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 7 d, 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.