AT-hook motif nuclear localized transcription factors function redundantly in promoting root growth through modulation of redox homeostasis.

Maintaining an optimal redox status is essential for plant growth and development, particularly when the plants are under stress. AT-hook motif nuclear localized (AHL) proteins are evolutionarily conserved transcription factors in plants. Much of our understanding about this gene family has been derived from studies on clade A members. To elucidate the functions of clade B genes, we first analyzed their spatial expression patterns using transgenic plants expressing a nuclear localized GFP under the control of their promoter sequences. AHL1, 2, 6, 7, and 10 were further functionally characterized owing to their high expression in the root apical meristem. Through mutant analyses and transgenic studies, we showed that these genes have the ability to promote root growth. Using yeast one-hybrid and dual luciferase assays, we demonstrated that AHL1, 2, 6, 7, and 10 are transcription regulators and this activity is required for their roles in root growth. Although mutants for these genes did not showed obvious defects in root growth, transgenic plants expressing their fusion proteins with the SRDX repressor motif exhibited a short-root phenotype. Through transcriptome analysis, histochemical staining and molecular genetics experiments, we found that AHL10 maintains redox homeostasis via direct regulation of glutathione transferase (GST) genes. When the transcript level of GSTF2, a top-ranked target of AHL10, was reduced by RNAi, the short-root phenotype in the AHL10-SRDX expressing plant was largely rescued. These results together suggest that AHL genes function redundantly in promoting root growth through direct regulation of redox homeostasis.