Tissue-Specific Developmental Changes in Lignin Deposition in Model Plants

Lignin is a plant cell wall phenolic polymer and the largest renewable source of aromatic carbon in nature. While lignin is essential for plant survival, little work has been carried out to understand its variation across plant tissues and developmental stages. Here, we combined microscopy, spectrophotometry, and mass spectrometry to compare lignin deposition in roots, stems, and leaves of the model plants Arabidopsis thaliana and Brachypodium distachyon across four developmental stages. Brachypodium accumulated more lignin and exhibited higher syringyl-to-guaiacyl (S/G) ratios than Arabidopsis in all stages and tissues. Lignin deposition increased across all tissues over development and was maintained during senescence, with stems and roots showing the largest lignin content and leaves contributing more substantially at senescence. Furthermore, lignification began with the deposition of G-units, followed by the accumulation of S-units, which became more predominant at later developmental stages in all tissues. Brachypodium contained more p-hydroxyphenyl (H) lignin than Arabidopsis, with the highest levels observed in roots compared to stems and leaves. Interestingly, while the S/G ratio in stems plateaued at maturity (R3 stage), roots of both species continued accumulating S-lignin during senescence (S4 stage). These results show that herbaceous monocots and dicots have different content and chemical compositions of lignin depending on the time of harvest, with implications for both biomass utilization and biological carbon sequestration.