Hormones

Hormones are small molecules that are released by one part of a plant to influence another part. The principal plant growth hormones are the auxins, gibberellins, cytokinins, abscisic acid, and ethylene. Plants use these hormones to cause cells to elongate, divide, become specialized, and separate from each other, and help coordinate the development of the entire plant. Not only are the plant hormones small in molecular weight, they are also active in the plant in very small amounts, a fact that made their isolation and identification difficult.

The first plant growth hormones discovered were the auxins. (The term auxin is derived from a Greek word meaning "to grow.") The best known and most widely distributed hormone in this class is indole-3-acetic acid. Fritz W. Went, whose pioneering and ingenious research in 1928 opened the field of plant hormones, reported that auxins were involved in the control of the growth movements that orient shoots toward the light, and that they had the additional, striking quality of moving only from the shoot tip toward the shoot base. This polarity of auxin movement was an inherent property of the plant tissue, only slightly influenced by gravity. Other less-investigated auxins include phenyl-acetic acid and indole-butyric acid, the latter long used as a synthetic auxin but found to exist in plants only in 1985.

The gibberellins are a family of more than seventy related chemicals, some active as growth hormones and many inactive. They are designated by number (e.g., GA₁ and GAL₂₀). GA₃ (also called gibberellic acid) is one of the most active gibberellins when added to plants. Slight modifications in the basic structure are associated with an increase, decrease, or cessation of biological activity: each such modified chemical is considered a different gibberellin.

Cytokinins are a class of chemical compounds derived from adenine that cause cells to divide when an auxin is also present. Of the cytokinins found in plants, zeatin is one of the most active.

Abscisic acid helps protect the plant from too much loss of water by closing the small holes (stomata) in the surfaces of leaves when wilting begins.

PLANT HORMONES AND THEIR FUNCTIONS
Hormone	Functions
Auxins (indoleacetic acid; IAA)	Stimulates shoot and root growth; involved in tropisms; prevents abscission; controls differentiation of xylem cells and, with other hormones, controls sieve-tube cells and fibers
Gibberellins	Stimulates stem elongation, seed germination, and enzyme production in seeds
Cytokinins	Stimulates bud development; delays senescence; increases cell division
Abscisic acid	Speeds abscission; counters leaf wilting by closing stomates; prevents premature germination of seeds; decreases IAA movement
Ethylene (gas)	Produced in response to stresses and by many ripening fruits; speeds seed germination and the ripening of fruit, senescence, and abscission; decreases IAA movement

The only known gas that functions as a plant growth hormone is the small C₂ H₂ molecule called ethylene. Various stresses, such as wounding or waterlogging, lead to ethylene production.

Major Effects of the Principal Plant Growth Hormones

Auxins.

Indoleacetic acid (IAA), produced primarily in seeds and young leaves, moves out of the leaf stalk and down the stem, controlling various aspects of development on the way. IAA stimulates growth both in leaf stalks and in stems. In moving down the leaf stalk, IAA prevents the cells at the base of the leaf from separating from each other and thus causing the leaf to drop (called leaf abscission). The speed of IAA polar movement through shoot tissues ranges from 5 to 20 millimeters per hour, faster than speeds for the other major hormones.

The growth responses of plants to directional stimuli from the environment are called tropisms. Gravitropism (also called geotropism) refers to a growth response toward or away from gravity. Phototropism is the growth response toward or away from light. These tropisms are of obvious value to plants in facilitating the downward growth of roots into the soil (by positive gravitropism) and the upward growth of shoots into the light (by positive phototropism, aided by negative gravitropism).

The role of auxin in controlling tropisms was suggested by Went and N. Cholodny in 1928. Their theory was that auxin moves laterally in the shoot or root under the influence of gravity or one-sided light. Greater concentration on one side causes either greater growth (in the case of the shoot) or inhibited growth (in roots). This Cholodny-Went theory of tropisms has been subject to refinement and question for decades. Evidence exists, for instance, that in some plants tropism toward one-sided light results not from lateral movement of auxin to the shaded side, but rather from production of a growth inhibitor on the illuminated side.

A widespread, though not universal, effect of IAA moving down from the young leaves of the apical bud is the suppression of the outgrowth of the side buds on the stem. This type of developmental control is called apical dominance: if the apical bud is cut off, the side buds start to grow out (released from apical dominance). If IAA is applied to the cut stem, the side buds remain suppressed in many plants.

In addition to enhancing organ growth, IAA also plays a major part in cell differentiation, controlling the formation of xylem cells and being involved in phloem differentiation. In its progress down the stem, IAA stimulates the development of the two main vascular channels for the movement of substances within the plant: xylem, through which water, mineral salts, and other hormones move from the roots; and phloem, through which various organic compounds such as sugars move from the leaves. In plants that develop a cambium (the layer of dividing cells whose activity allows trees to increase in girth), the polarly moving IAA stimulates the division of the cambial cells. Cut-off pieces of stem or root usually initiate new roots near their bases. As a result of its polar movement, IAA accumulates at the base of such excised pieces and touches off such root regeneration. In the intact plant, the shoot-tip toward shoot-base polar movement of IAA continues on into the root, where IAA moves toward the root tip primarily in the stele (the inner column of cells in the root).

Interesting effects of IAA have been found in a more limited number of plant species. Plants of the Bromeliad family, which includes pineapples, start to flower if treated with IAA. Some other plants typically produce flowers that can develop as either solely male or solely female flowers depending on various environmental factors: In several such species IAA stimulates femaleness.

Gibberellins (GAs).

