Morphological and physiological response of the root tip border cells to Fe²⁺ toxicity in rice

Toxicity of Fe2+ is one of the major constraints for lowland rice production in tropical and subtropical areas. However the mechanism of Fe-induced inhibition of root growth and the reasons for the spatial variations in Fe2+ sensitivity among the apical root zones are still poorly understood. The root tip is a primary site of Fe2+ toxicity in rice. The root border cells (BC), which originate from the root cap meristem by mitosis, can separately carry out metabolism and resist adverse stress through a series of distinct responses. In this study, the response mechanism of the root tip cells to Fe2+ toxicity were tested by comparing response of rice varieties Azucena (Fe-tolerance) and IR64 (Fe-sensitive) known to vary in Fe2+ resistance at a whole-root level. Results showed that Fe2+ toxicity inhibited the development of BC. However, compared to IR64, Fe2+ at 100~200 μmol/L was propitious to the development of BC in Azucena. With increase of Fe2+ concentration, the viability of the rice BC became lower, the cell wall of root tip outermost cells became thicker, and some characteristics of programmed cell death were observed in the cells (Fe-sensitive variety). Meanwhile, Fe2+ concentration affected the activities of peroxidase (POD), catalase (CAT), and superoxide dismutase (SOD). Higher enzyme activities were found in Fe-tolerance variety under high concentration Fe2+ (≥200 μmol/L) compared to control. These results indicated that, under the Fe2+ toxicity, root tip could resist Fe2+ toxicity by increasing BC, thickening cell wall, maintaining high POD, CAT and SOD activities.