(A) Enzymatic transformation of 5-Fluorocytosine (5-FC) into toxic 5-Fluoro-UMP by FCY and UPP enzymes. (B) Transactivation system used to express a conditional negative marker in the root cap. The Q0171 transgenic line contains a GAL4 activator gene (in T-DNA) driven by a minimal promoter (enhancer trap), which is transactivated in the root cap by a cell layer-specific gene X (black arrow). After crossing with a line containing the conditional ablation marker FCY-UPP driven by UAS, GAL4 binds to the UAS sequence, activating transcription of the GFP reporter gene as well as FCY-UPP (red arrows). (C) Effect of 5FC (3.8 mM) on GFP expression in the transgenic Q0171>>FCY-UPP line. The line contains a transactivating GFP marker (green) and an FCY-UPP fusion in the root cap. 5FC treatment abolishes expression of genes in the root cap (as visualized by a strong reduction in GFP signal) while keeping cells alive (as visualized by the absence of PI staining in the nuclei). The heat shock treatment control promotes cell death (visualized as PI nuclear staining). Images are focused on the surface of the lateral root cap. Magnifications of the images are shown in the inserts. Red: PI, green: GFP. Scale bars: 100 μm.