Hormone signaling is involved in all biological system. Hormones in general control the physical and emotional responses of an organism. A specific example of a hormone is auxin. Auxin is a plant hormone that plays an inportant role in several plant activities, such as: phototropism, gravitopism, apical dominance, fruit development, abscission, and root initiation. Auxin travels in a specific direction inside a plant, it is taken up by a particular cell at one end and release at the other end onto another cell. It binds specific receptors on specific cells to cause the plant to perform the abilities listed above. An example of such is ABP1 (Auxin binding protein 1). The binding of auxin to ARF1 (Auxin response factor 1) can also affect the gene transcription process; this overall affects the gene expression in the plant (Kimball).
In respect to phototropism and gravitropism, the hormone auxin is responsible for the bending of stems and roots of a plant in response to a gravitropic or phototropic stimulus.
Gravitopism are fast bending responses induced by gravity. Gravity also distributes auxin ,and is believed to cause the microtubule responses rather than just auxin itself. As for phototropism, light responses of microtubules show a fast reorientation into longitudinal array, which inhibits cell elongation at one side of the stem, causing the plant to bend in the opposite direction. Growth of the shoot apex usually inhibits the development of the later buds on the stem beneath, vice versa; auxin induces the formation of longitudinal microtubules in the roots, and the stem cell elongation is inhibited; the phenomenon is known as apical dominance. Young leaves and fruits produce auxin and so as long as they do so, they remain attached. When the auxin level declines an abscission layer forms at the base of the petiole or...