The PHR1-RSL2 module regulates root hair elongation induced by phosphate deficiency.

Glucose–TOR Signaling Regulates Root Hair Elongation in Arabidopsis via the RHD6-RSL4 Transcriptional Cascade

Root hairs are tubular protrusions of root epidermal cells that expand the root surface area to facilitate water and nutrient uptake. The target of rapamycin (TOR) kinase has been identified as a positive regulator of root hair elongation, and the RHD6-RSL4 bHLH transcriptional cascade is well established as a core module that governs root hair morphogenesis. However, whether TOR signaling acts upstream of the RHD6-RSL4 pathway and how glucose signals are integrated into this transcriptional regulatory network during root hair development remain incompletely understood. In this study, transcriptome profiling combined with pharmacological and genetic functional assays was performed to elucidate the TOR-mediated transcriptional regulatory pathway of root hair elongation in Arabidopsis. Chemical inhibition of TOR triggered genome-wide transcriptional reprogramming in seedling roots, including disruption of auxin and ethylene signal transduction and pronounced downregulation of hundreds of genes related to root hair development. Glucose-activated TOR signaling modulates the expression of root hair-specific (RHS) genes mainly through the core RHD6-RSL4 transcriptional cascade. The transcription of RSL1–RSL5 was strongly dependent on functional TOR activity, whereas RHD6 transcript abundance was specifically induced by glucose–TOR signaling under carbon-starvation recovery conditions. Genetic overexpression of either RHD6 or RSL4 partially rescued root hair elongation defects caused by TOR suppression, confirming that the RHD6-RSL4 cascade functions as a critical downstream transcriptional module of glucose–TOR signaling. Collectively, this work establishes a transcriptional framework in which glucose–TOR signals modulate root hair elongation via transcriptional activation of the master bHLH regulators RHD6 and RSL4.

SgPHR1-SgPAE1 module modulates root growth and increases low phosphorus tolerance in Stylosanthes guianensis.

Endophytic Flavobacterium promotes root hair development and enhances drought tolerance via an ERF–CEP5 hormonal regulatory module

The findings uncover a pathway that regulates RH growth as part of a broader, microorganism-dependent root system architecture plasticity under low-water conditions and highlight the potential of uncovering plant–microorganism mechanisms to strengthen crop resilience in a changing climate.

The circadian clock proteins PRR modulate root hair development via the RHD6/RSL module in Arabidopsis.

Root hairs, derived from trichoblasts, are critical for plant growth and environmental adaptation. Although environmental cues are known to influence root hair development, how endogenous timing systems such as the circadian clock integrate into the core transcriptional network governing root hair formation remains unclear. Here, we show that the circadian clock-associated protein PSEUDO-RESPONSE REGULATOR5 (PRR5) physically interacts with ROOT HAIR DEFECTIVE6 (RHD6) and RHD6 LIKE1 (RSL1), two basic helix-loop-helix transcription factors essential for root hair initiation. Genetic analyses suggest that PRR proteins contribute to root hair development under long-day conditions in Arabidopsis thaliana. Simultaneous disruption of PRR5, PRR7, and PRR9 results in defective root hairs, whereas PRR5 overexpression markedly increases root hair density and length. Transcriptomic and RT-qPCR analyses reveal that PRRs enhance the expression of RHD6, RSL1, and multiple downstream root hair-responsive genes, while modulating their temporal expression patterns. Furthermore, PRR5-mediated root hair promotion requires RHD6/RSL1, and PRR proteins enhance RHD6-dependent activation of the RSL4 promoter. PRRs also contribute to root hair development under phosphate-deficient and salt-stress conditions. Together, these findings establish a molecular framework in which PRR proteins regulate the RHD6/RSL network to coordinate root hair development and environmental responses.

Root hair formation capacity is associated with shoot growth under nutrient sufficiency and ETC3 induction under phosphate limitation in Arabidopsis

Root hairs enhance nutrient uptake by increasing root surface area. CAPRICE (CPC) and its homolog ENHANCER OF TRY AND CPC1 (ETC1) are R3-type MYB transcription factors that regulate root hair differentiation in Arabidopsis thaliana . Here, we report observations from a three-genotype comparison in Arabidopsis thaliana – wild type (WT), the root-hair-defective cpc etc1 double mutant, and the root-hair-enhanced CPC::CPC – grown hydroponically under nutrient-sufficient and deficient nitrogen (N), phosphorus (P) or potassium (K) conditions. Under nutrient-sufficient conditions, CPC::CPC exhibited increased leaf fresh weight and delayed bolting, relative to WT, accompanied by a higher leaf number. cpc etc1 also showed greater leaf fresh weight, attributable to increased weight per leaf. Flowering-related transcripts ( FT , CO , SOC1 ) showed no significant differences between WT and CPC::CPC . Under nutrient-deficient conditions, root hair number increased in WT under Low P and Low K, while CPC::CPC increased under Low P only; cpc etc1 remained low across conditions. At the transcriptional level, among CPC-family genes assayed in WT roots, expression of the CPC homolog ETC3 was upregulated under Low P, whereas CPC , ETC1 and TRY were not significantly altered. These results indicate that intrinsic root-hair formation capacity is associated with shoot growth and developmental timing under nutrient sufficiency and that phosphate limitation elicits a characteristic root hair response with selective ETC3 induction. These findings provide compact evidence linking epidermal cell-fate capacity to whole-plant traits and nutrient-responsive transcription.

Glutamine-Induced Root Hair Elongation Depends on Ethylene Signalling and the RHD6 Regulatory Pathway.

Glutamine (Gln), the first organic nitrogen (N) produced during primary N assimilation, is increasingly recognised as a signalling molecule in plants. Here, we show that Gln, supplied as the sole N source, promotes root hair elongation in Arabidopsis seedlings compared with NH4NO3. This response is dose-dependent and persists even under high NH4NO3 concentrations. Transcriptomic and gene expression analyses showed that Gln represses the negative regulator GLABRA2 while inducing the positive regulators ROOT HAIR DEFECTIVE6 (RHD6), RHD6-LIKE2, and RHD6-LIKE4, which control root hair development. Consistently, the rhd6-1 mutant failed to develop root hairs in response to Gln, demonstrating that Gln-induced root hair elongation depends on the RHD6 pathway. Gln also induces defence gene expression. Our pharmacological studies revealed that proper pectin methyl-esterification is required for both Gln-induced root hair elongation and defence gene expression, highlighting the importance of cell wall integrity in perceiving exogenous Gln. Furthermore, we show that Gln perception at the root apex triggers root hair elongation-a response that relies on defence hormone signalling, particularly ethylene, as demonstrated by pharmacological and genetic analyses. Together, these findings suggest that Gln integrates nutrient sensing, cell wall integrity, and defence hormone signalling to regulate root hair elongation in Arabidopsis.