Wood Formation in Trees

Abstract

Wood is the most important natural and endlessly renewable source of energy and therefore has a major future role as an environmentally cost-effective alternative to burning fossils fuels. The major role of wood is not only the provision of energy but also the provision of energy-sufficient material for our buildings and many other products. In addition, developing wood cells represent one of the most important sinks for excess atmospheric CO2, thereby reducing one of the major contributors to global warming. Wood is the fifth most important product of the world trade. Vast quantities of wood are logged by foresters to provide fuel, fibers (for pulp, paper products, and boards), and sawn timber (for house building and furniture) as commodities. The complex chemical makeup of wood (cellulose, hemicelluloses, lignin, and pectins) also makes it an ideal raw material for what could be a future “ligno-chemical” industry that could replace the petrochemical industry, in providing not only plastic and all kinds of chemical products, but also food and textile products. Resource analyses have led to the conclusion that wood and fiber needs over the next 40 years can only be met by logging 20% to 40% of the total present standing timber inventory in the natural forest (Food and Agriculture Organization). However, most agree that some of these forests should be either left completely alone or managed with only a minimum of wood extraction to preserve their environmental value. The solution is to try to accomplish what agriculture has been doing for the last few centuries: grow wood as a crop in the same way that we grow wheat and maize. The future of the world's forests as well as the forest products industry will therefore depend to a huge extent on our ability to domesticate wild tree species and tailor them for maximum economic yield in the highly controlled environments typical of agriculture. The domestication of forest trees must be accomplished rationally, using the best available modern scientific methods, to develop high-yielding, intensively managed plantation forests, occupying a small percentage of existing forested land. In particular, the safe and careful application of biotechnology (marker-assisted breeding, genetic engineering, and in vitro propagation) to forestry practices, should help develop genetically superior trees in a time span of only a few decades. Considering the important role that wood is foreseen to play in the near future, it is surprising to see that our understanding of how wood develops is far from complete. With a few exceptions, very little is known about the cellular, molecular, and developmental processes that underlie wood formation. Xylogenesis represents an example of cell differentiation in an exceptionally complex form. This process is controlled by a wide variety of factors both exogenous (photoperiod and temperature) and endogenous (phytohormones) and by interaction between them. It is driven by the coordinated expression of numerous structural genes (some of known function) involved in cell origination, differentiation, programmed cell death, and heartwood (HW) formation and by virtually unknown regulatory genes orchestrating this ordered developmental sequence. The presence of gene families and the extreme plasticity of the metabolism involved (as exemplified by the unusual behavior of plants with transformed cell walls; for review, see Fagard et al., 2000) add a further complexity to our understanding of the process of wood formation. The making of wood spans a broad range of topics. Rather than to attempt a detailed review of all recent works in the different fields of wood formation, it is an objective of this contribution to give a brief overview and generalized concepts of this developmental process. We first open this Update by reviewing the well-known stages leading to the formation of wood, starting from the activities in a secondary meristem (the vascular cambium) to programmed cell death via the maturation of cambial derivatives. Emphasis is made on the biosynthesis and deposition of the main cell wall compounds, as well as the mediation of this ordered developmental process by phytohormones and transcription factors. The considerable plasticity in anatomical, chemical and physical wood properties is then described in view of providing a unique opportunity to dissect the molecular and biochemical mechanisms involved in such differences. Finally, the current progress in the molecular biology of wood development is presented. Wood (secondary xylem) is manufactured by a succession of five major steps, including cell division, cell expansion (elongation and radial enlargement), cell wall thickening (involving cellulose, hemicellulose, cell wall proteins, and lignin biosynthesis and deposition), programmed cell death, and HW formation. The developmental biology of wood has been recently extensively reviewed and the following reading will provide the reader with ess