Gibberellins are plant hormones regulating plant development. Their discovery began with observations in Japanese rice fields, where rice plants exhibited unusually rapid, elongated growth, leading to their collapse, a condition called “bakanae” or “foolish seedling” disease. This was linked to the fungus Gibberella fujikuroi, which produces these compounds. Identifying gibberellins from this fungus revealed their widespread influence on plant physiology.
The Role of Gibberellins in Plant Growth
Gibberellins influence fundamental processes shaping plant growth and development. They promote stem and internode elongation by stimulating cell division and stretching. This increases plant height, particularly in dwarf varieties with a natural deficiency in these hormones.
Beyond vertical growth, gibberellins are involved in seed germination. They break seed dormancy, allowing the embryo to resume growth. This involves activating enzymes, such as α-amylase, in the seed’s aleurone layer, which break down stored starch into simple sugars to fuel the seedling.
Gibberellins also influence flowering, affecting the initiation and growth of floral buds. In some plants, they induce or accelerate flowering, particularly in those requiring environmental cues like cold temperatures (vernalization) or specific day lengths. They also contribute to floral organ differentiation and proper flower development.
They also contribute to fruit development, affecting fruit set and overall growth. For instance, gibberellins enhance fruit size and can facilitate seedless fruit development in some species. They influence fruit tissue expansion, contributing to the final marketable product.
How Gibberellins Work at the Molecular Level
At the molecular level, gibberellins modulate gene expression through a specific signaling pathway. The process begins when a gibberellin molecule binds to its receptor protein, GIBBERELLIN INSENSITIVE DWARF1 (GID1), primarily located within the nucleus of plant cells. This binding forms a complex.
This complex then targets DELLA proteins for degradation. DELLA proteins repress growth, acting as a “brake” on plant developmental processes. Once targeted, DELLA proteins are marked for destruction by the ubiquitin-proteasome pathway, removing their inhibitory effect. This allows previously suppressed growth-promoting genes to activate, enabling plant growth and development.
Commercial Uses in Agriculture
The ability of gibberellins to influence plant growth has led to their widespread use in agriculture, particularly in the form of gibberellic acid (GA3). In grape production, for example, GA3 increases individual berry size and lengthens grape clusters, leading to larger, more uniform bunches. This application helps to reduce compaction within the cluster, allowing berries to grow without restriction.
Another application is in barley malting for brewing. Gibberellins speed up barley grain germination by promoting rapid synthesis of α-amylase and other enzymes. These enzymes break down starch reserves, producing sugars necessary for fermentation and shortening the malting period.
Sugarcane production also benefits from gibberellin application. Spraying sugarcane stimulates the elongation of internodes, the sections of the stalk between leaf nodes. This increased stalk length translates to greater sugar accumulation, enhancing overall sugar yield.
Gibberellins also overcome dormancy in seed potatoes, leading to more uniform sprouting. They can be applied to citrus fruits, like navel oranges, to delay rind senescence and prevent post-harvest disorders, allowing for longer storage and extended marketing periods. This helps maintain fruit firmness and appearance for consumers.
Natural Production and Control in Plants
Gibberellins are naturally synthesized in actively growing tissues, including young leaves, developing seeds, the shoot apex, and roots. Their biosynthesis involves a series of enzymatic reactions occurring in cellular compartments, such as plastids, the endoplasmic reticulum, and the cytosol.
Plants control bioactive gibberellin concentration through a balance of synthesis and deactivation. Environmental signals regulate this balance. For instance, changes in light quality or cold temperatures can influence the expression of genes for gibberellin synthesis (e.g., GA20-oxidase, GA3-oxidase) or deactivation (e.g., GA2-oxidase). This environmental regulation ensures that processes like seed germination or flowering are initiated only when conditions are favorable for plant survival and growth.