Lignification and Lignin Manipulations in Conifers

Lignification is integral to wood formation and has been studied in great detail in conifers for decades. This effort has resulted in detailed knowledge about the chemical composition, structure and content of lignin in different cell and wood types. Lignin distribution and biochemical composition has been resolved at an ultra-structural level, and structural models for conifer lignin have been established. Recent years have seen significant advances in our molecular-level understanding of lignification, and in conifer monolignol biosynthesis. The majority of the genes involved have been identified and the molecular functions of several have been experimentally verified. Suppression of lignin-related genes confirmed that lignin is vital for plant fitness and vascular integrity in conifers and established that conifers do not tolerate substantial reductions in lignin content. Significant gaps in our understanding of conifer lignification nevertheless remain. Aspects of lignification about which we still know relatively little include: the regulatory cascades that trigger lignification, metabolic connections between monolignol biosynthesis and other metabolic processes, the cellular biology of monolignol biosynthesis, the transport of monolignols to the apoplast, the role of monolignol glucosides in lignification, the process of lignin initiation, and the interaction of lignin with other cell wall polymers such as non-cellulosic polysaccharides. These significant gaps in our understanding provide ample opportunity for new and exciting discoveries on lignification in conifers.