Selenite inhibited cadmium translocation and stimulated root growth of Brassica rapa L.: Regulation of element uptake, polysaccharide synthesis and crosslink, and cell wall enzymes

Previously studies have demonstrated that selenium (Se) can reduce cadmium (Cd) concentration in plants. However, its underlying mechanisms on relationships among Cd uptake/translocation, plant root growth, essential elemental uptake, cell wall components (CWCs), and cell wall enzymes were not fully elucidated. In this study, a hydroponic system was set up to explore the above relationships in a pakchoi variety (Brassica rapa L., LvYou 102) exposed to solo– or co– exposure of Cd (0.5 mg L–1) and Se (0.5 mg L–1) using a combined analysis of physiology and transcriptome. The results showed that Se reduced shoot Cd concentration and its translocation from roots to shoots. Se significantly stimulated root growth and root concentrations of potassium and magnesium, but reduced shoot concentrations of all tested essential elements as compared to the Cd treatment alone. Se significantly enhanced galacturonic acid (GalA) and total sugar concentrations in pectins but reduced that in hemicellulose I (HCI); while Se significantly increased GalA concentration but reduced total sugar concentration in hemicellulose II (HCII) when compared to the Cd treatment alone. Se enhanced activities of pectinesterase (PME), β–galactosidase (β–Gal), and xyloglucan endoglycosidase/hydrolase (XTH), but did not alter that of polygalacturonase (PG) and cellulase (CE) when compared to the Cd treatment alone. Under Cd exposure, Se in most cases down–regulated genes correlated with elemental uptake, degradation of pectins and hemicelluloses, and synthesis of cellulose and lignins, except for some up–regulated genes including XTH27, XTH22 and BraA10g 021100.3 C.