Phosphorus Deficiency Enhances the Tolerance of Rice to the Acidic Soil Stress Eriko Maejima 1 , Toshihiro Watanabe 1 , Mitsuru Osaki 1 , Tadao Wagatsuma 2 1 Research faculty of Agriculture, Hokkaido University, Sapporo, 060-8589, JAPAN (ericom@chem.agr. hokudai.ac.jp - nabe@chem.agr.hokudai.ac.jp - mosaki@chem.agr.hokudai.ac.jp) 2 Faculty of Agriculture, Yamagata University, Tsuruoka, 997-8555, JAPAN (wagatuma@tds1.tr.yama gata-u.ac.jp) INTRODUCTION A major mechanism of aluminium (Al) toxicity in plants is the dysfunction of the plasma membrane (PM) caused by Al binding tightly to the negative sites on the PM surface. Surface negativity in PM of root cells is mainly derived from the phosphate group of phospholipids, which is usually neutralised by calcium (Ca) ions contributing to maintain the integrity of membrane permeability (Shoemaker and Vanderlick, 2003). It has been suggested that Al replaces the membrane-bound Ca and tightly binds to the negative sites of the phosphate group of phospholipids, making the membrane rigid and gel-like, finally resulting in an increase in the PM permeability (Deleers et al., 1986). The ratio of phospholipids/sterols in root-tip membranes was lower in the tolerant rice cultivars compared with the sensitive ones (Khan et al., 2009). These results indicate that phospholipid concentration (proportion) in PM of root cells is an important factor in Al tolerance of rice. In light of these reports, it may be possible to enhance the Al tolerance of rice by decreasing the proportion of phospholipids in root cells. Phosphorus (P) starvation triggers membrane lipid remodelling in root cells, a process that replaces a significant portion of membrane phospholipids with non-P-containing galactolipids, presumably to use the phospholipids as an internal P reserve (Andersson et al., 2005). Therefore, we hypothesize that rice plants grown under low-P conditions will be more tolerant to Al compared with those grown under moderate P conditions. METHODS Plant growth For pretreatment, rice (Oryza sativa L. cv. Koshihikari) seedlings were grown in a culture solution with (0.34 mM) (+P) or without (0 mM) (−P) phosphorus for 2 weeks; subsequently, the lipid and mineral concentrations were determined. After phosphorus pretreatment, seedlings were transferred to a solution with (0.1 mM) or without (0 mM) AlCl 3 for a week; subsequently, the mineral concentrations and Al tolerance were determined. Analysis of phospholipids and galactolipids Phospholipids and galactolipids were extracted using the Bligh and Dyer method. The lyophilised root sample was homogenised in extraction solution (2-propanol/chloroform/H 2 O, 2:2:1, v/v/v). Phospholipids were quantified by measuring the P content in the lipid. To determine the galactolipid concentration, the lipid extracts were separated on silicagel thin layer chromatography plates. The lipid bands comprising sugars were visualised with 2% anthrone–sulphuric acid. The plates were scanned with a scanner and analysed using ImageJ (ver. 1.46, Wayne Raband, National Institutes, of Health USA; http://rsb.info.nih.gov/ij). RESULTS AND DISCUSSION The proportion of each lipid component was 85:9:5 (phospholipid:galactolipid:sterol) with +P pretreatment while it was 65:28:7 (phospholipid:galactolipid:sterol) with −P pretreatment, indicating that concentraons of phospholipids and galactolipids in roots of rice seedlings aer −P