Chemical messengers that are produced in one part of an organism are hormones. Plant hormones differ from animal hormones because they don't maintain homeostasis and there are no endocrine organs that produce hormones. When plants respond to stimuli it's controlled by hormones. There are five major hormones: Auxin, Gibberellins, cytokinins, abscisic acid, and ethylene.
Auxin affects meristematic growth, cell elongation in shoots, cell division, growth of lateral buds, production of xylem, development of fruits, and can help produce ethylene. Auxin causes the root to grow downward and shoot to grow upward. It's responsible for the bending of plants towards light and water. It also makes trees produce more xylem for thickness. Auxin causes plasma membrane to transport four ions out o the cell. Synthetic auxins are used to kill weeds. Agent Orange is used to kill unwanted trees or to control weeds. Agent Orange is not used anymore due to the fact it harmed animals as well as plants.
Gibberellins stimulate elongation of stem cells and cell division. It can also help to fix problems with faulty, odd plants. This hormone helps the development leaves, sexual flowering plants and helps differentiate phloem tissue. Also it can stimulate production of auxin, as well as make plants more responsive to auxin treatment.
Another hormone is cytokinin, which stimulates cell division. Cytokinin is involved in growth and production of fruits and seeds. It also prevents against dormancy and slows the aging of cut leaves or fruit.
The fourth main hormone is abscisic acid. Abscisic acid helps plants with environmental conditions. It's produced in mature leaves and can stop the apical meristem from dividing. Abscisic acid keeps seeds dormant until it is overcome by a stimulatory hormone. It promotes embryo growth and storage of proteins making the seed better prepared for after germination.