The differences of cell wall in roots between two contrasting soybean cultivars exposed to cadmium at young seedlings

The plant root cell wall (CW) is the first structure that comes into contact with extracellular cadmium (Cd), and it plays an important role in the absorption, immobilization, and translocation of Cd in the roots. However, the differences in the cell wall components between Cd-tolerant and Cd-sensitive cultivars are unclear. A hydroponic experiment was carried out to investigate the differences in the concentrations of Cd, total sugars, and uronic acid in pectin, hemicellulose 1, hemicellulose 2, cellulose, and lignin, as well as pectin methylesterase enzyme activity (PME) in the roots of two soybean cultivars that differ with respect to Cd tolerance exposed to 0 and 23 μM Cd treatments. The bound forms of Cd in the roots were found to differ between the two soybean genotypes; 50.2% of the Cd in the root cell wall accumulates in the pectin in the highly Cd-tolerant and low Cd-accumulating cultivar HX3, while 50.6% of the root cell wall Cd accumulates in cellulose in the Cd-sensitive and high Cd-accumulating cultivar BX10. The total sugar and uronic acid concentrations of the cell wall components increased in response to Cd stress, while the concentrations of total sugars and uronic acid in BX10 were higher than in HX3 (except for hemicellulose 1). Increased demethylation of pectin may be the main reason that Cd is mainly concentrated in the primary wall in HX3, because the PME activity was higher in HX3 than it was in BX10 under Cd treatment. Furthermore, BX10 had a higher lignin concentration after Cd treatment, and showed the same change in cellulose. Cd in the root cell wall of BX10 was fixed in the secondary cell wall, which may be a result of the coupling to cellulose and lignin. In conclusion, root cell walls in soybean cultivars that differ in Cd tolerance may possess different mechanisms to prevent Cd from entering cells, and the sequestration of Cd in different cell wall components may determine the differences in Cd tolerance between the two genotypes.