Plant growth hormones, or phytohormones, are small chemical messengers produced naturally by the plant. They regulate all aspects of the plant’s life cycle, from germination and growth to maturation and senescence. These compounds function at extremely low concentrations, coordinating cellular activities across the entire organism. The balance, concentration, and location of these hormones determine the plant’s final form and its adaptive responses to the environment.
The Three Primary Growth Stimulators
Auxins, Gibberellins, and Cytokinins are the main phytohormones that promote cell expansion, division, and overall growth. Auxin, primarily indole-3-acetic acid (IAA), is synthesized in the shoot apical meristem and young leaves. It is transported downwards to regulate cell elongation in the stem. Auxin’s mechanism involves the “acid growth hypothesis,” where it stimulates proton pumps in the cell membrane, acidifying the cell wall. This activates proteins called expansins that loosen the cell wall structure, allowing the cell to expand from internal turgor pressure.
Gibberellins (GAs) promote stem elongation by stimulating both cell division and cell stretching, particularly in the internodes. GAs also break seed dormancy by triggering the synthesis of hydrolytic enzymes, such as $\alpha$-amylase, which break down stored starch for the developing embryo. Cytokinins are primarily synthesized in the roots and travel upward through the xylem, where their main function is to promote cell division (cytokinesis).
Cytokinins work synergistically with auxins to control differentiation. A high cytokinin-to-auxin ratio promotes shoot development, while a low ratio favors root initiation. Cytokinins counteract apical dominance, which is driven by high auxin concentrations in the shoot tip suppressing lateral bud growth. They promote lateral bud break and delay chlorophyll breakdown, helping maintain the plant’s vigor.
Hormones Governing Stress and Maturation
Ethylene and Abscisic Acid (ABA) are two major hormones associated with slowing growth, initiating aging, and responding to environmental threats. Ethylene is a unique gaseous hormone that controls maturation processes, particularly in climacteric fruits like bananas and tomatoes. It triggers the expression of genes for cell wall-degrading enzymes and pigment changes, leading to fruit softening and color development.
Ethylene is also involved in senescence, the aging process that leads to leaf drop (abscission) and flower wilting. Abscisic Acid functions as the plant’s internal alarm system, synthesized primarily in response to drought stress. When water is scarce, ABA binds to receptors on the guard cells surrounding the stomata.
This binding initiates a signaling cascade that causes the efflux of ions like potassium ($\text{K}^+$) from the guard cells, leading to a loss of turgor pressure. As water leaves the guard cells via osmosis, the stomata close, reducing water loss through transpiration. ABA also maintains seed dormancy, ensuring germination only occurs when environmental conditions are favorable.
Hormonal Interaction and Developmental Processes
Plant development is controlled not by a single hormone, but by the ratio and spatial distribution of multiple phytohormones. This interaction is exemplified by tropisms, which are directional growth responses to external stimuli like light (phototropism) and gravity (gravitropism). These responses are governed by the Cholodny-Went theory, which posits that a stimulus causes the asymmetric redistribution of auxin across an organ.
In phototropism, blue light is perceived by photoreceptors called phototropins, which signal auxin movement to the shaded side of the stem. The higher auxin concentration on the shaded side causes those cells to elongate faster than cells on the light side, resulting in the stem bending toward the light source. Gravitropism follows a similar logic: specialized cells in the root cap (statocytes) sense gravity and redistribute auxin to the lower side of the shoot and the root.
However, the result is opposite because shoots and roots have different sensitivities to auxin. High auxin concentration on the lower side promotes elongation in the shoot (causing it to grow up), but inhibits elongation in the root (causing it to curve down). The transition to flowering is another process involving multiple hormones, regulated by environmental cues and the transmission of a signal called florigen. Gibberellins promote this transition, particularly in long-day plants or those requiring a cold period, by influencing the genetic pathways that control the switch from vegetative to reproductive growth.
Manipulating Plant Hormones in Practice
The functions of phytohormones are widely exploited in agriculture and horticulture through the application of synthetic compounds. Synthetic auxins like Indole-3-butyric acid (IBA) are sold commercially as “rooting hormone” powders to induce the formation of adventitious roots on stem cuttings (vegetative propagation). Synthetic auxins are also used at high concentrations as selective herbicides, such as 2,4-D, which causes uncontrolled growth in broadleaf weeds while leaving most grasses unharmed.
Gibberellins are applied commercially to increase the size of seedless grapes, causing fruit cells to elongate and clusters to loosen, improving air circulation. In post-harvest management, Ethylene gas is utilized to ripen climacteric fruits uniformly after they are picked green for transport. Conversely, the chemical 1-methylcyclopropene (1-MCP) blocks ethylene receptor sites on fruit and cut flowers, delaying ripening and senescence during storage and shipping.