Rachel Spicer and N. Michele Holbrook Harvard University
Heartwood formation occurs in nearly all known tree species and marks the senescence of secondary xylem, such that it no longer functions in water transport or carbohydrate storage in the living tree. The process of heartwood formation is driven by the death of parenchyma cells oriented both axially and radially within the stem. These cells exhibit a remarkable range of longevities (from 2 to 200 years, depending on species and site), but the actual cause of death is not known. This research is designed to test the hypothesis that parenchyma cell death during heartwood formation is a form of programmed cell death (PCD), and drives an important stage in woody plant development. This hypothesis will be tested through the application of molecular biological techniques to wood development, including the in situ detection of fragmentation of nuclear DNA (a hallmark of PCD) via a fluorescent-labeling assay (TUNEL assay), as well as tests for the formation of DNA fragments using gel electrophoresis. Changes in cell ultrastructure (i.e., changes in the morphology of cell membranes, mitochondria, and the nucleus) during parenchyma cell death will also be compared against those associated with other examples of PCD in plants. Species to be studied represent a range of xylem structures and physiologies, including a conifer (Eastern hemlock, Tsuga canadensis), a ring-porous angiosperm (white ash, Fraxinus americana), and a diffuse-porous angiosperm (bigtooth aspen, Populus grandidentata). Results from this work will create the foundation for a new, biological model of heartwood formation in trees.