Modified lignin promotes plant development by nitrate assimilation and iron-dependent metabolic reprogramming

In recent years, agricultural development has received considerable attention due to the growing global demand for sustainable food production and environmental stewardship. Trace elements and nitrogen are essential nutrients that critically regulate plant seedling growth. Lignin, the second most abundant natural organic macromolecule after cellulose, is abundantly present in the low-cost byproduct from industrial biorefineries. Here, the modified lignin compounds were prepared by grafting urea onto lignin via the Mannich reaction, which could promote rice seedling growth through the slow release of nitrogen during lignin degradation. The as-prepared modified lignin showed strong ion-chelating properties, which can simultaneously provide nitrogen nutrition and metal trace elements. Then, multi-technique characterization strategies for the resulting lignin-based complexes were performed to systematically evaluate their physicochemical properties, including DLS, TEM, GPC, FTIR, XPS, and ¹³C NMR. Multi-dimensional analytical strategies were also applied to analyze plant growing status, such as root/shoot dimensions and biomass, chlorophyll quantification and EdU staining. Last, RNA sequencing analysis unraveled the underlying molecular mechanisms of L-Urea/Fe on seedlings, including enhanced nutrient reservoir activity and significant upregulation of pathways related to nitrate transport, assimilation and response. Furthermore, α-amylase activity, iron-binding activity and FAD-binding (a key cofactor for iron-dependent oxidoreductases), were also significantly increased in seedlings. This work clarified the interactive regulatory network of nitrogen utilization, iron homeostasis and antioxidant defense of plants by modified lignin, providing solid evidence for the utilization of eco-friendly lignin-derived fertilizers to boost plants seedling growth.