Systematic analysis of the <scp>GRAS</scp> family genes in <i>Arabidopsis thaliana</i> reveals a role for <i>SCL14</i> in primary root growth and redox homeostasis

The GRAS family of proteins is evolutionarily conserved plant-specific transcription regulators involved in a diverse array of biological processes. Despite extensive research in the past decades, the functions of many GRAS family genes are still unclear, even in the case of Arabidopsis thaliana. In this study, we asked if these genes play a role in root growth and development. Although most of them were expressed in the root tip, we found that only a mutation in SCL14 caused root growth retardation. Transcriptome analysis revealed that glutathione metabolism was among the processes most significantly affected by the scl14 mutation. In line with this finding, we demonstrated that the scl14 mutant had an elevated level of hydrogen peroxide in its root apical meristem. The short-root phenotype of scl14 was exacerbated by H₂O₂ but ameliorated by glutathione and was nearly completely rescued by the overexpression of GSTU25, which was downregulated in scl14. In further experiments, we demonstrated that SCL14 directly regulates glutathione metabolism genes, including GSTU25. Based on these results, we conclude that SCL14 has a positive role in root growth and development and that it does so by maintaining redox homeostasis via modulating glutathione metabolism. We also obtained results that suggest that SCL14 promotes root growth mainly by stimulating mitotic activity in the meristem, but its role in stem-cell maintenance also contributes to root growth, especially under conditions that lead to oxidative stress.