Silicon alleviates aluminum toxicity by promoting citrate secretion and reducing aluminum accumulation in the cell wall of soybean roots

Silicon (Si) participates in the alleviation of plants to aluminum (Al) toxicity. However, the mechanisms of Al detoxification by Si needs to be further clarified, especially in soybean. We investigated the roles of citrate secretion and cell wall polysaccharides in Si-triggered soybean resistance to Al stress. Results showed that applying 2 mM Si alleviated elongation inhibition of soybean roots exposed to 100 or 200 µM Al for 24 h. Si application increased citrate exudation from roots under Al exposure, thereby reducing Al accumulation. Furthermore, Si promoted citrate synthesis through activating citrate synthase and inhibiting aconitase under Al stress. Quantitative real-time PCR assay showed that Si enhanced the expression level of citrate transporter genes (GmMATE13 and GmMATE47) under Al stress. In addition, Si supply reduced Al content in cell wall and in cell wall polysaccharides (pectin and hemicellulose 1) under Al exposure. Meanwhile, Si addition decreased the contents of pectin and hemicellulose 1, which were the main binding sites for Al in cell wall, under Al exposure. Furthermore, Si increased the degree of pectin methylesterification by inhibiting pectin methylesterase activity under Al stress, thereby reducing the ability of pectin to bind Al. These results suggest that Si can promote citrate efflux by regulating the synthesis and transport of citrate, thus chelating the toxic Al cations and alleviating Al toxicity. Besides, Si can also regulate the cell wall properties to reduce Al deposition in the cell wall of root tip, thereby improving Al tolerance of soybean.