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

Highlights: • Cd reduces endogenous auxin levels in Arabidopsis. • Exogenous applied auxin NAA increases Cd accumulation in the roots but decreases in the shoots. • NAA increases cell wall hemicellulose 1 content. • Hemicellulose 1 retains Cd and makes it difficult to be translocated to shoots. • NAA rescues Cd-induced chlorosis. -- Abstract: 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