Induction of Microsomal Membrane Proteins in Roots of an Aluminum-Resistant Cultivar of Triticum aestivum L. under Conditions of Aluminum Stress

Three-day-old seedlings of an Al-sensitive (Neepawa) and an Al-resistant (PT741) cultivar of Triticum aestivum were subjected to Al concentrations ranging from 0 to 100 [mu]M for 72 h. At 25 [mu]M Al, growth of roots was inhibited by 57% in the Al-sensitive cultivar, whereas root growth in the Al-resistant cultivar was unaffected. A concentration of 100 [mu]M Al was required to inhibit root growth of the Al-resistant cultivar by 50% and resulted in almost total inhibition of root growth in the sensitive cultivar. Cytoplasmic and microsomal membrane fractions were isolated from root tips (first 5 mm) and the adjacent 2-cm region of roots of both cultivars. When root cytoplasmic proteins were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, no changes in polypeptide patterns were observed in response to Al stress. Analysis of microsomal membrane proteins revealed a band with an apparent molecular mass of 51 kD, which showed significant accumulation in the resistant cultivar following Al exposure. Two-dimensional gel analysis revealed that this band comprises two polypeptides, each of which is induced by exposure to Al. The response of the 51-kD band to a variety of experimental conditions was characterized to determine whether its pattern of accumulation was consistent with a possible role in Al resistance. Accumulation was significantly greater in root tips when compared to the rest of the root. When seedlings were subjected to Al concentrations ranging from 0 to 150 [mu]M, the proteins were evident at 25 [mu]M and were fully accumulated at 100 [mu]M. Time-course studies from 0 to 96 h indicated that full accumulation of the 51-kD band occurred within 24 h of initiation of Al stress. With subsequent removal of stress, the polypeptides gradually disappeared and were no longer visible after 72 h. When protein synthesis was inhibited by cycloheximide, the 51-kD band disappeared even when seedlings were maintained in Al-containing media. Other metals, including Cu, Zn, and Mn, failed to induce this band, and Cd and Ni resulted in its partial accumulation. These results indicate that synthesis of the 51-kD microsomal membrane proteins is specifically induced and maintained during Al stress in the Al-resistant cultivar, PT741.