Genotypical differences in aluminum resistance of maize are expressed in the distal part of the transition zone. Is reduced basipetal auxin flow involved in inhibition of root elongation by aluminum?

Short-term Al treatment (90 mu M Al at pH 4.5 for 1 h) of the distal transi tion zone (DTZ; 1-2 mm from the root tip), which does not contribute signif icantly to root elongation, inhibited root elongation in the main elongatio n zone (EZ; 2.5-5 mm from the root tip) to the same extent as treatment of the entire maize (Zea mays) root apex. Application of Al to the EZ had no e ffect on root elongation. Higher genotypical resistance to Al applied to th e entire root apex, and specifically to the DTZ, was expressed by less inhi bition of root elongation, Al accumulation, and Al-induced callose formatio n, primarily in the DTZ. A characteristic pH profile along the surface of t he root apex with a maximum of pH 5.3 in the DTZ was demonstrated. Al appli cation induced a substantial flattening of the pH profile moreso in the At- sensitive than in the Al resistant cultivar. Application of indole-3-acetic acid to the EZ but not to the meristematic zone significantly alleviated t he inhibition of root elongation induced by the application of Al to the DT Z. Basipetal transport of exogenously applied [H-3]indole-3-acetic acid to the meristematic zone was significantly inhibited by Al application to the DTZ in the Al-sensitive maize cv Lixis. Our results provide evidence that t he primary mechanisms of genotypical differences in Al resistance are locat ed within the DTZ, and suggest a signaling pathway in the root apex mediati ng the Al signal between the DTZ and the EZ through basipetal auxin transpo rt.