Root growth in Arabidopsis depends on the amount of glutathione and not the glutathione redox potential

Activity of the root apical meristem and hence plant growth strictly depends on glutathione homeostasis. Despite compelling evidence for this dependency based on glutathione depletion, the cause for the growth arrest had remained unclear. Meristem control may depend on either the absolute amount of glutathione or on the glutathione redox potential (EGSH). To unambiguously distinguish those two options, we characterized an allelic series of six Arabidopsis mutants affected in glutamate-cysteine ligase, which catalyses the first step for the biosynthesis of reduced glutathione (GSH). When grown under the same conditions, even mutants with 20% and 50% of wild-type GSH amounts are slightly stunted. The most severely compromised mutants, zir1 and rml1, were crossed with either gr1, which lacks cyto-nuclear glutathione disulfide reductase and was used to induce a pronounced shift in EGSH, or with bir6, which has a diminished glutathione consumption and thus exhibits slightly increased levels of GSH. Based on theoretical considerations, these levels are not expected to shift the EGSH to any significant extent. Our study shows that deleting GR1 in the zir1 or rml1 background does not result in an obvious phenotypic change. By contrast, deleting BIR6 was sufficient to suppress the growth arrest in rml1 and to attenuate the growth restriction in zir1. These findings demonstrate that root growth is dependent on the availability of sufficient amounts of GSH, and not affected by pronounced changes in EGSH. This insight provides a decisive step towards understanding the mechanisms underpinning the proposed role of glutathione in growth control.