Molecular Mechanisms of Biostimulants in Promoting Tomato Seedling Growth: Linking Chemical Structure to Physiologic Function

Humic acids (HAs), tea polyphenols (TPs), and fulvic acids (FAs) are bioactive substances known for their antimicrobial, antioxidant, and plant growth-promoting properties. However, the chemical structures and molecular mechanisms underlying their growth-promoting effects remain unclear. Solid-state ¹³C nuclear magnetic resonance analysis reveals that FA has the most aromatic and oxygen-containing groups, followed by HA and TP, with a molecular weight ranking of HA > TP > FA. Transcriptomic and metabolomic analyses reveal that these biostimulants unregulated genes related to IAA, C and N metabolism, and pathogen resistance, leading to enhanced synthesis of amino acids, lipids, vitamins, and sugars while improving N and P utilization efficiency. Small molecular weight and aromatic compounds with carboxyl groups and phenolic hydroxyl (especially those with high O/C or (O + H)/C ratios) are crucial for stimulating hormone synthesis and nutrient uptake. These findings provide valuable insights into developing novel fertilizers and artificially regulating humic substances.