IRON DEFICIENCY RESHAPES THE POPLAR ROOT AND EXUDATE METABOLITE PROFILES AND DRIVES EXUDATE-MEDIATED IRON MOBILIZATION
Iron (Fe) deficiency under alkaline conditions limits Fe bioavailability and reshapes plant metabolism. We asked how Fe deficiency at pH 7.5 alters the metabolite profiles of roots and root exudates of Populus x canescens, and whether exudates mobilize Fe at high pH. Hydroponically grown plants were deprived of Fe for 20 and 27 days. Roots and exudates, collected in nutrient solution and in distilled water, were profiled by UHPLC-QTOF MS, and metabolites were annotated using MS/MS molecular networking (MetGem) and public spectral libraries. Fe deficiency caused significant global shifts in both root and exudate metabolomes. In roots, phenylpropanoids increased at 27 days (proanthocyanidins, neochlorogenic acid, naringenin), whereas the salicylate glycoside salicortin decreased, suggesting a redirection of phenylpropanoid flux. In nutrient-solution exudates, eight flavonoids, including quercitrin, accumulated strongly at 27 days, and catecholic coumarins such as fraxetin were detected in roots and exudates. Exudates from Fe-deficient plants increased Fe mobilization from Fe(III) oxides at pH 7.5, and quercitrin solubilized Fe as efficiently as fraxetin. Alkaline Fe deficiency thus reconfigures poplar root and exudate metabolomes, and exuded flavonoids, not only coumarins, may contribute to Fe mobilization at high pH.