Biology
Plant Growth Regulators: Quick Revision
A brief note on Plant Growth Regulators: the five groups and their chemistry, the classic experiments, what each regulator does, the traps in matching questions, with a worked example, practice, a program and questions to check yourself.
Written by StemPy LabA brief note on Plant Growth Regulators. Read it before a test. Try each question at the end before you open its answer.
The five groups
| Regulator | Kind of molecule | Examples |
|---|---|---|
| Auxin | indole compound | IAA, IBA (natural); NAA, 2,4-D (synthetic) |
| Gibberellin | terpene | GA3 (over 100 known) |
| Cytokinin | adenine derivative | kinetin, zeatin |
| Ethylene | gas | ; ethephon releases it |
| Abscisic acid | carotenoid derivative | ABA |
- Promoters: auxins, gibberellins, cytokinins. Inhibitors: abscisic acid, and mostly ethylene.
The classic experiments
- Darwin: the coleoptile tip senses one-sided light, and the bending is below the tip.
- Boysen-Jensen: the signal crosses gelatin but not mica, so it is a chemical.
- Paal: a tip put on one side in the dark makes the stump bend away from that side.
- Went: auxin collected in agar blocks. The Avena curvature test measures the angle, and a bigger angle means more auxin.
- Kurosawa: bakanae of rice, caused by Gibberella, led to gibberellins.
- Skoog and Miller: kinetin from autoclaved herring sperm DNA. Zeatin came from maize and coconut milk.
What each regulator does
- Auxin: rooting of cuttings, flowering in pineapple, apical dominance, parthenocarpy, 2,4-D kills dicot weeds, xylem differentiation, prevents early fruit and leaf drop and promotes the drop of old ones.
- Gibberellin: longer stem, grape stalks, bolting, malting, sugarcane yield, delays senescence.
- Cytokinin: cell division, new leaves and chloroplasts, lateral shoots, overcomes apical dominance, delays senescence.
- Ethylene: ripening and climacteric, senescence and abscission, breaks dormancy (peanut, potato), deep-water rice, root hairs, flowering in pineapple and mango, female flowers in cucumber.
- ABA: closes stomata, stress hormone, seed dormancy and desiccation tolerance, antagonist of GA.
A worked example
A farmer wants the lateral buds of young plants to grow, without any cutting of the tips. Which regulator?
- Auxin from the apical bud keeps the lateral buds quiet.
- A cytokinin overcomes this, so a cytokinin is the answer.
Practice with new numbers
A program to try
The program sorts jobs into the five groups and counts the jobs of each group.
Mistakes to avoid
- Auxin only promotes. A high dose inhibits, and roots need the least.
- Giving gibberellin the work of ethylene in deep-water rice.
- Kinetin or zeatin. Kinetin came from DNA, and zeatin is the natural one.
- Climacteric in every fruit. Citrus and grapes have none.
Check yourself
- Which regulators delay senescence?
- Which regulator is used to kill dicot weeds?
- What does mica show in the experiment of Boysen-Jensen?
- Which regulator is the antagonist of gibberellin in seeds?
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