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.