Silica biomineralization in plants alters the structure of lignin

Biomineralization of silica is a major process in plants, which may contribute 3-10% of tissue dry weight. For reasons that remain unclear, plants containing silica are less sensitive to abiotic and biotic stress. In particular, the mechanisms of silica deposition and stress amelioration are still not fully understood. Silica resides mostly in the extracellular volume (the apoplast) which is made of the lignocellulosic cell wall. In a previous work we showed that synthetic lignin catalyses the formation of silica nanoparticles at RC-OSi(OH)₃ positions. Since the phenolic O-4 position is the most reactive during lignin polymerization, the binding sites form at the expense of β-O-4 lignin backbone bonds. Therefore, synthetic lignin becomes more branched when polymerized in the presence of silicic acid, as compared to lignin polymerized without silicic acid. To study lignin-silica relationships in the plant, we extracted lignin from stems of wild type sorghum and compared it to lignin extracted from mutants exhibiting high and low silica contents. The thermal stability of both non-extracted biomass and extracted lignin was measured using thermogravimetric analysis (TGA). High-silica biomass was thermally less stable than low-silica biomass, suggesting lower content of ether (β-O-4) linkages. This interpretation was supported by gas chromatography-mass spectroscopy (GC-MS). Fourier transform infrared (FTIR) and X-ray photoelectron spectra (XPS) indicated lignin with C-O-Si modifications in all genotypes and further showed silicic acid binding to lignin phenolics and carbonyl moieties. Our results show that lignin extracted from genotypes with native-silicon levels have higher affinity to silicic acid, catalysing silica deposition through Si-O-4 (Si-phenoxyl) bonds, and suggest that the presence of silicic acid during in vivo lignin polymerization reduces β-O-4 ether linkages.