Plant Hormones

Abscisic Acid (ABA)

Abscisic acid, commonly known as ABA, was initially identified and studied under different names: dormin and abscicin II. Once it was determined that these two substances were the same compound, it was renamed abscisic acid, reflecting its high concentration in newly fallen (abscised) leaves. ABA is a plant growth regulator (PGR) composed of a single chemical compound typically produced in the leaves, derived from chloroplasts, particularly under stress conditions. It functions primarily as an inhibitory agent, affecting processes such as bud growth and seed and bud dormancy. ABA influences the apical meristem, leading to bud dormancy and transforming the last set of leaves into protective bud coverings. Although initially thought to play a role in natural leaf drop, further research has disproven this theory.


In temperate plant species, ABA contributes to leaf and seed dormancy by inhibiting growth. As ABA levels decrease from seeds or buds, growth resumes. In some plants, growth begins as ABA levels fall and gibberellin levels rise. Without ABA, buds and seeds might start growing during warm winter periods and be damaged by subsequent freezing. ABA dissipates slowly from tissues, and its effects are gradually counteracted by other plant hormones, providing a delay in growth and protection against premature development.


During fruit maturation, ABA accumulates in seeds, preventing premature germination. Its effects are diminished by cold temperatures or water washing, which helps release seeds and buds from dormancy.  ABA also plays a crucial role in managing water stress in plants. When water is scarce, roots signal the leaves to produce ABA precursors. These precursors move to the roots, which then release ABA into the vascular system. This ABA travels to the leaves, modulating potassium and sodium uptake in guard cells, leading to stomatal closure. ABA is present throughout the plant, and its concentration in various tissues influences its hormonal functions. Its degradation affects metabolic reactions, cellular growth, and the production of other hormones.


Seeds start with high ABA levels, which decrease before and during germination. As plants develop shoots and leaves, ABA levels rise again, slowing growth in more mature areas. ABA production and degradation are influenced by environmental stresses such as water scarcity or predation, triggering specific cellular responses. Scientists continue to explore the intricate interactions and effects of ABA and other phytohormones.

Auxins (IAA)

Auxins are plant hormones that significantly influence cell enlargement, bud formation, and root initiation. They also stimulate the production of other hormones and, in combination with cytokinins, regulate the growth of stems, roots, and fruits, and facilitate the transformation of stems into flowers. As the first class of growth regulators discovered, auxins primarily affect cell elongation by modifying cell wall plasticity.


Auxin levels decrease in light and increase in darkness. They promote the division of cambium cells and lead to the differentiation of secondary xylem in stems. Auxins inhibit the growth of buds located lower on the stem while encouraging the development of lateral and adventitious roots. The process of leaf abscission begins when the growing point of a plant stops producing auxins.


In seeds, auxins regulate protein synthesis as they develop within the flower post-pollination, leading to fruit formation that houses the developing seeds. While auxins are essential for various plant functions, they can be toxic in high concentrations, with dicots being more susceptible than monocots. This toxicity has led to the creation of synthetic auxin herbicides, such as 2,4-D and 2,4,5-T, used for weed control.


Auxins like 1-Naphthaleneacetic acid (NAA) and Indole-3-butyric acid (IBA) are frequently applied to stimulate root growth in plant cuttings. The most prevalent auxin in plants is indoleacetic acid (IAA). The balance between auxins and cytokinins in plants is consistently maintained (A/C = const.).

Cytokinins

Cytokinins are a class of plant hormones that play a crucial role in cell division and shoot formation. Initially referred to as kinins when they were first isolated from yeast cells, cytokinins are now recognized for their ability to delay tissue aging, mediate auxin transport throughout the plant, and influence internodal length and leaf growth.


These hormones work synergistically with auxins, and the balance between these two hormone groups is essential for regulating major growth phases throughout a plant’s life. Cytokinins counteract the apical dominance caused by auxins and, together with ethylene, facilitate the abscission of leaves, flower parts, and fruits. The relationship between auxins and cytokinins remains consistent (A/C = const.).

Ethylene

Ethylene is a gaseous plant hormone produced through the Yang Cycle from the breakdown of methionine, a compound found in all cells. This hormone has very low water solubility and does not accumulate within cells but diffuses out and escapes into the atmosphere. Its effectiveness as a hormone depends on the balance between its production rate and its escape rate.


Ethylene is generated more rapidly in cells that are growing and dividing quickly, particularly in the dark. In newly germinated seedlings and areas of new growth, the rate of ethylene production can exceed its rate of escape, leading to higher ethylene levels and inhibiting leaf expansion. When the new shoot is exposed to light, phytochrome reactions signal a reduction in ethylene production, allowing for leaf growth.


Ethylene influences both cell growth and cell shape. When a growing shoot encounters an obstacle underground, ethylene production increases significantly. This boost in ethylene prevents cell elongation and causes the stem to swell, creating a thicker stem that can apply more pressure against the obstruction and continue its upward growth. If the shoot remains obstructed and ethylene levels stay high, it can alter the stem’s geotropic response, enabling it to grow around the obstacle.


Research indicates that ethylene affects stem diameter and height. For example, when tree stems are subjected to lateral stress from wind, increased ethylene production results in thicker, sturdier trunks and branches. Ethylene also plays a key role in fruit ripening. As seeds mature, ethylene production ramps up, leading to a climacteric rise in ethylene levels just before seed dispersal. Additionally, the nuclear protein Ethylene Insensitive2 (EIN2), regulated by ethylene, influences other hormones, including ABA and stress hormones.

Gibberellins (GA)

Gibberellins are a diverse group of chemicals produced naturally in plants and fungi. They were first identified by Japanese researchers, including Eiichi Kurosawa, who observed that a chemical from the fungus Gibberella fujikuroi caused abnormal growth in rice plants.


Gibberellins play a crucial role in seed germination by influencing enzyme production that mobilizes stored food for new cell growth, primarily through modulation of chromosomal transcription. In cereal grains like rice, wheat, and corn, seeds have an aleurone layer surrounding the endosperm tissue. When the seed absorbs water, it stimulates the production of gibberellins. These gibberellins are transported to the aleurone layer, prompting it to produce enzymes that break down stored food reserves in the endosperm, which are then used by the growing seedling.


Gibberellins are also known for promoting the bolting of rosette-forming plants by increasing internodal length. They stimulate flowering, cellular division, and post-germination growth in seeds. Additionally, gibberellins counteract the growth inhibition and dormancy effects caused by abscisic acid (ABA).

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