Gibberellin (GA) is a class of plant hormones that acts as a powerful regulator of growth and development throughout a plant’s life cycle. These compounds are naturally produced by plants and fungi, with over 130 different forms identified. The history of gibberellin traces back to the early 20th century in Japan, where rice farmers noticed a disease causing seedlings to grow excessively tall and weak, a condition they called “Bakanae” or “foolish seedling” disease. Research identified the fungus Gibberella fujikuroi as the cause, and the active substance isolated was named gibberellin. This discovery revealed that plants themselves use similar chemicals to control their stature and developmental transitions.
Core Functions in Vegetative Growth
The most noticeable effect of gibberellin on a plant is its influence on vegetative growth, particularly the elongation of stems and overall plant height. Gibberellin promotes significant growth by stimulating both cell division and the subsequent expansion of those cells, primarily within the internodes, which are the segments of the stem between leaf attachment points. This action causes the stem to rapidly lengthen, a process especially evident in genetically short or dwarf plant varieties.
In plants that normally grow as a compact rosette close to the ground, gibberellin triggers a dramatic extension of the main stem called bolting. This rapid elongation is necessary for the plant to elevate its flowers for pollination and seed dispersal. The hormone achieves this by activating enzymes that loosen the cell wall structure, allowing the internal turgor pressure to expand the cells significantly.
Triggering Key Life Cycle Events
Gibberellin plays a significant role in managing a plant’s developmental timing, serving as a chemical signal for important transitions beyond simple growth. One of its primary roles is to break seed dormancy and initiate germination. Upon water absorption, the embryo releases gibberellin, which travels to the aleurone layer, the outer part of the endosperm.
In the aleurone layer, the hormone triggers the production of hydrolytic enzymes, such as $\alpha$-amylase. These enzymes digest the starch and other stored food reserves within the seed’s endosperm, converting them into sugars that provide the necessary energy for the embryo’s initial growth. Gibberellin also influences the transition to reproduction, promoting flowering in certain species, particularly those that require long daylight periods. Additionally, the hormone can induce parthenocarpy, which is the development of fruit without prior fertilization, resulting in seedless fruit.
The Cellular Signaling Mechanism
The way gibberellin works at a molecular level is by overcoming the mechanisms that normally restrain plant growth. In the absence of gibberellin, specialized proteins known as DELLA proteins accumulate and act as repressors, blocking the activity of growth-promoting genes. The presence of active gibberellin initiates a specific molecular cascade that overcomes this repression.
The gibberellin hormone first binds to a receptor protein called GIBBERELLIN INSENSITIVE DWARF1 (GID1). This binding creates a complex that attracts an enzyme called an E3 ubiquitin ligase. The ligase then tags the DELLA repressor proteins with a small molecule called ubiquitin, a process called polyubiquitination. This tagging marks the DELLA proteins for destruction by the cell’s waste disposal system, the 26S proteasome. Once the DELLA repressors are degraded, the growth-promoting genes are free to activate, leading to the observed effects of stem elongation, germination, and flowering.
Agricultural and Horticultural Uses
The powerful growth-regulating effects of gibberellin have been widely adopted in commercial agriculture and horticulture to enhance crop production and quality. Synthetic gibberellins, such as gibberellic acid ($\text{GA}_3$), are routinely applied to various crops to manipulate their development.
One prominent application is in the production of table grapes, where treatment with gibberellin increases the length of the fruit stalk, allowing the berries more space to grow larger and resulting in looser, better-formed bunches. Gibberellin is also used to induce seedless fruit development in crops like grapes and citrus, taking advantage of its ability to promote parthenocarpy.
In the brewing industry, it is applied to barley to ensure rapid and uniform germination, speeding up the malting process by inducing the synthesis of $\alpha$-amylase enzyme. Furthermore, in crops like sugarcane, gibberellin is sprayed to stimulate the elongation of internodes, which increases the height of the stalk and significantly boosts the total yield of sugar stored in the stem. The hormone can also be used to break dormancy in seed potatoes, leading to uniform sprouting and a synchronized harvest.