A1 is a major component of soils, and free A1 ions solubilized at pH values of 5.0 and below are a major growthlimiting factor. A1 inhibits root growth within 1 to 2 h of the start of exposure, and also interferes with the uptake of severa1 elements and water from root apices. Although many different mechanisms of A1 toxicity have been hypothesized, they have not yet been fully characterized (for reviews, see Foy, 1974;Foy et al., 1978;Haug, 1984; Taylor, 1988; Rengel, 1992;Kochian, 1995).The primary site of A1 accumulation and toxicity in the plant is reported to be the root meristem, which consists of actively dividing cells (Rincón and Gonzales, 1992;Ryan et al., 1993). At the cellular level, A1 is mainly localized in the cell wall, although some is found in the nucleus. Although the ultimate target that mediates A1 toxicity has not yet This work was supported in part by grants-in-aid for general scientific research (no. 05660074), co-operative research (no. 07306020), and for scientific research in priority areas (no. 05278106) from the Ministry of Education, Science, Sports and Culture of Japan, by the Ohara Foundation for Agricultura1 Science, and by a SUNBOR from the Suntory Institute for Bioorganic Research.* Corresponding author; e-mail yoko@rib.okayama-u.ac.jp; fax 81-86-434-1210.been identified, an increase in the rigidity of the actin network (Grabski and Schindler, 1995) and the inhibition of phospholipase C (Jones and Kochian, 1995) have been proposed as possible intracellular A1 target sites that may be involved in root growth (for reviews, see Rengel, 1992;Delhaize and Ryan, 1995;Kochian, 1995).We have previously investigated the responses of tobacco (Nicotiana tabacum L.) suspension culture cells (Yamamoto et al., 1994;Ono et al., 1995) to Al. The sensitivity of tobacco cells to A1 changes depending on the phase of growth. Cells at the logarithmic phase are sensitive to Al, whereas cells at the stationary phase are resistant to it. Logarithmic-phase cells start to accumulate A1 after about 10 h of exposure, and there is a good correlation between the extent of growth inhibition by A1 and the amount of A1 accumulated in the cells. However, since the stationary-phase cells do not take up Al, it appears that accumulation of Al,, such as that which occurs in actively growing cells (Yamamoto et al., 1994), is a prerequisite for A1 toxicity. Although we have not determined the cellular sites to which A1 binds, the localization of A1 in tobacco cells has been estimated by staining with hematoxylin, which indicated the presence of A1 over the entire cell surface and in the nucleus. Because A1 that has accumulated in cells is not released by chelators (EDTA and citrate), but is instead retained in cells after digestion of the cell walls by protoplasting enzymes, it seems likely that A1 is either absorbed by the cell or is tightly bound to the plasma membrane or some minor part of the cell wall that remains after digestion (Yamamoto et al., 1994).It has been reported that the peroxidation of lipids is ca...
