Response of root border cells to drought stress and their effects on physiological characteristics of roots of black soybean seedlings

Root border cells and root cap cells secrete mucilage into root rhizosphere which acts as a chemical, physical and biological interface between roots and their surrounding environment. The effects of root border cells (RBCs) and root mucilage on protection of roots from drought stress were carried out by two cultivation methods, a shaking cultivation method to remove RBCs from root tips and a static cultivation method to maintain RBCs adhered to the root tips. Compared to the control, the root elongations are decreased by about 18.1~21.6% and 28.8~30.4% , respectively under the treatments of the roots with 11% and 22% PEG, while the thicknesses of the mucilage layer around RBCs are increased by about 62.8%~140.7% and 133.8%~172.7%. These results suggest that the drought stress inhibits RBCs development and enhances mucilage secretion. The physical removal of RBCs from root tips results in a more severe drought-induced inhibition of root elongation, relative electric conductivity and malondialchehyche (MDA) content. The proline content in roots of black soybean with RBCs removed is significantly lower than that in roots with RBCs surrounding. These results suggest that RBCs adhered to the root tips tend to enhance RBCs mucilage secretion, decrease the damage to root cells, promote root growth, and thereby promote drought tolerance in black soybean seedlings.