Plant cells are delimited by an extra-cellular matrix called the cell wall. It is a complex structure, composed of a network of cellulose microfibrils interconnected by hemicellulose chains and incrusted by pectin and protein. Cells that are dead at maturity may impregnate their walls with lignin or suberin, which makes walls impervious to water. The exact relationships between various cell wall components are not fully understood. However, it is well established that the cell wall structure and composition changes during cell development, affecting its shape, size and function. In plants, the cell function depends heavily on the nature of its cell wall. This is true for the living cells as well as for cells that undergo a programmed cell death as a part of their differentiation, like most xylem cells. Wood fibers that we use are nothing but the cell walls remaining following xylem cell death. The nature and composition of the wood fibers are determined well before cell death, of course, and they are formed in stages. Following cell division in the cambium, the primary wall is formed while the cell grows in size. The growth is co-ordinated among cells and plasmodesmatal connections are maintained and formed to ensure symplastic continuity. While many cell types do not form any additional wall layers, most xylem cells and some supportive cell types outside the xylem produce a thick secondary cell wall layer. It is variability within this layer that most strongly affects wood fiber properties. Some xylem cells may form, in addition, a thin tertiary cell wall layer or, in the case of tension wood fibers, the gelatinous layer, with distinct properties. Thus, xylem cell wall formation is a complex process that requires co-ordinated activities of many enzyme systems. Despite a high level of interest in them, most enzymes involved in cell wall formation in the xylem are still not known.

Using high throughput sequencing of ESTs from the developing xylem region in poplar, we isolated a number of cDNAs that are potentially involved in the process of cell wall formation. These include cDNAs encoding carbohydrate metabolising enzymes such as cellulose synthase, xyloglucan endo transglucosyldase (XET), xylan hydrolase and cellulase, expansins, various cytoskeleton proteins and Rec13-like small GTPases. To investigate the role of these genes in the process of wood fiber formation and their effects on fiber properties we have adopted a broad approach. Firstly, we make proteins in heterologous systems and produce polyclonal antibodies in egg yolk. The antibodies are subsequently used to identify each protein in plant extracts, for example on western blots, and in sections where we can immunolocalise it in various cell types. With this technique we are able to determine where and when during xylem cell development a given enzyme is expressed. This is essential for establishing each enzyme’s role in fiber biosynthesis. Intracellular enzyme localisation will be followed by the immuno gold TEM technique. In parallel, we produce GFP-tagged fusion proteins in suspension cultures. In this system, we can observe intracellular enzyme localisation in vivo with laser confocal microscopy and test several signaling molecules for their involvement in various proceses, such as enzyme secretion. Secondly, we transform poplar to obtain enzyme overproducing and down-regulated trees. These trees will be used to determine the effects of each enzyme on the cell wall structure and composition. We also use Arabidopsis, as a model system to study cell wall formation in secondary xylem. When grown in appropriate conditions, Arabidopsis will produce secondary xylem composed of vessel elements and fibers. We found extensive natural variability in secondary xylem of Arabidopsis that can serve as a basis for discovering QTLs. In addition, mutagenesis in Arabidopsis opens up possibilities of finding new genes involved in fiber biosynthesis.

Selected publications

 

Mellerowicz, E.J., Riding R.T., Coleman W.K. and Little C.H.A. () Periodicity of cambial activity in Abies balsamea. I. Effects of temperature and photo period on cambial dormancy and frost hardiness. Physiologia Plantarum 85: 515-525.

 

Mellerowicz, E.J., Riding R.T. and Little C.H.A. () Periodicity of cambial activity in Abies balsamea. II. Effects of temperature and photo period on the size of nuclear genome in fusiform cambial cells. Physiologia Plantarum 85: 526-530.

 

Mellerowicz, E.J., Riding R.T. and Little C.H.A.() Nucleolar activity in fusiform cambial cells of Abies balsamea: effect of season and age. American Journal of Botany 80: .

 

Lloyd, A.D., Mellerowicz E.J., Chow C.H., Riding R.T. and Little C.H.A. () Fluctuations in ribosomal RNA gene content and nucleolar activity in the cambial region of Abies balsamea (Pinaceae) shoots during reactivation. American Journal of Botany 81: .

 

Zhong, Y., Mellerowicz E.J., Lloyd A.D., Leinhos V., Riding R.T. and Little C.H.A. () Seasonal variation in the nuclear genome size of ray cells in the vascular cambium of Fraxinus americana. Physiologia Plantarum 93: 305-311.

 

Mellerowicz, E.J., Riding R.T. and Greenwood M.S. () Nuclear and cytoplasmic changes associated with maturation in the vascular cambium of Larix decidua. Tree Physiology 15: 443-449.

 

Lloyd, A.D., Mellerowicz E.J., Riding R.T and Little C.H.A. () Changes in nuclear genome size and relative ribosomal RNA gene content in the vascular cambium of Abies balsamea during the annual activity-dormancy transition. Canadian Journal of Botany 74: 290-298.

 

Uggla, C., Mellerowicz E.J. and Sundberg B. () Endogenous indole-3-acetic acid controls cambial growth by positional signaling in Pinus sylvestris (L.). Plant Physiology 117: 113-121.