In vivo measurement of GGT activity in wild-type and ggt1 knockout A. thaliana roots. GGT activity is measured in vivo as the release of p-nitroaniline by Arabidopsis roots placed in a spectrophotometric cuvette containing 4.6 mM γ-glutamyl-p-nitroanilide; the solution was cycled through home-made piping from the cuvette to the spectrophotometer cell using a peristaltic pump (see Materials and methods for details). Plants incubated with 100 μM glutathione were harvested after 3 h and 6 h of incubation; single plant roots were transferred to a 5 ml tube containing 4.4 ml of bioassay solution. The change in absorbance at 407 nm induced by releasing p-nitroaniline into the assay medium was recorded for 10 min. Inset graph: spectrophotometric recording of the assay solution in contact with wild-type or ggt1 roots. Values are means ±SE of at least five replicates.