Auxin, or indole-3-acetic acid, was the first plant hormone discovered. It was discovered by chance, in a human nutrition study conducted in the early twentieth century. One subject in that study had a disease that caused him to excrete large amounts of auxin in his urine. For a long period of time following that initial discovery, auxin was thought to be the sole plant hormone. Considerable research was conducted on the effects of exogenous auxin application on plant development and on internal levels of auxin like activity, with the goal of explaining the regulation of plant growth and development.
In plants, auxin is synthesized primarily in the apical meristem. It is actively transported basipetally, from the apical meristem toward the root. In vegetative tissues, this basipetal movement of auxin has three broad developmental roles that regulate plant architecture. The first is enhancing cell elongation and the differentiation of xylem elements below the apical meristem. The second is apical dominance, which is the suppression of lateral bud outgrowth at nodes below the apex in the current season's shoot. The third major function is the stimulation of root growth.
In perennials, auxin production affects development in woody tissues. Its production in the vascular cambium leads to the differentiation of xylem elements. It is also responsible for apical control, which is the regulation by the apex of shoot and spur development in the established woody structure of the plant. A secondary site of auxin synthesis is in developing leaves and fruit. Following synthesis, auxin moves basipetally from the leaf blade or fruit into the petiole or pedicel. There, its role is to maintain the abscission layer in a "young" state, so that the leaf or fruit remains attached to the plant. As leaf or fruit senescence occurs, auxin levels decline, and cells differentiate into an abscission zone, leading to leaf or fruit drop.
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