Lignin-Carbohydrate Nano-Sized Structures: An Evidence of Intracellular Lignin Biosynthesis?

Area of Science:

• Plant Biology
• Biochemistry
• Materials Science

Background:

• The precise spatial arrangement of secondary cell wall polymers in plants remains poorly understood.
• Previous models lack clarity on the intracellular origins and deposition of key components like lignin.

Purpose of the Study:

• To investigate the spatial localization and formation process of lignin within the secondary cell wall of Norway spruce.
• To propose a new model for secondary cell wall structure based on experimental findings.

Main Methods:

• Isolation and characterization of spherical nanoparticles from differentiating xylem using size exclusion chromatography.
• Visualization of nanoparticles via Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM).
• Compositional analysis of nanoparticles using pyrolytic Gas Chromatography-Mass Spectrometry (GC-MS), proton Nuclear Magnetic Resonance (¹H NMR) spectroscopy, and nitrobenzene oxidation.

Main Results:

• Spherical nanoparticles (20-50 nm) were observed on the surface of parallel-organized cellulose microfibrils.
• Lignin was detected within these nanoparticles during early cell wall formation.
• Analysis suggests lignin forms as a lignocarbohydrate complex intracellularly, likely within the Golgi apparatus and vesicles.

Conclusions:

• A new model for secondary cell wall structure is proposed, featuring cellulose microfibrils encased by lignin-carbohydrate complexes.
• These complexes possess a carbohydrate outer layer and an aromatic lignin core.
• Lignin formation is hypothesized to occur intracellularly before deposition into the cell wall.