Exogenous auxin alleviates cadmium toxicity in Arabidopsis thaliana by stimulating synthesis of hemicellulose 1 and increasing the cadmium fixation capacity of root cell walls

Auxin is involved in not only plant physiological and developmental processes but also plant responses to abiotic stresses. In this study, cadmium (Cd²⁺) stress decreased the endogenous auxin level, whereas exogenous auxin (α-naphthaleneacetic acid, NAA, a permeable auxin analog) reduced shoot Cd²⁺ concentration and rescued Cd²⁺-induced chlorosis in Arabidopsis thaliana. Under Cd²⁺ stress conditions, NAA increased Cd²⁺ retention in the roots and most Cd²⁺ in the roots was fixed in hemicellulose 1 of the cell wall. NAA treatment did not affect pectin content and its binding capacity for Cd²⁺, whereas it significantly increased the content of hemicellulose 1 and the amount of Cd²⁺ retained in it. There were highly significant correlations between Cd²⁺ concentrations in the root, cell wall and hemicellulose 1 when the plants were subjected to Cd²⁺ or NAA+Cd²⁺ treatment for 1 to 7d, suggesting that the increase in hemicellulose 1 contributes greatly to the fixation of Cd²⁺ in the cell wall. Taken together, these results demonstrate that auxin-induced alleviation of Cd²⁺ toxicity in Arabidopsis is mediated through increasing hemicellulose 1 content and Cd²⁺ fixation in the root, thus reducing the translocation of Cd²⁺ from roots to shoots.