Exploring Structure–Function Relationship of Lignin Derivatives to Enhance Its Biostimulant Potential

Lignin is a promising biostimulant for enhancing plant growth and stress tolerance. In this study, bamboo kraft lignin (BKL) and its oxygen-alkali-modified derivative (OBKL) were applied to maize seedlings to elucidate the structure-function relationship. The increased hydrophilic groups of OBKL improved its capacity to enhance water and nutrient uptake, resulting in a 50% increase in root elongation, 65% enhancement in photosynthesis, and a 106% rise in transpiration. Low-molecular-weight OBKL promoted chlorophyll synthesis and stress resilience, whereas high-molecular-weight lignin suppressed growth. Transcriptomics analysis revealed that OBKL upregulated 1333 genes, including key transcription factors such as AP2/ERF, WRKY, MYB, and NAC, which are involved in regulating cell proliferation, tropisms, carbon metabolism, and stress responses. Genomes and Gene Ontology analysis further identified plant hormone signal transduction regulated by lignin as a major pathway. These findings reveal that OBKL is an effective biostimulant for improving crop productivity and environmental adaptability.