HOMEOBOX PROTEIN 24 mediates the conversion of indole‐3‐butyric acid to indole‐3‐acetic acid to promote root hair elongation

Abstract

Summary 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.

PUB30‐mediated downregulation of the HB24‐SWEET11 module is involved in root growth inhibition under salt stress by attenuating sucrose supply in Arabidopsis

“…Previous study has found that the loss‐of‐function of the HB24 gene affects root hair growth but not root growth on conventional medium (Zhao et al ., 2021). However, under moderate salt stress (i.e.…”

“…However, under moderate salt stress (i.e. 125 mM NaCl), the roots of 5‐d‐old hb24‐1 and hb24‐2 mutants grew significantly shorter than those of the wild‐type (Col‐0), while the overexpression lines ( HB24OE6‐4 and HB24OE7‐9 ; Zhao et al ., 2021) showed opposite phenotypic response, suggesting that HB24 was specifically involved in salt stress‐induced root growth inhibition (Fig. 1a,b).…”

“…(2017). The hb24‐2 was constructed by CRISPR/Cas9 (Zhao et al ., 2021). The CDS sequence of HB24 and PUB30 was cloned into pCAMBIA1300 vectors.…”

“…The pub30-1 (SALK_012549) and pub30-3 (SALK_073739) were reported by Zhang et al (2017). The hb24-2 was constructed by CRISPR/Cas9 (Zhao et al, 2021). The CDS sequence of HB24 and PUB30 was cloned into pCAMBIA1300 vectors.…”

“…The CDS sequence of HB24 and PUB30 was cloned into pCAMBIA1300 vectors. The complementation lines of HB24:HB24-GFP#1 and HB24:HB24-GFP#2, and overexpression lines 35S:HB24OE6-4, 35S:HB24OE7-9, and pSuper: HB24-MYC were reported by Zhao et al (2021). The hb24-1 pub30-1, double-mutant hb24-1 sweet11, sweet11 HB24OE7-9, and pub30-1 pSuper:HB24-MYC were obtained by cross genetics.…”