Plant Responses - Hormones Chapter 39

  Plants, like animals, can sense changes in their environments. However, they lack a nervous system and cannot respond immediately. Their responses are limited to events that occur as a result of the activity of a plant's hormonal system.   Hormones are “chemical messengers” - they carry information from one part of an organism's body to another. This information is carried in chemical form.   Typically, these bits of chemical information result in changes in growth patterns in plants (because plants lack the two principle animal effectors - muscles and glands.)   Hormones also influence flowering and fruit production.

Phototropism   A plant growing on a windowsill will grow toward light.   This recognition of light and bending is termed phototropism. It is adaptive.

  If the cells of a plant growing toward light are inspected, it can be seen that the cells on the dark side of the plant are larger.   The different cellular growth patterns make plants bend toward the light.   In order for plants to grow toward light, their shoot tips are necessary (a discovery made by Charles Darwin and his son Francis).

  They also learned that the shoot grew straight up if an opaque cover was placed on the end of the shoot.   Darwin and his son speculated that some "signal" was transmitted downward from the tip to the meristems in the shoot.

  In 1913, Danish botanist Peter Boysen-Jensen continued Darwin's experiments.   In one group of seedlings, he inserted a block of gelatin between the tip and the rest of the shoot. This gelatin block allowed chemicals to pass through.   (Gelatin is like microscopic “Swiss Cheese”!   In the other group he placed a thin piece of impermeable mica between the tip and the rest of the shoot. This is what he discovered:

  In 1913, Dutch botanist Fritz Went discovered the transmitted factor and he named it auxin.   Auxin diffuses to the dark side of a shoot and promotes cell growth there. The lighted side, which experiences reduced auxin, shows a slower growth rate.   The most common auxin is IAA (Indoleacetic Acid) - a chemical that plants make from the amino acid tryptophan (Name the other 19, out loud, now, please).

  Botanists have since identified five different types of plant hormones. Three of these - the auxins, cytokinins, and gibberellins - are actually hormone classes. Abscisic acid and ethylene are single chemicals.   General statements concerning plant hormones: They often affect growth. They are produced in very small quantities. Their effect is profound. Their effects are limited to target cells.   Plant hormones exert their effects by altering the expression of genes, by activating or inhibiting enzymes, or by changing properties of membranes or cell walls.

  Plant hormones affect plant growth by causing target cells to elongate or divide (mitosis).  

The Auxins   The chief function of the auxins is to promote the elongation of developing shoots.   The most common auxin is IAA (Indoleacetic acid).   The major site of auxin synthesis is the apical meristem at the tip of the shoot.   When IAA is produced, it migrates downward and causes elongation of the cells in the stem

  Lower than normal concentrations of IAA results in slower stem growth and increased root growth.   Higher concentrations of IAA promote stem growth and diminish root growth.   Auxins can also trigger the development of vascular tissue and induce cell division in the vascular cambium.   Auxins are also found in seeds and promote the development of the fruit. Some plants (tomatoes, cucumbers, eggplants) can be made to produce fruits without fertilization if they are sprayed with auxin. (The fruits are seedless!)   Auxins are also known to affect gravitotropism. An auxin shield results in the growth of roots that do not grow toward gravity:

  Auxins seem to exert their influence by weakening cell walls. Auxins may stimulate certain proteins in a plant cell's plasma membrane to pump hydrogen ions into the cell wall. These hydrogen ions combine with (and activate) enzymes that break the bonds that hold cellulose molecules together. When the cell swells with water (turgor), the cell elongates.   Botanists attribute auxin movements in plants to active transport mechanisms - they seem to have specific target tissues and they are faster than diffusion.  

The Cytokinins   This is the plant hormone group that promotes cytokinesis (AKA cell division) - making it a pretty easy-to-remember hormone.   Most often associated with mitosis in roots, embryos, and fruits.   The most commonly cited effect of the cytokinin hormones is to promote lateral branch growth in the absence of auxin production in stem tips. To explain, if a plant is left to grow with an intact terminal bud, its growth habit is to increase in length, with a smaller amount of lateral stem development. If, however, the stem tip is CUT OFF (negating auxin production), then there is no inhibitory effect on the axillary buds and the plant grows laterally. Cytokinins, transported upward from the root, activate the axillary buds, making the plant grow more "bushy".

  Most plant growth can be explained as a combination of the effects of auxins and cytokinins. Cytokinins traveling upward from the roots counter the effects of auxins being produced in the apical shoot meristems. Growth in the lower parts of the plant are more prominently lateral because of their proximity to the cytokinins, whereas apical dominance remains the growth pattern in the higher parts of the plant.   Cytokinins also retard the aging of flowers and fruits. This aging is termed senescence.

The Gibberellins   The gibberellins are a category of at least 70 different hormones. They are produced in both shoot and root tip meristems.   The function most likely to be addressed on the A.P. exam is the role they play in the lengthening of the stem between nodes (and the growth of leaves).   Gibberellins also influence fruit development. They (like auxins) can be sprayed on certain flowers to induce fruit development without seeds – apples, currants, eggplants.   Gibberellins are known to influence seed germination. If they are sprayed on seeds, they can cause germination in the absence of factors that are normally required for seed germination to begin (such as colder temperatures, increased photoperiod).

Abscisic Acid   Abscisic acid (ABA) is produced in buds and inhibits cell division in apical meristems and cambium. It is useful in maintaining dormant periods. It also signals the formation of bud scales, which protect buds from harsh conditions.   ABA is also generated to help maintain seed dormancy. A downpour or monsoon will leach out ABA, enabling germination. This helps to insure that seeds do not germinate until sufficient water is present in the environment.   It is often helpful to think of the effects of Abscisic Acid to counter (or balance) the effects of the Gibberellins.   Abscisic acid also helps plants deal with adverse conditions. If a plant is dehydrated, ABA causes the stomata to close and remain closed until more water is available.

Ethylene   Plants produce ethylene, a gas, to function as a hormone that triggers a variety of aging responses - the most important is the ripening of fruit.   Fruit ripens when: Cell walls are weakened or broken down. The color of the skin changes (sometimes) Drying occurs.   Ethylene is produced in the cells of the fruit to encourage all fruits nearby to ripen at the same time.   Stored fruits can be stored under high CO2 conditions to counter the effects of ethylene and retard ripening.   The change of color that occurs in trees in autumn is also a function of ethylene.

  The separation of leaves from their stems in the autumn (abscission) is regulated by ethylene.   Separation occurs at a site known as the abscission layer at the base of the leafstalk (petiole). This layer is dominated by parenchyma cells. Under the influence of ethylene, these parenchyma cells are sealed off from the vascular bundles (their source of water and nutrients). When the parenchyma cells die, the abscission layer weakens until the leaf falls from the tree noiselessly and floats daintily to the ground. (Note: During the summer, auxin prevents abscission).

How are plant hormones different than animal hormones?   Plant hormones are not produced in glands (but they are transported around the plant to act on distant tissues).   Plant hormones are not specific in their action. Or more simply, they can produce different responses in different tissues.   Plant hormones are small, simple molecules but animal hormones are large, complex molecules   There are many types of animal hormones but there are only five classic plant hormones.