Unravelling cell wall formation in the woody dicot stem

Populus is presented as a model system for the study of wood formation (xyl ogenesis). The formation of wood (secondary xylem) is an ordered developmen tal process involving cell division, cell expansion, secondary wall deposit ion, lignification and programmed cell death. Because wood is formed in a v ariable environment and subject to developmental control, xylem cells are p roduced that differ in size, shape, cell wall structure, texture and compos ition. Hormones mediate some of the variability observed and control the pr ocess of xylogenesis. High-resolution analysis of auxin distribution across cambial region tissues, combined with the analysis of transgenic plants wi th modified auxin distribution, suggests that auxin provides positional inf ormation for the exit of cells from the meristem and probably also for the duration of cell expansion. Poplar sequencing projects have provided access to genes involved in cell wall formation. Genes involved in the biosynthes is of the carbohydrate skeleton of the cell wall are briefly reviewed. Most progress has been made in characterizing pectin methyl esterases that modi fy pectins in the cambial region. Specific expression patterns have also be en found for expansins, xyloglucan endotransglycosylases and cellulose synt hases, pointing to their role in wood cell wall formation and modification. Finally, by studying transgenic plants modified in various steps of the mo nolignol biosynthetic pathway and by localizing the expression of various e nzymes, new insight into the lignin biosynthesis in planta has been gained.