Homeobox Protein 24 Mediates the Conversion of Indole-3-Butyric Acid to Indole-3-Acetic Acid to Promote Root Hair Elongation

The study identified HOMEOBOX PROTEIN 24 (HB24) as a key transcription factor involved in the conversion of indole‐3‐butyric acid (IBA) to indole‐3‐acetic acid (IAA), which is crucial for root hair elongation in Arabidopsis thaliana seedlings. Loss of HB24 function led to defective root hair growth, highlighting its role in regulating the expression of INDOLE‐3‐BUTYRIC ACID RESPONSE 1 (IBR1), an enzyme essential for the IBA-to-IAA conversion. The application of exogenous IAA or the auxin analogue 1‐naphthalene acetic acid (NAA) rescued the root hair growth defects in hb24 mutants, whereas IBA did not, further supporting HB24's role in this conversion process. Additionally, IAA was found to increase HB24 expression, indicating a feedback loop in the regulation of root hair elongation.