Auxins: Auxin, or IAA (indoleacetic acid), promotes plant growth by facilitating the elongation of developing cells. Auxin does this by increasing the concentration of H+ in primary cell walls, which, in turn, activates enzymes that loosen cellulose fibers. The result is an increase in cell wall plasticity. In response, turgor pressure causes the cell wall to expand, thus generating growth. Auxin is produced at the tips of shoots and roots, where, in concert with other hormones, it influences plant responses to light (phototropism) and gravity (geotropism). In addition, auxin is active in leaves, fruits, and germinating seeds. Structurally, auxin is a modified tryptophan amino acid. After synthesis from tryptophan, it is actively transported (using ATP) from cell to cell in a specific direction (polar transport), by means of a chemiosmotic process.

Auxin and Phototropism:

•  Auxin is produced in the apical meristem, moves downward by active transport into the zone of elongation, and generates growth by stimulating elongation. 
• When all sides of the apical meristem are equally illuminated, growth of the stem is uniform and the stem grows straight. 
• When the stem is unequally illuminated, auxin moves downward into the zone of elongation but concentrates on the shady side of the stem. Auxin that would have normally accumulated on the sunny side ends up on the shady side.
• The higher concentration of auxin in the shady side of the stem causes differential growth; that is, since auxin generates growth by stimulating elongation, the shady side grows more than the sunny side. When the shady side grows more than the sunny side, the stem bends toward the light. 

Auxin and Gravitropism:

•  Gravitropism (or geotropism), the response to gravity by stems and roots, is not well understood. In general, both auxin and gibberellins are involved, but their action depends on their relative concentrations and the target organ (root or stem). The role of auxin appears to agree with the following: 
• If a stem is horizontal, auxin produced at the apical meristem moves down the stem and concentrates on its lower side. Since auxin stimulates cell elongation, growth of the lower side is greater than that of the upper side, and the stem bends upward as it grows. 
• If a root is horizontal, auxin is produced at the apical meristem (root tip), moves up the roots, and, as in stems, concentrates on the lower side of the root. However, in roots, auxin inhibits growth. This is because concentrations of auxin are higher in roots than in stems. 

Abscisic Acid: Abscisic acid (ABA) is a growth inhibitor. In buds, it delays growth and causes the formation of scales in preparation for overwintering. In many species of plants, ABA maintains dormancy in seeds. Dormancy in these seeds is broken by an increase in gibberellins or by other mechanisms that respond to environmental cues such as temperature or light. In some desert species, seed dormancy is overcome by the leaching of ABA from seeds by rains. Although ABA is named for the process of abscission, its influence on the abscission of leaves, flowers, and fruits is controversial.

Ethylene: Ethylene is a gas that promotes the ripening of fruit. During the later stages of fruit development, ethylene gas fills the intercellular air spaces within the fruit and stimulates its ripening by enzymatic breakdown of cell walls. Ethylene is also involved in stimulating the production of flowers. In addition, ethylene (in combination with auxin) inhibits the elongation of roots, stems, and leaves and influences leaf abscission, the aging and dropping of leaves.

Rachel Putri Utomo
Block F, Honors Biology
Mr. Kevin Quick
Academic Year 2013-2014