• Indole-3-acetic acid (IAA) is a predominant form of active auxin in plants. In addition to de novo biosynthesis and release from its conjugate forms, IAA can be converted from its precursor indole-3-butyric acid (IBA).
• The IBA-derived IAA may help drive root hair elongation in Arabidopsis thaliana seedlings, but how the IBA-to-IAA conversion is regulated and affects IAA function requires further investigation.
• In this study, HOMEOBOX PROTEIN 24 (HB24), a transcription factor in the zinc finger-homeodomain family (ZF-HD family) of proteins, was identified. With loss of HB24 function, defective growth occurred in root hairs. INDOLE-3-BUTYRIC ACID RESPONSE 1 (IBR1), which encodes an enzyme involved in the IBA-to-IAA conversion, was identified as a direct target of HB24 for the control of root hair elongation. The exogenous IAA or auxin analogue 1-naphthalene acetic acid (NAA) both rescued the root hair growth phenotype of hb24 mutants, but IBA did not, suggesting a role for HB24 in the IBA-to-IAA conversion.
• Therefore, HB24 participates in root hair elongation by upregulating the expression of IBR1 and subsequently promoting the IBA-to-IAA conversion. Moreover, IAA also elevated the expression of HB24, suggesting a feedback loop is involved in IBA-to-IAA conversion-mediated root hair elongation.

中文翻译：

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HOMEOBOX PROTEIN 24 介导吲哚-3-丁酸转化为吲哚-3-乙酸以促进根毛伸长

• Indole-3-乙酸 (IAA) 是植物中活性生长素的主要形式。除了从头生物合成和从其缀合物形式释放之外，IAA 还可以从其前体吲哚-3-丁酸 (IBA) 转化。
• IBA 衍生的 IAA 可能有助于推动拟南芥幼苗的根毛伸长，但如何调节 IBA 到 IAA 的转化并影响 IAA 功能需要进一步研究。
• 在这项研究中，HOMEOBOX PROTEIN 24 (HB24) 是蛋白质锌指同源域家族 (ZF-HD 家族) 中的一种转录因子。随着HB24功能的丧失，根毛出现生长缺陷。INDOLE-3-BUTYRIC ACID RESPONSE 1 ( IBR1 ) 编码参与 IBA 到 IAA 转化的酶，被确定为 HB24 控制根毛伸长的直接靶标。外源 IAA 或生长素类似物 1-萘乙酸 (NAA) 都挽救了hb24突变体的根毛生长表型，但 IBA 没有，这表明HB24在 IBA 到 IAA 的转化中起作用。
• 因此，HB24通过上调IBR1的表达并随后促进 IBA 到 IAA 的转化参与根毛伸长。此外，IAA 还提高了HB24的表达，表明反馈回路参与 IBA 到 IAA 转换介导的根毛伸长。