Auxin distribution in root tips revealed by GUS staining. The use of the reporter gene line DR5:GUS allowed the microscopical visualization of the auxin distribution. Whole mount preparation of roots were stained and observed by optical microscopy. A) Optical microscopy images of DR5-GUS-stained root meristems from seedlings grown for 4 days in the magnet; the staining shows the distribution of auxin in the root tip. From top-left to bottom right: samples from the 1 g external control, 0 g*, 1 g* and 2 g* positions in the magnet. B) Quantitative study of the GUS staining by measuring the integrated optical density (I.O.D.) in the four-day experiment. Statistically significant differences in IOD (p < 0.05), compared to the control, are indicated with a ‘§’ symbol when compared with the 1 g external control, and with ‘#’ when compared with the 1 g* control.