Arabidopsis γ-glutamylcyclotransferase affects glutathione content and root system architecture during sulfur starvation

γ-Glutamylcyclotransferase initiates glutathione degradation to component amino acids l-glutamate, l-cysteine and l-glycine. The enzyme is encoded by three genes in Arabidopsis thaliana, one of which (GGCT2;1) is transcriptionally upregulated by starvation for the essential macronutrient sulfur (S). Regulation by S-starvation suggests that GGCT2;1 mobilizes l-cysteine from glutathione when there is insufficient sulfate for de novo l-cysteine synthesis. The response of wild-type seedlings to S-starvation was compared to ggct2;1 null mutants. S-starvation causes glutathione depletion in S-starved wild-type seedlings, but higher glutathione is maintained in the primary root tip than in other seedling tissues. Although GGCT2;1 is induced throughout seedlings, its expression is concentrated in the primary root tip where it activates the γ-glutamyl cycle. S-starved wild-type plants also produce longer primary roots, and lateral root growth is suppressed. While glutathione is also rapidly depleted in ggct2;1 null seedlings, much higher glutathione is maintained in the primary root tip compared to the wild-type. S-starved ggct2;1 primary roots grow longer than the wild-type, and lateral root growth is not suppressed. These results point to a role for GGCT2;1 in S-starvation-response changes to root system architecture through activity of the γ-glutamyl cycle in the primary root tip. l-Cysteine mobilization from glutathione is not solely a function of GGCT2;1.