The PHR1-RSL2 module regulates root hair elongation induced by phosphate deficiency.
Phosphorus (P) is an essential macronutrient for plant growth and development. Root hairs enhance P acquisition as inorganic phosphate (Pi) from soil by expanding the root surface area, and their elongation is a key adaptive response to low Pi availability. However, the transcriptional regulators that couple Pi starvation signaling to root hair elongation remain largely unknown. Here, we demonstrate that PHOSPHATE STARVATION RESPONSE1 (PHR1), the central transcription factor of the Pi starvation response, positively regulates Pi deficiency-induced root hair elongation in Arabidopsis. RNA-seq analysis of root tips identified ROOT HAIR DEFECTIVE 6-LIKE 2 (RSL2), a bHLH transcription factor governing root hair elongation, as a prominent PHR1-regulated target. We show that PHR1 binds to the promoter of RSL2 to activate its expression, and genetic analysis confirms that RSL2 acts downstream of PHR1. Further RNA-seq analysis revealed that RSL2 regulates cell wall remodeling genes, among which XYLOGLUCAN ENDOTRANSGLUCOSYLASE/HYDROLASE 26 (XTH26) was identified as a key target. RSL2 binds to the promoter of XTH26 to upregulate its transcription, and XTH26 overexpression partially rescues the reduced root hair length of both phr1 and rsl2. Collectively, our findings delineate a PHR1-RSL2 transcriptional module that orchestrates root hair elongation under Pi deficiency, thereby contributing to enhanced Pi acquisition.