Overexpression of AHL proteins enhances root hair production by altering the transcription of RHD6-downstream genes.

AT-HOOK MOTIF NUCLEAR LOCALIZED (AHL) proteins occur in all sequenced plant species. They bind to the AT-rich DNA sequences in chromosomes and regulate gene transcription related to diverse biological processes. However, the molecular mechanism underlying how AHL proteins regulate gene transcription is poorly understood. In this research, we used root hair production as a readout to study the function of two Arabidopsis AHL proteins, AHL17, and its closest homolog AHL28. Overexpression of or greatly enhanced root hair production by increasing the transcription of an array of genes downstream of RHD6. RHD6 is a key transcription factor that regulates root hair development. Mutation of completely suppressed the overproduction of root hairs by blocking the transcription of -activated genes. The overexpression of or , however, neither affected the transcription of nor the accumulation of RHD6 protein. These two AHL proteins also did not directly interact with RHD6. Furthermore, we found that three members of the Heat Shock Protein70 family, which have been annotated as the subunits of the plant Mediator complex, could form a complex with both AHL17 and RHD6. Our research might reveal a previously unrecognized mechanism of how AHL proteins regulate gene transcription.