Al tolerance mechanism in plants. The focus of our research is to examine whether ABA is involved in the Al-induced secretion of citrate. our research to identify the relationship of ABA and Al induced citrate exudation in soybean roots : 1)Split-root experiments (Part A and Part B) show that endogenous ABA in the roots of Part A exposed to Al elevated and Al induced citrate exudation in soybean roots, endogenous ABA in the roots of Part B without Al reduced and no citrate exudation in soybean roots. 2)Exogenous ABA application had no effect on Al induced citrate exudation, and didn't affect the Al induced increase of citrate content nor citrate synthase activity. 3)Al-tolerant cultivar secreted more citrate than Al-sensitive cultivar while the ABA content in the roots of both cultivars had no differences under same Al stress. 4)By time course experiment, both citrate secretion rate and endogenous ABA content increased with increasing of Al treatment time during 0-12 h. When endogenous ABA content dropped during the next 6 h pulse (12-18 h) free of Al, citrate secretion rate continued to increase. These results suggested that ABA increased the Al tolerance in soybean, but not by increasing the Al induced citrate exudation from soybean roots. 3. ABA and system of antioxidation under Al stressAn important response to stress by plant is the production of reactive oxygen free radicals, these reactive oxygen species can directly or indirectly activated membrane lipid peroxidation, causing membrane damage and destruction. ABA involved in biotic and abiotic stress caused by oxidative stress, but the effect of ABA on oxygen species generation, lipid peroxidation and antioxidant enzyme activity on soybean roots under Al stress remains unclear, the results showed that Al stress increased the MDA content,O₂^·– generated rate,the content of H₂O₂ in roots and leaves of soybean. Exogenous ABA treatment inhibited the increase in MDA content,O₂^·– generated rate,the content of H₂O₂ induced by Al in roots and leaves of soybean. ABA inhibitors fluridone enhanced the O₂^·–generated rate,H₂O₂ and MDA content in roots and leaves of soybean under Al stress, Al induced the increasing of the activity of POD,SOD,CAT activity. ABA increased the activity of POD; reduced SOD,CAT activity; ABA inhibitors fluridone increased the antioxidant protective enzymes SOD,CAT activity, reduced POD activity. Because of the site of Al toxicity is the roots, ABA is probably a signal molecule transmission between aboveground and underground under Al stress and ABA blocked activities of CAT,SOD and elevated activities of POD. ABA might activate an H₂O₂-mediated pathway, a POD-dependent antioxidative mechanism to protecting the plant against Al-induced oxidative stress.4. the role of NO in alleviating Al toxicity and the relationship between signaling molecule ABAAl induced increase in the NO content in soybean roots. NO donor SNP exhibited significantly greater root elongation and reversed the inhibition of fluridone in root elongation under Al stress. Exogenous SNP enhanced the Al-induced increase in the ABA content in soybean roots, the NO scavenger cPTIO completely reversed the effect of NO on the ABA content in the presence of Al. In the signaling pathway of alleviating Al toxicity, NO plays an positive role in internal source level of ABA, NO alleviated the inhibition of root elongation under Al stress may be due to the arising endogenous ABA content. NO promoted the endogenous ABA content and redued the poisoning effect of Al. However, ABA content improve inhibited NO synthesis. Exogenous ABA and fluridone respectively inhibited and raised NO generation induced by Al. Thus, in the signaling pathway of alleviating Al toxicity, ABA plays a regulatory role of feedback inhibition in the level of the source of NO.