Unravelling cell wall formation in the woody dicot stem

Abstract

Populus is presented as a model system for the study of wood formation (xylogenesis). The formation of wood (secondary xylem) is an ordered developmental process involving cell division, cell expansion, secondary wall deposition, lignification and programmed cell death. Because wood is formed in a variable environment and subject to developmental control, xylem cells are produced that differ in size, shape, cell wall structure, texture and composition. Hormones mediate some of the variability observed and control the process of xylogenesis. High-resolution analysis of auxin distribution across cambial region tissues, combined with the analysis of transgenic plants with modified auxin distribution, suggests that auxin provides positional information 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 biosynthesis of the carbohydrate skeleton of the cell wall are briefly reviewed. Most progress has been made in characterizing pectin methyl esterases that modify pectins in the cambial region. Specific expression patterns have also been found for expansins, xyloglucan endotransglycosylases and cellulose synthases, pointing to their role in wood cell wall formation and modification. Finally, by studying transgenic plants modified in various steps of the monolignol biosynthetic pathway and by localizing the expression of various enzymes, new insight into the lignin biosynthesis in planta has been gained.

Functional genomics and cell wall biosynthesis in loblolly pine

“…Only recently have EST projects in pine and poplar begun to provide information on the genes expressed in woody plants in wood-forming tissues, to address the question of whether there may be unique genes expressed during secondary xylem formation (Allona et al, 1998;Sterky et al, 1998;Mellerowicz et al, 2001, this issue). There has also been little information on the possible number of unique genes present in pines and other gymnosperms that are not found in herbaceous or woody angiosperms.…”

Structure, organization, and functions of cellulose synthase complexes in higher plants

“…In aspen, the degree of polymerization of cellulose chains in the primary cell wall is approximately 4,200. In contrast, 9,200 glucose residues form cellulose chains in the secondary cell walls of aspen wood (Mellerowicz et al, 2001).…”

“…The most remarkable one is concerning the quantities of cellulose and lignin. The amount of lignin in secondary cell walls of trees like Populus trichocarpa (poplar) is 19-21%, whereas in the primary cell wall lignin is absent (Mellerowicz et al, 2001). The content of cellulose, expressed in dry weight, in the primary cell wall of poplars is 20-30%, whereas in the secondary cell wall it is 40-50% (Mellerowicz et al, 2001).…”

“…The amount of lignin in secondary cell walls of trees like Populus trichocarpa (poplar) is 19-21%, whereas in the primary cell wall lignin is absent (Mellerowicz et al, 2001). The content of cellulose, expressed in dry weight, in the primary cell wall of poplars is 20-30%, whereas in the secondary cell wall it is 40-50% (Mellerowicz et al, 2001). For instance, some clones of Eucalyptus grandis cultivated in Brazil have a cellulose content of 43.9 -49.7% in the secondary cell wall (Gomide et al, 2005).…”

“…In some special cases, when the developing angiosperm wood is under tension stress, a gelatinous layer (G layer) is formed instead of the S2 or S3 layer (Timell, 1969;Mellerowicz et al, 2001;Pilate et al, 2004). The G layer is composed almost exclusively of highly crystalline axially oriented microfibrils of cellulose.…”