SHADE AVOIDANCE 4 Regulates Magnesium Deficiency‐Induced Root Hair Development by Regulating Auxin Transport in <i>Arabidopsis</i>

Root hairs enlarge root surface and facilitate plant exploration for edaphic resources, particularly under nutrient-limiting conditions. While magnesium (Mg) deficiency is known to markedly stimulate root hair development, the underlying molecular mechanisms remain poorly characterised. Here, we elucidate the critical role of SHADE AVOIDANCE 4 (SAV4) in regulating Mg deficiency-induced root hair elongation through modulating epidermal auxin level in Arabidopsis. Root hair elongation under Mg deficiency was significantly suppressed in sav4 mutant, and SAV4 epidermal-specific expression partially rescued its root hair elongation defect. Further analysis revealed that SAV4 expression was upregulated in root tips under Mg deficiency. Attenuated root hair elongation in sav4 mutants is explained by reduced basipetal auxin transport in the root epidermis, caused by an inability to maintain the abundance of the membrane-localised auxin efflux carrier PIN2. The endocytosis of PIN2 is delayed in Arabidopsis roots under Mg deficiency, which is released in sav4 mutant and thus impairs basipetal auxin transport. Our findings establish a molecular framework wherein SAV4-mediated dynamics of PIN2 at the epidermal membrane under Mg deficiency ensures proper basipetal auxin transport, thereby facilitating root hair elongation.