Auxin Gradients Are Associated with Polarity Changes in Trees

摘要

Wood grain pattern is determined by the orientation of elongated fusiform initial cells in the vascular cambium, the cylinder of soft-walled meristematic cells whose daughter cells differentiate to become new xylem and phloem (1). The orientation of the initials and the resulting grain pattern can remain approximately constant for the life of the tree, but the initials retain the ability to reorient in response to injury or other developmentally relevant events. This response is critical to the health of the tree because the grain direction is the principle direction of water and assimilates movement through the stem. Theories of wood grain patterning propose that the orienting signal is either mechanical strain in the cambium (2) or a concentration gradient of the plant hormone auxin in the plane of the cambium (3-5). We show that an auxin gradient exists in cambial tissue undergoing grain reorientation, following model predictions.