Silicon alleviates aluminum stress in alfalfa via mucilage-mediated reduction of aluminum accumulation in root cell wall

The objectives of this study is to investigate whether silicon (Si) could alleviate aluminum (Al) stress in alfalfa seedlings grown hydroponically under acidic pH conditions and its regulatory mechanism. As a result, it was confirmed that the growth inhibition due to Al stress could be alleviated when Si was supplied together with Al (Al + Si treatment). MDA and H2O2 contents of shoots and roots were significantly decreased in the Al + Si treatment compared with plants exposed to Al treatment alone. In addition, antioxidant enzyme activities of SOD, APX, and CAT were decreased by Al stress, but were recovered to the control level by Al + Si treatment. The Al content of roots and root cell wall polysaccharide components, pectin and hemicellulose, were significantly decreased in the Al + Si treatment compared with Al treatment alone. Mucilage exudation and beta-xylosidase activity decreased in the Al + Si treatment compared with plants exposed to Al treatment alone. However, MsXylosidase gene expression increased in the Al + Si treatment than with Al treatment alone, which suggests that Si could partially replace the role of mucilage and could also be partially involved in MsXylosidase gene expression. Taken together, results of morphological, physiological, biochemical and molecular analyses demonstrate that Si effectively alleviates Al toxicity in alfalfa plants grown under acidic soil associated with reduced Al accumulation in the cell wall.