Auxin‐Dependent Activation of RHD6‐RSL4 Cascade Promotes Root Hair Growth Under Boron Deficiency in Arabidopsis Primary Root Apices

Plants rapidly adjust their root system architecture to enhance resource uptake and cope with fluctuating soil environments. One such adaptive response is the development of root hairs (RH), which increase the effective root surface area and thereby improve water and nutrient acquisition. Here, we show that Arabidopsis seedlings respond rapidly to boron (B) deficiency by increasing both RH density and length at the root tip. This response is strongly correlated with enhanced auxin signaling in the root apex, as revealed by the auxin-responsive reporters IAA2::GUS and DR5rev:GFP. Our findings support the idea that B deficiency leads to auxin accumulation in the root tip, likely mediated by enhanced shoot-to-root auxin transport. Furthermore, by using pharmacological and genetic approaches, we provide evidence that enhanced auxin signaling via TIR1/AFBs in the RH zone activates the RHD6-RSL4 cascade to promote RH elongation under B deficiency. In contrast, the increase in RH density under B-deficient conditions appears to involve additional auxin-independent regulatory mechanisms.