Crop root Exudate Composition-Dependent Disassembly of Lignin-Fe-Hydroxyapatite Supramolecular Structures: A Better Rhizosphere Sensing Platform for Smart Fertilizer Development

Abstract

A one-pot/one-step process of lignin-Fe coordination is implemented to synthesize lignin-Fe-hydroxyapatite (HA) supramolecular structures, which can be disassembled and subsequently surface-engineered by root exudate-derived organic acids. The extent of disassembly is superior to the corresponding HCl-induced pH change, suggesting that organic acids directly hamper non-covalent interactions involving lignin components and ferric ions. This disassembly coincides with the facilitated co-release of crop nutrients (i.e., ferric ions) and stimulants (i.e., lignin components) in a time-dependent manner. The resulting exposed core HAs are further surface-engineered with organic acids, thus accelerating the release of crop nutrients (i.e., calcium and phosphate ions) through the particle dissolution. Finally, lignin-Fe-coated HAs significantly enhances Zea mays growth rates under heat and salt stress compared to bare ones, suggesting that the increased organic acid secretion by the roots under abiotic stresses accelerates the disassembly and subsequent dissolution of the coated particles. Given that conventional rhizosphere-responsive fertilizers are mainly based on H+ ion-mediated structural changes, the approach provides a more effective means to detect the rhizosphere and crop stress state by exploiting the nature-inspired relationship between small organic acids and supramolecular polyphenolic assemblies.