Structural variation of humic-like substances and its impact on plant stimulation: Implication for structure-function relationship of soil organic matters

Here, five aromatic monomers, one bearing a long alkyl chain [3-pentadecylphenol (3-PP)], the second bearing a polycyclic aromatic hydrocarbon [dihydroxynaphthalene (DHN)], the third bearing an organic amine [l-3,4-dihydroxyphenylalanine (l-DOPA)], the fourth bearing a carboxylic acid [vanillic acid (VA)], and the fifth bearing a phenol [catechol (CA)] were oxidatively coupled to produce four humic-like substances (3-PP, DHN, l-DOPA, and CAVA products) to mimic the diverse organic architectures of natural humus. Analysis using several methods, including SEM, EPR, elemental analysis, FT-IR-ATR, ¹³C NMR and anti-oxidant capability, revealed that each of the monomeric structures was well incorporated into the corresponding humic-like substances. Seed germination acceleration and NaCl-involved abiotic stress resistance of Arabidopsis thaliana were then tested to determine whether the different structures resulted in different levels of plant stimulation. The l-DOPA, CAVA and DHN-based materials showed enhanced stimulatory activities compared with no treatment, whereas the effects of the 3-PP-based materials were meager. Interestingly, high-resolution (15 T) ESI FT-ICR mass spectrometry-based van Krevelen diagrams clearly showed that the presence of molecules with H/C and O/C ratios ranging from 0.5 to 1.0 and 0.2 to 0.4, respectively, could be connected with such biological actions. Here, the l-DOPA sample showed the highest content of such molecules, followed by the CAVA, DHN and 3-PP samples. Next, the ability of l-DOPA and CAVA products to induce resistance in A. thaliana to a pathogen-related biotic stress was tested to confirm whether the proposed molecular features are associated with multi-stimulatory actions on plants. The expression level of pathogenesis-related protein 1 and inspection of plant morphology clearly revealed that both the l-DOPA and CAVA products stimulate plants to respond to biotic stresses. Size-exclusion chromatography together with NMR and IR data of both the materials strongly suggests that lignin-like supramolecular assemblages play an important role in versatile biological activities of humus.