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Duraminization is the ultimate process of differentiation of wood cells and leads to the formation of the heartwood or duramen. It is responsible for the principal chemical (durability) and esthetic (color) properties which condition its utilization. Depending on the species, the characteristics of the heartwood are extremely variable. Some species, such as the walnut and the larch, produce highly colored heartwood which cannot be clearly differentiated from the sapwood in terms of color or durability (poplar).
The study of the formation and variability of the characteristics of heartwood is being approached by our group in species with well differentiated heartwood such as the cherry tree, teak, larch and walnut. For example, the walnut produces high quality wood which is colored and very sought after for cabinetmaking. We have chosen it as a model to study the physiologic and genetic mechanisms which regulate the production of phenol compounds synthesized during the formation of heartwood, and which are responsible for the color of the wood.
In general, the formation of heartwood is studied using descriptive (histology and analysis of the image of its extension), analytic (biochemistry and molecular biology) and genetic (selection and improvement programs) techniques.
In the walnut tree, our research mainly targets the study of phenol compounds (phenylpropanoids, flavonoids and naphtoquinones) (Burtin et al., 1998) and the enzymatic systems which control their syntheses (chalcone synthase, flavanone 3-hydrolase, dihydroflavonol 4-reductase and beta-glucosidase) (Duroux et al., 1998). At the level of the transition zone, beta-glucosidases may play a dominant role in the synthesis of juglone, a naphtoquinone characteristic of heartwood. The purification and determinaiton of the biochemical characteristics of this class of enzymes are in progress.
The development of molecular biological techniques is much more recent due to the problems inherent in the small numbers of living cells and the large quantities of phenol compounds present id old wood tissue (Label et al., 2000, Beritognolo et al., 2002).
We have constructed a library of cDNA which is reprresentative of the genes expressed in the sapwood-heartwood transition zone. Over 2000 clones have already been sequenced and annotated in the framework of collaborations with UREFV (INRA-Bordeaux) and the HTIRC (Purdue University, USA).
The comparison of expressend sequences in the different wood tissues (cambial zone, xylem and transition zone) and the utilization of microarray technology will make it possible to see which metabolic pathways are activated or repressed at all stages of differentiation of the wood, and also during infection of trunks by Erwinia, which is a major menace for the production of walnuts in France. This infection causes branches used for carpentry to fall; up to 80% of the trees are affected in certain areas in Isere. Based on the ESTs from resistant genotypes (J. nigra, J. regia x J. nigra) which are available only to our group, we can envisage collaborations with California (which is also affected) (UC Davis, Prof. G. McGranahan and A. Dandekar).
On the longer term, fundamental and technical knowledge acquired on the formation of heartwood in the walnut tree could be transferred to other species in order to study other aspects of duraminization. Because of the more and more restrictive European measures regulating chemical treatment of wood, the natural durability and the identification of the metabolic pathways which are involved constitute a research axis which may have important economic and ecologic consequences. The characterization and utilization of molecules of interest such as the phenolic molecules found in the duramen of certain species (tannins, flavonoids, naphtoquinones) with strong antioxidant properties which are toxic for pathogenic organisms (bacteria, fungi, insects) represent a future pathway for enhancing the value of wood products.