Abstract
The composition and structure of lignin in different tissues—phellem (cork), phloem and xylem (wood)—of Quercus suber was studied. Whole cell walls and their respective isolated milled lignins were analyzed by pyrolysis coupled with gas chromatography/mass spectrometry (Py-GC/MS), two-dimensional nuclear magnetic resonance spectroscopy (2D-NMR) and derivatization followed by reductive cleavage (DFRC). Different tissues presented varied p-hydroxyphenyl:guaiacyl:syringyl (H:G:S) lignin compositions. Whereas lignin from cork has a G-rich lignin (H:G:S molar ratio 2:85:13), lignin from phloem presents more S-units (H:G:S molar ratio of 1:58:41) and lignin from xylem is slightly enriched in S-lignin (H:G:S molar ratio 1:45:55). These differences were reflected in the relative abundances of the different interunit linkages. Alkyl-aryl ethers (β–O–4′) were predominant, increasing from 68% in cork, to 71% in phloem and 77% in xylem, as consequence of the enrichment in S-lignin units. Cork lignin was enriched in condensed structures such as phenylcoumarans (β-5′, 20%), dibenzodioxocins (5–5′, 5%), as corresponds to a lignin enriched in G-units. In comparison, lignin from phloem and xylem presented lower levels of condensed linkages. The lignin from cork was highly acetylated at the γ-OH of the side-chain (48% lignin acetylation), predominantly over G-units; while the lignins from phloem and xylem were barely acetylated and this occurred mainly over S-units. These results are a first time overview of the lignin structure in xylem, phloem (generated by cambium), and in cork (generated by phellogen), in agreement with literature that reports that lignin biosynthesis is flexible and cell specific.
Highlights
Lignin is the second most abundant polymer in vascular plants
The results provide a first time overview of the differences in the lignin structure in the xylem and phloem tissues originating from the vascular cambium, and in the cork generated by the phellogen
This study reports the differences in composition and structure of the lignins from different tissues—cork, phloem and xylem—of Q. suber
Summary
Lignin is the second most abundant polymer in vascular plants. Lignin deposition in the cell wall has a major importance for plant physiology and development: (i) by acting as the mechanical support of plant organs, it allows an upright growth and large sizes; (ii) it provides strength and rigidity to the cells; (iii) it allows transport of water and solutes in the vascular system due to its hydrophobicity and mechanical resistance; and (iv) it is associated to protection against pathogens (Boudet, 2000; Donaldson, 2001; Boerjan et al, 2003; Vanholme et al, 2008).Lignin Differs between TissuesRevealing the lignin structure and the lignification mechanism has been the subject of extensive research along the years. In the last decade other monomers have been recognized as participating in lignin polymerization, including hydroxycinnamic acids and aldehydes, as well as coniferyl and sinapyl acetates or coumarates (e.g., Ralph et al, 2004a; Grabber et al, 2010; Ralph, 2010) After their synthesis, the lignin monomers are transported to the cell wall where they are polymerized in a combinatorial fashion by free-radical coupling mechanisms in a reaction mediated by peroxidases, generating a variety of structures within the lignin polymer (Boerjan et al, 2003; Ralph et al, 2004a). The lignin molecule has a high chemical flexibility, i.e., the plant produces a lignin with a specific composition depending on the precursors that are been deposited in the lignifying zone (Boudet, 2000)
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