A genomic approach to investigate developmental cell death in woody tissues of Populus trees

One EST library in POPULUSDB originates from woody tissues of the Populus stem where xylem fibers undergo cell death. Analysis of EST abundances and library distribution within the POPULUSDB revealed a large number of previously uncharacterized transcripts that were unique in this library and possibly related to the death of xylem fibers. The in silico analysis was complemented by a microarray analysis utilizing a novel Populus cDNA array with a unigene set of 25,000 sequences.In silico analysis, combined with the microarray analysis, revealed the usefulness of non-normalized EST libraries in elucidating transcriptional regulation of previously uncharacterized physiological processes. The data suggested the involvement of two novel extracellular serine proteases, nodulin-like proteins and an Arabidopsis thaliana OPEN STOMATA 1 (AtOST1) homolog in signaling fiber-cell death, as well as mechanisms responsible for hormonal control, nutrient remobilization, regulation of vacuolar integrity and autolysis of the dying fibers.The woody tissues of angiosperm trees, the xylem fibers and vessels, are formed from the lateral meristem of the stem, the vascular cambium. In contrast to vessel elements, which differentiate very rapidly close to the vascular cambium, fiber differentiation is a relatively slow process involving initial expansion of the cells in both the radial and longitudinal dimensions, followed by extensive synthesis of the secondary cell walls. The final phase in maturation of both vessel elements and fibers is cell death and autolysis of the cell contents.Xylem-cell death involves a range of morphological and nuclear changes in a strictly spatially and temporally coordinated and programmed fashion [1,2]. The programmed cell death (PCD) of xylem has been analyzed in detail in an in vitro system of Zinnia elegans, in which mesophyll cells of Zinnia transdifferentiate into xylem vessels commonly called as tracheary elements in a semi-synchronized manner [3]. In Zi