We define the collective resistance that plants and plant materials pose to deconstruction from microbes and enzymes as “biomass recalcitrance.” This trait developed in terrestrial

Biomass Recalcitrance: Deconstructing the Plant Cell Wall for Bioenergy. Edited by Michael. E. Himmel © 2008 Blackwell Publishing Ltd. ISBN: 978-1-405-16360-6

plants during evolutionary maturation, in part, as a consequence of their moving from the protection of the aquatic environment.

Although little is known about the definitive steps involved or the intermediate forms explored, modern plants possess many systems for protection. The first line of defense in most plants is the epidermis, or outer layer of the plant anatomy. In grasses, this layer usually contains dense collections of thick-walled cells, as well as specialized cells that secrete waxy or oily materials. In trees, the bark presents a considerable physical, as well as chemical, barrier to all but the most dedicated assault.

Plant defense systems extend to the structure and organization of vascular tissue and even of the cell wall. Buried in the cell wall are the elementary fibrils that harbor the cellulose core (4). Even cellulose poses a significant barrier to enzyme action, where the highly ordered and water-excluding nature of the crystallite is sufficient to significantly retard cellulase action. This point is made especially clear when considering that the processive cellulase, cellobiohydrolase II, has been estimated from kinetic data to break about 14 bonds per second (5). Cell wall microfibrils are surrounded by sheaves of hemicellulose that, in turn, is covalently linked to lignins. This matrix of heteropolymers in which cellulose is embedded is certainly the dominant reason why plant biomass has resisted low-cost chemical and enzymatic treatment schemes.