Plant cell wall aromatics: influence on degradation of biomass

Plant cell walls are limited in bioconversion by aromatic constituents. Chemical and structural characteristics of specific cell walls in a variety of plants have been investigated in conjunction with their biodegradability using microscopic methods. Histochemistry, ultraviolet absorption microspectrophotometry, and response of cell walls to microorganisms and specific enzymes identified significant aromatics contributing to recalcitrance. Monocotyledonous plants, such as grasses, and dicotyledonous ones, such as alfalfa, have lignin-type compounds within the secondary walls and middle lamella of cell walls. Cell walls that are heavily lignified as shown by positive staining reactions with acid phloroglucinol and UV absorption near 280 nm appear to be the most resistant, with examples of vascular tissues in both monocots and dicots. Grasses, however, and especially warm-season species, are rich in low molecular weight phenolic acids ester-linked to sugars within their cell walls, occurring in both lignified and non-lignified cell walls. In non-lignified tissues of grasses, phenolic acids prevent microbial degradation and appear to be a major barrier to biodegradation, particularly in warm-season grasses, such as corn, millet, and bermudagrass. Modifications in amounts, types, and linkages of cell wall aromatics, either in naturally occurring mutants or cultivars developed by plant breeding, have effectively improved the biodegradability of lignocellulose. Microbial delignification by cellulase-less white-rot fungi as well as pretreatment with commercial ferulic acid esterases improve biodegradation of lignocelluloses. Cellulosic bast fibers, such as those from flax stems, are mostly free of lignin and are degraded by cellulases without further pretreatment. Recovery of aromatics after pretreatment or other processing means could provide value-added compounds and improve the economics of bioconversion. (C) 2008 Society of Chemical Industry and John Wiley & Sons, Ltd