Gibberellic acid (GA) is a plant hormone (phytohormone) that acts as a regulator of growth and development across the plant kingdom. It governs the timing and rate of various growth processes. By stimulating cell division and elongation, GA helps determine a plant’s overall architecture and influences key biological transitions, coordinating internal genetic programs with external environmental cues.
Chemical Identity and Natural Origin
Gibberellic acid belongs to the family of compounds called gibberellins, which are chemically classified as diterpenoids. The active form, $\text{GA}_3$, is one of over 125 known gibberellins identified in plants and microorganisms. These tetracyclic molecules are derived from the mevalonate pathway.
The discovery of GA traces back to the 1920s and a fungal disease of rice known as “bakanae,” or “foolish seedling disease.” Scientist Eiichi Kurosawa confirmed that the symptoms—excessive stem elongation and pale color—were caused by a substance secreted by the fungus Gibberella fujikuroi. This substance was isolated and named gibberellin, with $\text{GA}_3$ being the first characterized. Gibberellins are produced endogenously by all higher plants, primarily in young leaves, root tips, and developing seeds.
Driving Vegetative Growth
Gibberellic acid strongly influences vegetative growth, particularly stem elongation. The hormone promotes both cell division in the meristems and cell expansion within the internodes. This elongation occurs because GA alters the properties of the plant cell walls, making them more flexible.
When the cell wall loosens, turgor pressure forces the cell to expand longitudinally, resulting in increased stem height. This action is clearly seen in the reversal of genetic dwarfism; applying GA to dwarf varieties, such as maize or pea, restores them to their normal stature. In rosette plants, which have short internodes, a surge in GA concentration triggers bolting, a rapid stem elongation that precedes flowering. This process is temperature-dependent, with warmer conditions often leading to higher natural GA production.
Controlling Life Cycle Transitions
Gibberellic acid initiates key transitions throughout a plant’s life cycle, notably by breaking seed dormancy to allow germination. In seeds requiring cold or light exposure, GA acts as the internal signal, triggering the mobilization of stored energy reserves.
The hormone moves from the embryo to the aleurone layer, stimulating the production of hydrolytic enzymes like alpha-amylase. This enzyme breaks down stored starch into simple sugars, fueling the seedling’s growth. GA also influences flowering, especially in plants requiring long day lengths or cold exposure (vernalization). Applying GA can bypass the cold requirement, inducing early flowering in biennial plants like cabbage or beet.
GA is also involved in fruit development and growth. GA application can induce parthenocarpy—the development of fruit without prior fertilization—resulting in seedless varieties in crops like grapes or tomatoes. By promoting cell division and expansion, GA increases the final size and quality of the produce.
Commercial and Horticultural Applications
Gibberellic acid is one of the most widely used plant growth regulators in modern agriculture and horticulture. Its commercial form, often $\text{GA}_3$, is produced industrially through the fermentation of the Gibberella fujikuroi fungus. This synthesized hormone is applied via foliar sprays and seed soaks to achieve specific production goals.
In viticulture, GA increases the size of individual grapes and elongates fruit clusters, which loosens the bunch and reduces rot. For the brewing industry, treating barley seeds with GA promotes uniform malting by accelerating alpha-amylase production, ensuring consistent starch breakdown. Growers also use it on citrus fruits, such as navel oranges, to delay rind senescence, keeping the fruit firmer and extending storage life. Furthermore, applying GA to crops like sugarcane stimulates internode elongation, increasing total stem biomass and the yield of stored sucrose.