Cyclin B1 expression in root tips revealed by GUS staining. The use of the reporter gene line CYCB1:GUS allowed the microscopical visualization of the expression of the cyclin B1 gene. Whole mount preparations of roots were stained and observed by optical microscopy. A) 2 days’ and B) 4 days’ growth; from left to right: 1 g external control, 0 g*, 1 g* and 2 g*. C) and D) Quantitative study of the expression of the cyclin B1 gene by measuring the integrated optical density (I.O.D.) in the two-day (C) and in the four-day (D) experiments. 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. GUS staining show an overall decrease in the expression of this gene (which is usually considered to be a marker of cell proliferation)in the magnet. This is especially clear in samples from the simulated microgravity position (0 g*).