The Interaction Of Salicylic Acid And Ca²⁺ Alleviates Aluminum Toxicity In Glycine Max

In acidic soils, aluminum（Al） toxicity is a major constraint for crop production. Al is a most abundant metal in the earth’s crust and is solubilized as the free Al³⁺ion under acidic conditions.The main symptoms of the rapid inhibition of growth in plant roots showed that the toxic effect of Al on plant roots was mainly root tips.A wide range of toxic effects of Al ions has been demonstrated in plants, although the mechanisms of Al toxicity have not been elucidated.As the modified crops of acid soil, soybean has been playing an important role in the study of Al resistance.Calcium(Ca²⁺), as a second messenger, directly or indirectly regulates various biochemical processes, including abiotic stress responses, and triggers the response of organisms to stress. Therefore, it is significant to further understand the mechanism of Ca²⁺ signal transduction and the mechanism of plant resistance to stress.Most importantly, salicylic acid（SA） has a certain regulation effect on the resistance to Al of soybean. Therefore, soybean（Glycine max L. cv Jiyu 70） was used to test the mechanism of Al tolerance. The influences of Ca²⁺ and SA on root elongation, Al accumulation, and citrate secretion in soybean plant were determined. And the activities of antioxidative enzymes in Al-exposed soybean roots is also investigated. The possible involvement of Ca²⁺ in regulating Al-induces salicylic acid（SA） release and the role of SA in regulating Al-induces cytosolic Ca²⁺ accumulation in soybean root was identified. The effect of Ca²⁺ on the change of mitochondria and the activities of Phospholipase in soybean roots under Al stress was detected.Relative root growth（RRG） was used a parameter to compare the Al tolerance in soybean.The results indicated that the elongation of roots was severally inhibited when these roots were exposed to 30 μM Al, the inhibition of root elongation by Al was ameliorated by Ca²⁺ but aggravated by Ca²⁺ inhibitors, such as verapamil（VP）. Furthermore, 1.0 m M Ca Cl2 reduced Al accumulation in roots. citrate secretion from these roots increased with the addition of either 1.0 m M Ca²⁺ or 10μM SA.In addition, SA can restore the stimulating effects of Ca²⁺ on root elongation, citrate secretion. Thus, both Ca²⁺ and SA contribute to Al tolerance in soybean.When treated with 30 μM Al and 10μM SA, the content of cytosolic Ca²⁺ increased gradually within 4 h, and then decreased at 12 h. SA upregulated the expression of four genes that possibly encode calmodulin-like proteins within 4 h, and then gradually decreased as time prolong. It indicated that exogenous SA could activate Ca²⁺ signal transduction system and involved in mechanism of Al tolerance through increasing the level of Ca²⁺, upregulating the expression of four genes that possibly encode calmodulin-like proteins. Furthermore, Ca²⁺ supplements can rapidly increase Al-induced accumulation of free-SA or conjugated SA（SAG）, while Ca²⁺ inhibitors delayed the accumulation of SA for more than 8 h.Antioxidant system enzymes can control or even eliminate Al-induced reactive oxygen species（ROS）, and then relieve oxidative stress. 1.0 m M Ca²⁺ could release the inhibition of Al in time duration and the ability decrease in the time. Exogenous Ca²⁺ decreased the content of MDA and H2O2 under Al stress. Enzymatic analysis showed that Ca²⁺ and SA stimulate the activities of superoxidase（SOD）, peroxidase（POD）, and ascorbate peroxidase（APX） in Al-treated roots.n addition, SA could restore the stimulating effects of Ca²⁺ on activities of antioxidative enzymes. The results indicated that Ca²⁺ can alleviate the damage of Al to root tip membrane and alleviate the oxidative stress caused by Al.Ca²⁺ plays an important role in the mitochondrial function system.The function of mitochondria and the structure of microtubules are destroyed under Al stress.The reorganization of microtubules is the beginning of the inhibition of root elongation in plant. The mitochondria from the cells gradually shifted to the cell wall under Al stress, and Ca²⁺ could alleviate the inhibition effect of Al on the count of mitochondria. Ca²⁺ could promote the activity of PLC and PLD and enhanced the relative expressions of plc and pld in phospholipid signaling pathway. Ca²⁺ could relieve the damage of Al to the mitochondrial function and phospholipid signaling pathway, most importantly it could release the toxicity of Al on cell and the disruption of microtubule structure in the mechanism of Al tolerance.In conclusion, Ca²⁺ and SA act as primary signaling molecules and respond to Al stress in soybean. They coordinately regulate root elongation, Al content, citrate secretion, and oxidative stress. The root cells of soybean facilitate Ca²⁺-mediated pathway for the accumulation of SA. Therefore, SA can mediate soybean’s response to Al stress by increasing cytosolic Ca²⁺ concentration and the expression of Ca²⁺-related genes, such as Ca M-like. Thus, the interaction between Ca²⁺ and SA influences Al tolerance in soybean.