Aluminum (Al) toxicity is an important factor in determining the distribution of plant species and ecotypes in the natural habit at crop productivity in the acid soils. However, the mechanism of Al-induced inhibition of root growth and the reasons for the spatial variations in Al sensitivity among the apical root zones are still poorly understood. The root tip is a primary site of Al toxicity in senior plants. The root border cells, which originate from the root cap meristem by mitosis, can separately carry out metabolism and resist adverse stress through a series of distinct responses after being detached from the root tips. Furthermore, the root border cells also play many crucial roles in protecting the root tip from the abiotic stress, which especially can participate in detection and tolerance of aluminum (Al) toxicity. The toxin effects of Al³⁺ on the border cells and response mechanism of the root border cells to Al³⁺ toxicity were tested by comparing response of soybean varieties Zhechun No. 2 (Al-tolerance) and Huachun No. 18 (Al-sensitive) known to vary in Al resistance at a whole-root level. The response of the root border cells to Al toxicity and its mechanism of resisting Al toxicity clearly were further investigated.
The study shows that the higher viability was found in the root border cells clinging to the root tip than the detached border cells under Al³⁺ stress. After being exposed to Al³⁺ for 24 h, the border cells clinging to the root tip from two varieties significantly reached more than 74% viability, and that of the detached border cells was lower when exposed to Al³⁺ for 12 h. The viability of Zhechun No. 2 and Huachun No. 18 was just 44.58% and 26.16% respectively at 400 μmol/L Al³⁺. No matter clinging to the root tip or detached, the viability of the soybean border cell became lower and lower with increase of Al³⁺ concentration and treatment time. However, the viability of the detached border cells in Al-tolerance variety had an inflexion at 200 μmol/L Al³⁺ treated for 6 h, i.e., higher concentration Al³⁺（≥200 μmol/L）was propitious to the border cells viability. With increase of Al³⁺ concentration, both the PME activity and relative root elongation inhibit rates increased, and those of the Al-sensitive variety were higher than Al-tolerance. At the same time, Al³⁺ concentration was influential on secretion of mucilage and the mucilage layer became much thicker with increase of Al³⁺ concentration. Under different Al³⁺ concentration and treatment time, these results indicated that Al can speed up death of the border cells. The higher penetrative was propitious to the viability of the border cells and the exfoliate cell mortality was correlated with production of mucilage under greater Al³⁺ concentration. Al binding to the cell walls, root elongation inhibited and secretion of mucilage represented an important pathway in the response of the soybean root cap to Al toxicity. Results also suggest that the Al-resistance mechanisms in the border cells levels were consistent with those of the soybean whole plant levels.