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Wood cells accumulate the major proportion of terrestrial biomass in their thick cell walls, composed of cellulose, matrix polysaccharides and lignin. By interacting with cellulose and lignin, matrix polysac- charides affect mechanical and chemical properties of cell walls in developing and mature wood cells, which determines wood quality traits, such as fiber length, wood and fiber strength and durability, and properties important in industrial wood processing such as pulping and saccharification. Our research is aiming at elucidating these matrix functions and effects in wood cells.

Matrix polysaccharides are synthesized and modified by Carbo- hydrate Active Enzymes (CAZYmes) and by enzymes acting on non-carbohydrate types of side chains.These enzymes reside either in the Golgi apparatus where they synthesise matrix polymers, or in cell walls where they modify them post-synthet- ically.We analyzed the genomic complement of CAZYmes in Populus and identified the ones highly expressed in developing wood. Similar analyses are currently being carried out in Picea. Co-expression analyses help us to identify novel unknown function genes that may be involved in matrix biosynthesis or modification. Most studied genes belong to large multi-gene families whose members may have distinct or overlapping functions, or else be non-functional copies.Their functions are studied using transgenic Populus and Arabidopsis.
Another tool used to study the role of different matrix com- ponents in wood properties is the transgenic overexpression of various microbial enzymes having defined specificity to matrix components in Populus. These enzymes, when targeted to cell walls,modify specific chains of the matrix,revealing their intricate interactions among different wall components and their overall role during wood cell development.

Matrix modification affects wood cell expansion and thus regulates the size and shape of wood cells. Fibers and vessel elements have different ways of expansion. Fibers elongate by intrusive tip growth and radially expand by symplastic growth. In contrast, vessel elements do not elongate, but they radially expand by a combination of radial intrusive and symplastic growth. Accordingly, a highly regulated modification of matrix in developing wood cells is required to achieve these different effects.

A model of growth stress generation and XET action in maturing fibers. Crystallization of cellulose microfibrils (single beige rods) around xyloglucan (yellow) would generate longitudinal tensional stress within the macrofibril (large beige rods) during fiber maturation. XET enzyme (blue) is present associated with xyloglucan between wall layers. Any displacement between the wall layers driven by the shrinkage of cellulose macrofibrils will be quickly stopped by the XET-mediated transglycosylation to a free xyloglucan acceptor thus creating a cross-link between cell wall layers.

Activity of XET enzyme in the specific cell wall layers visualized by the incorporation of the fluorescent substrate to the XET transglycosylation product. Such activity could be detected in tension wood fibers years after their initiation, indicating that the matrix is being modified even in dead cells.We have found that pectin matrix, which cements wood cells together, is extensively modified during xylogenesis. Pectin methyl esterases (PME) remove methyl ester side chains, which makes the pectin matrix susceptible to degradation by pectate lyases or polygalacturonases, and prone to formation of intra-molecular cross-links involving calcium ions that rigidify the walls.We have found that pectin desterification by PME1 inhibits the intrusive tip growth of fibres and radial expansion of vessel elements. Surprisingly, pectin integrity was found to affects wood extractability and saccharification even though pectin is a minor component of wood.This suggests that pectin and xylan could be interconnected in wood cell walls.l

Hemicelluloses bind to cellulose fibrils by hydrogen bonding and potentially can cross-link them, which is thought to deter- mine wall mechanical properties. Indeed, the modification of xyloglucan by xyloglucan endotransglycosylases (XETs) affected wood cell growth.We found that they mediate incorporation of newly synthesized xyloglucan to the tightly-bound xyloglucan cell wall network, and that they also act on cell walls after cell death. Such long-lived enzymatic activity could mediate devel- opment of growth stresses in the wood. We have recently discov- ered that substantial xylan transglycosylase activity accompanies secondary wall deposition. This activity unexpectedly was found to affect the direction of cellulose microfibrils deposition in secondary walls, the mechanism of which is now being studied..
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