Hormonal Horticulture: A Breakdown of Plant Hormones, Part 2
Plant hormones, or phytohormones, are biomolecules that control a plant’s growth and development. They are involved in signaling and cell-to-cell communications that help a plant respond to environmental stimuli. There are five classic types of hormones—auxins, cytokinins, gibberellins, ethylene and abscisic acid—and new classes are still being discovered and synthesized.
All hormones work together as a signaling network to regulate a plant’s defense responses. This is the second installment in a three-part series on plant hormones. This time we focus on gibberellins, cytokinins and abscisic acid.
Gibberellins (GAs) are a large group of diterpenoid molecules that have a diverse range of properties and effects on plants. More than 120 gibberellins have been isolated and identified as naturally occurring compounds in nature. Out of these, only a few types are actually biologically active, with the remaining forms being inactivated forms for storage, or precursors to the active forms. They are used by plants, algae, fungi and bacteria. Some gibberellins act as natural plant hormones and plant growth promoters, with the most important in plants being GA1 and GA3.
Among their many roles, gibberellins elongate plant stalks and stems. If a plant is under a canopy and not getting enough light, production of this hormone initiates so the plant can stretch towards the light.
This elongation is caused by cell division triggered by the presence of active gibberellins. As terpenes, gibberellins fight pathogens and environmental stresses while producing vital biomolecules. Gibberellins also contribute to fruit and flower development and induction, and break the dormancy of seeds while controlling their germination.
Once in the presence of water, gibberellins trigger the conversion of starch reserves in seeds to glucose sugars that act as usable energy for the plant’s embryo. Essential gibberellins activity is found in the primary and secondary meristems—both in shoots and roots—where undifferentiated stem cells are influenced to produce.
While gibberellins are found throughout the entire anatomy of a plant, they are synthesized in certain organelles and transported via the vascular tissues to where they are needed. The sites of their biosynthesis occur in the endoplasmic reticulum, plastids and the cytosol (the broth of the cellular soup), and are facilitated by a system of enzymes.
Certain dwarf plants are bred or genetically modified to deactivate gibberellin synthesis, limiting plant height by decreasing internodal length. Certain chemicals such as paclobutrazol, a plant growth regulator, inhibit gibberellin synthesis, thereby slowing or stopping vertical growth and initiating early fruitset production.
Gibberellin supplements are available in the agricultural industry. Most commercial versions are a product of fermentation of the fungus Gibberella fujikuroi. If a plant is sprayed with gibberellins, it shoots and stretches upward drastically. A cold-pressed kelp extract used as a fertilizer supplement will supply a good amount of biologically active gibberellins to plants.
Cytokinins are hormones derived from adenine that stimulate cell division. This is also known as cytokinesis. They play a role in nutrient uptake as they promote new shoot growth and flower sites, and they also play smaller roles in all phases of plant growth and development. In autumn, cytokinins can delay leaf senescence, or programmed plant aging, and are capable of spreading apical dominance to the lateral meristems and away from the main central stalk.
Cytokinins are synthesized throughout a plant but primarily in the root tips where they are then transported via the xylem tissues up to the shoots. Seeds are loaded with them because their embryos require cell division to grow larger. The most prominent and biologically active cytokinins are called zeatin, named after the corn plant from which they were first isolated.
Cytokinins and the other phytohormones interdepend on each other. Their dynamic interplay is a complex, highly-evolved system with checks and balances. For example, if the top of a plant is pinched off, auxins are removed, thereby inhibiting gibberellin synthesis. This in turn signals the plant to initiate the production of cytokinins in the lateral meristems (side tops), making the plant bushier, rather than lankier.
Abscisic acid (ABA) is a phytohormone that acts as a natural growth inhibitor. Although it has no role in abscission, it keeps its name for historical emphasis. When it was first isolated, it was thought to be responsible for the abscission (dropping) of ripened fruits and leaves. In recent years, the true character of the molecule has been uncovered. Abscisic acid facilitates situations when a plant needs to close its stomata, like in the desert or any low humidity environment. A plant with low abscisic acid levels is only capable of flourishing in high humidity. Abscisic acid synthesis is kicked into high gear if a plant is dehydrated and showing water stress.
As mentioned previously, gibberellins are responsible for breaking seed dormancy upon exposure to water, while abscisic acid is the hormone that actually holds the dormancy steady while a seed waits for spring’s rainfall. This natural balance prevents premature germination.
Missed Part I? You can find it here.
Or, head to Part III.