Negative feedback regulation of GLABRA1 contributes to epidermal cell patterning in the Arabidopsis root

GLABRA1 (GL1), which encodes an R2R3 MYB transcription factor, is a key regulator of trichome patterning in the aerial organs of Arabidopsis (Arabidopsis thaliana). Although it has been generally assumed that GL1 functions exclusively in shoots and is not expressed in roots, reverse transcription polymerase chain reaction (RT-PCR) analysis has revealed that GL1 is indeed expressed in roots. To investigate whether GL1 plays a role in root epidermal patterning, we analyzed the effects of gl1 mutations in sensitized genetic backgrounds. Our findings show that gl1 mutants enhance the root epidermal phenotype of a weak allele of the werewolf (wer) mutant and suppress the phenotype of the caprice (cpc) mutant. We also demonstrate that the GL1 promoter is active in N-position epidermal cells, and that the GFP-GL1 fusion protein is predominantly localized in the nucleus of N-position cells. Furthermore, we provide evidence that GL1 expression is positively regulated by WER, GLABRA3, ENHANCER OF GLABRA3, and TRANSPARENT TESTA GLABRA1, while negatively regulated by CPC, TRIPTYCHON, and GLABRA2 (GL2). Notably, GL2, which is positively regulated by GL1, moderately represses GL1 expression, and both GL1 and GL2 are positively regulated by WER in N-position cells. These findings suggest that a negative feedback regulation of GL1 expression via GL2 contributes to the fine-tuning of non-hair cell fate determination in Arabidopsis root epidermis.