Gibberellins

The discovery of gibberellins is credited to Ewiti Kurosawa who found that a fungus was responsible for abnormal rice seedling growth, called the "foolish seedling" disease. The fungus secreted a chemical that caused the rice plants to grew abnormally long, and then collapse from weakness. The fungus was Gibberella fujikuroi, hence the hormone name. Many seeds contain a variety of different gibberellins. Over 100 different gibberellins (organic acids synthesized from mevalonic acid) are known. Gibberellins are produced in roots and younger leaves. Most effects of gibberellins are shown only in concert with auxins.

Gibberellin Functions

  • Gibberellins work with auxins to promote rapid elongation and division of stem tissue. It is thought that gibberellins particularly promote expansin activity. This is seen in:

    • Bolting of biennials

    • Reversal of genetic dwarfism

  • Gibberellins promote flowering in biennials during the first growing season, a process called bolting.

  • Gibberellins are important to breaking dormancy after imbibition of water by the seed coat. Gibberellins signal germination activities. In particular application of gibberellins to seeds will counter the normal environmental cues, such as exposure to low temperatures. They stimulate RNA to promote synthesis of enzymes that convert stored nutrients (starches) to sugars needed for rapid cell respiration during germination. Absiscic acid can counter the effect of gibberllin to keep seeds dormant.

  • Gibberellins stimulate some fruit enlargement (e.g., grapes with longer internodes), and may counter the effects of herbicides.

Abscisic Acid

Abscisic acid (ABA) is a hormone that functions by inhibiting growth activities in times of environmental stress rather than by promoting growth. It often serves as an antagonist to the other growth promoting hormones in plants. Abscisic acid, which is also synthesized from mevalonic acid, got its name from the erroneous belief that it promoted the formation of abscission layers in leaves and fruits. It does not, although leaf abscission accompanies dormancy in many plants.

Abscisic Acid Functions

  • ABA promotes seed dormancy activities. ABA levels are high when seeds mature, promoting lowered metabolism and synthesis of proteins needed to withstand the dehydration associated with dormancy. Seeds germinate when ABA is degraded by some environmental action. Desert seeds must have the ABA washed out of the seed coat; temperate area plants have ABA degraded by light-stimulated enzymes. In other cases breaking dormancy is relative to the ratio of ABA (which keeps seeds in dormancy) and gibberellins (which promote germination).Low levels of ABA in maturing seeds promotes premature germination.

  • ABA is also referred to as the stress activity hormone. For example, ABA promotes stomata closure during leaf water deficit conditions by activating K+ ion transport out guard cells. This involves signal transduction pathways with Calcium secondary messengers. ABA in this case originates in roots, and detects low water level in root tissues. ABA moves upward into leaves and activates stomatal closure.

  • ABA derivatives, called dormins, are used in commercial nurseries to keep materials to be shipped in dormant conditions. The dormancy can be reversed with gibberellins.

Ethylene

Ethylene is the sole growth regulator known which is a gas. (It is a small hydrocarbon molecule, easily synthesized in chemistry labs). The benefits of ethylene as a fruit ripener were known for centuries prior to it being identified as a plant product in the early 1900's. Chinese gardeners knew centuries ago that fruits ripened better in rooms with burning incense. Citrus growers used kerosene stoves in the rooms in which they ripened their fruit. During the era of gas lamps, leaking lamps along city streets often promoted leaf abscission. Today, grocer warehouses have ethylene rooms that are used for ripening most of our produce, which is shipped unripe. Immature fruits are firmer and less subject to damage. Thanks to ethylene rooms, we have "mature green tomatoes". Ethylene is produced in many plant organs, though best studied in fruits. Ethylene affects many aspects of growth and development in tissues throughout the plant, but emphasis is on fruit maturation, leaf abscission and senescence. Ethylene is synthesized from the amino acid methionine. High auxin concentrations promote an intermediate step in this pathway which activates an enzyme in the tonoplast to convert the intermediate into ethylene. Toxic substances such as air pollutants also trigger the intermediate step. Ethylene is also produced by bruised tissues. Although the benefit of this is unknown, it's probably an artifact of the wounding response.

Ethylene Functions

  • Ethylene promotes ripening in fruits by triggering chemical reactions that degrade the pectins of the middle lamella, softening fruit, and promoting the conversion of stored starches and/or oils into sugars that attract seed dispersers. Some fruit ripening involves a great increase in the rate of cell respiration with concurrent high O2 uptake. This function of ethylene (fruit ripening) has extensive agricultural impact. Biotechnology has been used to alter sensitivity to ethylene in some crops to facilitate harvest and shipping of fruits.

  • Ethylene promotes female flower production in some members of the Cucurbitaceae, whereas high gibberellins may promote formation of male flowers.

  • An unusual function of ethylene occurs when germinating seeds encounter mechanical stress. A shoot tip that encounters an immovable object will grow around the object by changing its growth direction. This occurs through differential elongation of cell walls. When the shoot tip can not penetrate the mechanical obstacle, ethylene is synthesized, slowing cell wall expansion. The walls thicken (more resistant to the pressure) and a stem curvature to bypass the obstacle via horizontal growth. (Essentially the shoot is growing around the obstacle.) The stem tip pushes upward periodically to test for the obstacle. If the obstacle is encountered, ethylene starts another cycle for horizontal growth. If no obstacle is encountered, the shoot resumes negative geotropic growth.

  • Ethylene is responsible for initiating the programmed death (apoptosis) of sclerenchyma and xylem vessels and tracheids. Death involves intense cellular activity to degrade and salvage materials of the cytoplasm.

  • Ethylene is the direct cause of leaf and fruit abscission. Ethylene promotes the degradation of the cell walls in the abscission zone cells. The production of ethylene in the abscission zone cells is triggered by the declining levels of auxin in leaves as summer ends. This auxin/ethylene interaction is also used commercially. Fruit growers may spray auxins on fruits to prevent the fruits from falling to the ground prior to harvest. Concentration is important; very high auxin level promotes ethylene production. High concentrations of CO2 inhibit ethylene production so CO2 is used to prevent fruit ripening in grocer warehouses.

Brassinolides (Brassinosteroids)

Brassinolides is a plant steroid discovered in pollen of members of the mustard family, and best studied in Arabadopsis. Chemically they are very similar to animal steroid hormones. Brassinosteoroids activate signal transduction pathways the promote cell elongation and cell division. Brassinosteroids promote differentiation of xylem tissue, and perhaps other tissues, too. Brassinosteroids can also retard leaf abscission. Absence of brassinolides results in dwarf plants. It is difficult to study them because their effects overlap those of auxin and gibberellins.

Salicylic Acid

Salicylic acid is known to activate defense genes against pathogen invaders. Salicylic acid, a phenolic extract from willow bark, was long used as an analgesic. It is now prepared commercially and is the active ingredient of aspirin.

Jasmonates

Jasmonates are a group of fatty acid derivatives. They appear to have a role in seed germination, root growth, and the storage of protein (especially in seeds). Synthesis of defense proteins may be triggered by jasmonates.

Systemin

Systemin is a small peptide found in wound tissue. It may stimulate defense activities in other parts of the plant to prevent more wounding.

Oligosaccharins

Oligosaccharins are short chain sugars in cell walls that may have a role in defense against pathogens. They may also help regulate growth, differentiation and flower development all by activating signal transduction pathways.

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