Fungal Laccase-Catalyzed Oxidation of Naturally Occurring Phenols for Enhanced Germination and Salt Tolerance of Arabidopsis thaliana: A Green Route for Synthesizing Humic-like Fertilizers

Abstract

Fungal laccases have been highlighted as a catalytic tool for transforming phenols. Here we demonstrate that fungal laccase-catalyzed oxidations can transform naturally occurring phenols into plant fertilizers with properties very similar to those of commercial humic acids. Treatments of Arabidopsis thaliana with highly cross-linked polyphenolic products obtained from a mixture of catechol and vanillic acid were able to enhance the germination and salt tolerance of this plant. These results revealed that humic-like organic fertilizers can be produced via in vitro enzymatic oxidation reactions. In particular, the root elongation pattern resulting from the laccase products was comparable to that resulting from an auxin-like compound. A detailed structural comparison of the phenol variants and commercial humic acids revealed their similarities and differences. Analyses based on SEM, EFM, ERP, and zeta-potential measurement showed that they both formed globular granules bearing various hydrophilic/polar groups in aqueous and solid conditions. Solid-phase C NMR, FT-IR-ATR, and elemental analyses showed that more nitrogen-based functional and aliphatic groups were present in the commercial humic acids. Significant differences were also identifiable with respect to particle size and specific surface area. High-resolution (15 T) FT-ICR mass spectrometry-based van Krevelen diagrams showed the compositional features of the variants to be a subset of those of the humic acids. Overall, our study unraveled essential structural features of polyaromatics that affect the growth of plants, and also provided novel bottom-up ecofriendly and finely tunable pathways for synthesizing humic-like fertilizers.

One-Pot Transformation of Technical Lignins into Humic-Like Plant Stimulants through Fenton-Based Advanced Oxidation: Accelerating Natural Fungus-Driven Humification

“… 29 Solid-state NMR analyses have clearly demonstrated heterogeneous mixtures of aromatic and aliphatic groups to constitute one of the key structural features of commercial humic acids. 8 Thus, the use of Fenton-based oxidations of lignin powders would be a scalable way to effectively transform aromatic lignins into humic-like structures containing aliphatic groups. This method also overcame our previous humification method, 8 which hardly generated aliphatic structures.…”

“… 8 Thus, the use of Fenton-based oxidations of lignin powders would be a scalable way to effectively transform aromatic lignins into humic-like structures containing aliphatic groups. This method also overcame our previous humification method, 8 which hardly generated aliphatic structures.…”

“…Seed germination and NaCl-related salt tolerance tests were based on the methodologies previously described. 8 Briefly, the seeds were germinated on the MS media, each of them containing commercial humic acids, lignin, and lignin variants. Radicle and cotyledon emergence after 2 and 4 days of the incubation were monitored and photographed with a microscope (Olympus).…”

“… 7 Polymeric products obtained from oxidative polymerization of natural phenols such as catechol and vanillic acid were proven to be effective at stimulating Arabidopsis plants. 8 …”

“… 14 In addition, phenols and carboxylic acids in aromatic polymers have been proven to directly stimulate plants, indicating that oxygen incorporation leading to the formation of such hydrophilic functional groups allows lignins to directly boost the stimulation of plant growth and development. 8 …”