Aluminum (Al) and manganese (Mn) toxicity are two main constraints to crop production in some acid soils. In the present study, hydroponic experiments were conducted to investigate the phytoxicity caused by the metals Al and Mn in the Al-sensitive soybean (Glycine max) cultivar Jilin62. Results showed that more than 80% of the total cellular Al accumulated in the cell walls of the root tissue. Excessive Mn supply significantly increased the Al level in the root and aggravated Al toxicity, and as the result leading to a stronger inhibition of the root-elongation rate and root-biomass production. No obvious change of Al concentration was detected in root-cell organelles and soluble fractions. Interestingly, Al-induced increase in cell wall pectin content in the root was further enhanced by addition of Mn, which might explain how Mn aggravated Al toxicity. In addition, it was found that high levels of Al significantly alleviated Mn toxicity symptoms like brown spots on the old leaves, crinkle on the young leaves and shoot growth inhibition. This Al reduced Mn toxicity was related to the decrease of Mn content in shoots under combined Al and Mn stress. Lanthanum (La 3+ ), which has similar chemical properties as Al, can also reduce Mn toxicity and Mn accumulation in the plant. Thus, it is speculated that Al exerts an antagonistic effect on Mn uptake, and thus leads to alleviated Mn toxicity.

Chemical injury is partly due to free radical lipid peroxidation, which can induce oxidative stress and produce a large number of reactive oxygen species (ROS). Oxaloacetic acid is an important intermediary in the tricarboxylic acid cycle (TCA cycle) and participates in metabolism and energy production. In our study, we found that oxaloacetate (OA) effectively alleviated liver injury which was induced by hydrogen peroxide (H2O2) in vitro and carbon tetrachloride (CCl4) in vivo. OA scavenged ROS, prevented oxidative damage and maintained the normal structure of mitochondria. We further confirmed that OA increased adenosine triphosphate (ATP) by promoting the TCA production cycle and oxidative phosphorylation (OXPHOS). Finally, OA inhibited the mitogen-activated protein kinase (MAPK) and apoptotic pathways by suppressing tumor necrosis factor-α (TNF-α). Our findings reveal a mechanism for OA ameliorating chemical liver injury and suggest a possible implementation for preventing the chemical liver injury.

To further understand the process of Al-induced citrate secretion from soybean roots, the effect of protein synthesis inhibitor, anion channel blockers, and citrate carrier inhibitors on Al-induced citrate exudation was investigated in Al-resistant soybean cultivar PI 416937. Citrate exudation from roots increased with the increase of Al concentration from 10 to 50 µM and initiated after 4 h of Al exposure. Protein synthesis inhibitor, cycloheximide (CHM; 25 µM) completely inhibited Al-induced citrate secretion during 12-h exposure, suggesting that novel protein synthesis was necessary in Al-induced citrate efflux. Also both anion channel blocker anthracene-9-carboxylic acid (A-9-C) and citrate carrier inhibitor mersalyl acid (Mersalyl) significantly reduced citrate secretion, suggesting that both anion channels in plasma membrane and citrate carriers in mitochondria membrane were the rate limiting factors of Al dependent citrate release. However, Al-induced citrate secretion was insensitive to anion channel blockers phenylglyoxal (PG), 4,4'-diisothiocyanostibene-2,2'-disulfonat (DIDS) and citrate carrier inhibitor pyridoxal 5'-P (PP).Additional key words: aluminum stress, soybean, anion channel blockers, citrate carrier inhibitors.