Gibberellins — The Plant's Growth Hormone for Stretching and Waking Up
Imagine a plant that has been sitting low to the ground, its stem short and compact. Then, with the right signal — longer days, warmer weather — it suddenly shoots upward, producing a tall flowering stalk. That dramatic stretch is gibberellins at work.
Gibberellins are a class of plant growth regulators (PGRs) best known for promoting stem elongation, especially in rosette plants (like cabbage or lettuce) that bolt to produce flowers. They also break dormancy in seeds and buds, and trigger seed germination.
The Intuition
Think of gibberellins as the "green light" signal that tells a plant: It's time to grow tall and reproduce. Without enough gibberellin, a plant stays dwarfed and compact. Apply it externally, and you can make a dwarf plant grow to normal height — a classic experiment that first revealed gibberellin's role.
In seeds, gibberellins act like an alarm clock. They are produced by the embryo and diffuse into the endosperm, where they trigger the production of enzymes (like α-amylase) that break down stored starch into sugar. That sugar fuels the growing seedling until it can photosynthesize on its own.
The Precise Statement
Gibberellins are diterpenoid acids — a family of over 100 related compounds — synthesised in young tissues: shoot tips, young leaves, root tips, and developing seeds. Their major physiological effects include:
- Stem elongation — by promoting both cell division and cell elongation in the subapical meristem.
- Bolting — the rapid elongation of a flowering stalk in rosette plants (e.g., cabbage, beet, sugarcane).
- Seed germination — by mobilising stored food reserves (starch → sugars) via enzyme induction.
- Breaking dormancy — in both seeds and buds, replacing the need for cold treatment (vernalisation) or light.
Gibberellin (GA) acting in the aleurone layer of the seed triggers the enzyme α-amylase, which breaks down stored starch into sugars, which then fuel the growing embryo.
Key Exam Points
- Gibberellins are not the same as auxins — auxins promote cell elongation in stems too, but gibberellins are far more potent for internode elongation and are the primary driver of bolting. …