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Structural Characterization of Lignin from Different Types of Wood in Cactaceae Species by NMR, Py-GC/MS and ATR-FTIR: a Biological Aproach
Lignin confers resistance to the cellular wall to withstand the negative pressures during transpiration and plays an important role in the defense against pathogens. The acquisition of lignin biosynthesis is a fundamental adaptation which gave plants the ability to colonize terrestrial ecosystems and its evolution has been parallel to that of tracheophytes. Thus, lignin composition has been used as a marker for characterizing major plant groups. Evolitionarily, the Cactaceae family is considered as one of the most surprising radiations of succulent plants in the New World angiosperms. The evolution of succulent growth form in this family was greatly influenced by internal anatomical novelties in the stem, particularly those of Wood (i.e., increased ability of wood for water retention, very specialized tracheary elements with a limited secondary cell wall extension and a decreased lignification rate, translated into a low wood accumulation). Here we studied the lignin structure from different types of wood in four Cactaceae species with different stem morphologies (Pereskia lychnidiflora, tree/fibrous wood; Opuntia streptacantha and Pilosocereus chrysacanthus, tree/succulent fibrous wood; Ferocactus hamatacanthus, cylindrical stem/dimorphic wood) in order to determine their relationship with the wood anatomy in an evolutionary-adaptive context. Wood lignin was isolated and analyzed by pyrolysis coupled with gas chromatography and mass spectrometry (Py-GC/MS), two-dimensional nuclear magnetic resonance spectroscopy (2D-NMR) and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). In dimorphic wood, the ferulates and the so-called γ-acetylated guaiacyl-syringaresinol complexes act as nucleation sites for lignification and as cross-links between lignin and carbohydrates at the WBT-fiber junctions. The exceptional predominance (>90%) of the syringil units in dimorphic wood lignin of the most succulent species is considered a structural defense mechanism. Evolutionarily, the generalized low γ-acetylation (≤ 5%) found in the wood lignin is evidence of the high specialization of the wood elements in the conduction/storage of water.