The wood-forming tissues of trees offer an attractive experimental system in which to correlate tissue-specific expression with stages in a defined developmental gradient. In the December 4 Proceedings of the National Academy of Sciences, Magnus Hertzberg and colleagues from the Umeå Plant Science Center in Umeå, Sweden, describe a transcript-profiling study of xylogenesis in poplar trees (Proc Natl Acad Sci USA 2001, 98:14732-14737).

They collected tangential sections across the wood development region in Populus tremula X Populus tremuloides (hybrid aspen). Samples were hybridized to cDNA microarrays containing nearly three thousand unique ESTs from hybrid aspen. Hierarchical clustering allowed them to define 539 genes grouped into classes with developmentally regulated expression patterns.

Genes expressed in the cambial meristem and early expansion zones encode regulators of cell cycling, cell expansion, tip growth, and primary cell wall biosynthesis. The regions involved in late expansion and secondary cell-wall biosynthesis...