Produced in young leaves, developing seeds, and probably in root tips, the biologically active GAs, such as GA₁ and GA₃, move in shoots without polarity and at a slower rate than IAA down the stems where they cause elongation. In roots they show root-tip toward root-base polar movement—the opposite of IAA. Their effect on stem elongation is particularly striking in some plants that require exposure to long days in order to flower. In such plants the stem elongation that precedes flowering is caused by either long days or active GAs and is so fast that it is called bolting. A similar association of light effects and active GAs is found in seeds that normally require light or cold treatment to germinate. GAs can substitute for these environmental treatments. In cereal seeds, GA, produced by the embryos, moves into the parts of the seeds containing starch and other storage products. There the GA triggers the production of various specific enzymes such as alpha-amylase, which breaks down starch into smaller compounds usable by the growing embryos. In the flowers that can develop as either male or female, active GAs cause maleness (the opposite effect to that of auxin). Not surprisingly, in view of the relatively large amounts of GAs in seeds, spraying GAs on such seedless grape varieties as Thompson produces bigger and more elongated grapes on the vines.

Cytokinins.

Produced in roots and seeds, the cytokinins' often-reported presence in leaves apparently results from accumulation of cytokinins produced by roots and moved to the shoot through the xylem cells. Research using pieces of plant tissue growing in test tubes revealed that adding cytokinins increased cell divisions and subsequently the number of shoot buds that regenerated, while increasing the amount of added IAA increased the number of roots formed. The test-tube cultures could be pushed toward bud or root formation by changing the ratio of cytokinin to IAA. The growth of already-formed lateral buds on stems could be stimulated in some plants by treating the lateral buds directly with cytokinins. With IAA from the apex of the main shoot inhibiting outgrowth of the lateral buds and with cytokinins stimulating their outgrowth, the effects of the two hormones on lateral buds suggests a balancing effect like that seen in root/shoot regeneration in the tissue cultures. Treatment with cytokinins retards the senescence of leaves, and naturally occurring leaf senescence is accompanied by a decrease in native cytokinins. When the movement of cytokinins such as zeatin through excised petioles was tested in the same sort of experiment that showed IAA moving with polarity at 5 to 10 millimeters per hour, cytokinins showed the slower rate of movement and the lack of polarity characteristic of GAs. However, through root sections, zeatin movement was nonpolar, unlike the movement of GAs.

Abscisic Acid.

Abscisic acid is found in leaves, roots, fruits, and seeds. In leaves that are not wilting, the hormone is mostly in the chloroplasts. When wilting starts the abscisic acid is released for movement to the guard cells of the stomates. Abscisic acid moves without polarity through stem sections and at the slower rate typical of GAs and cytokinins.

As its name implies, abscisic acid stimulates leaf or fruit abscission in many species, as evidenced by faster abscission from treating with the hormone and by increases in the amount of native abscisic acid in cotton fruits just prior to their natural abscission. Abscisic acid's most investigated effect, however, is its protection of plants from too much water loss (wilting) by closing the stomates in leaves when wilting starts. The onset of wilting is accompanied by fast increases in the abscisic acid levels in the leaves and subsequent closure of the stomates. Spraying the leaves with abscisic acid causes stomate closure even if the leaves are not wilting. In seeds, abscisic acid prevents premature germination of the seed.

Ethylene Gas.

Ethylene gas is produced by many parts of plants when they are stressed. Also, normally ripening fruits are often rich producers of ethylene. Among ethylene's many effects are speeding the ripening of fruits and the senescence and abscission of leaves and flower parts; indeed, it is used commercially to coordinate ripening of crops to make harvesting more efficient. Ethylene gas releases seeds from dormancy. If given as a pretreatment, it inhibits the polar movement of auxin in stems of land plants (but, surprisingly, increases auxin movement in some plants that normally grow in fresh water). Ethylene moves readily through and out of the plant. The stimulation of flowering in pineapple and other bromeliads by spraying with IAA, mentioned earlier, is due to ethylene produced by the doses of auxin applied. Despite its frequent production by plants, ethylene is apparently not essential for plant development. Mutations or chemicals that block ethylene production do not prevent normal development.

Interactions of Hormones

In addition to the many effects on development of individual plant growth hormones, a sizeable number of effects of one hormone on another have been found. For example, IAA alone can restore the full number of normal tracheary cells in the xylem, but to restore the full number of sieve-tube cells in the phloem zeatin is needed in addition to IAA. Similarly, to restore the full number of fibers in the phloem, GA must be added along with IAA.

Hormones affect each other's movement, too. Mentioned above was the decrease in IAA movement from pretreatment with ethylene. Similarly, abscisic acid decreases the basipetal polar movement of IAA in stems and petioles. Therefore, in view of IAA's role as the primary inhibitor of abscission in plants, the abscisic acid-induced decrease in IAA movement down the leaf stalk toward the abscision zone probably explains at least part of abscisic acid's role as an accelerator of abscission. In other cases, increases in IAA basipetal movement have resulted from GA or cytokinin treatment. The nonpolar movement typical of cytokinins was changed to polar movement when IAA was added, too.

William P. Jacobs

Bibliography

Abeles, Frederick B., Page W. Morgan, and Mikal E. Saltveit, Jr. Ethylene in Plant Biology, 2nd ed. San Diego, CA: Academic Press, 1992.

Addicott, Fredrick T., ed. Abscisic Acid. New York: Praeger Publishers, 1983.

Davies, Peter J., ed. Plant Hormones: Physiology, Biochemistry, and Molecular Biology. Boston: Kluwer Academic Pulishers, 1995.

Jacobs, William P. Plant Hormones and Plant Development. Cambridge, England: Cambridge University Press, 1979.