Aspen genes with significantly altered expression in Al-treated root tips compared to control root tips. A Root growth rates of plants used for the gene expression profiling. Roots were treated with 500 μM Al for 6 h, 54 h, and 246 h. During the 246 h treatment, the culture medium was exchanged every 2 d to maintain a constant Al stress. Means and standard error values from four independent roots are shown. Different letters indicate significant differences between treatments (ANOVA; p < 0.05). Number of up-regulated genes (B) and down-regulated genes (C) after 6 h, 54 h, and 246 h of Al treatment (≥ 2-fold change in expression).