Rucete ✏ Campbell Biology In a Nutshell
Unit 6 PLANT FORM AND FUNCTION — Concept 39.2 Plants Use Chemicals to Communicate
Plants use hormones—chemical messengers produced in tiny amounts—to coordinate growth, development, and responses to their environment. These hormones interact in complex networks that regulate everything from seed germination to fruit ripening.
What Are Plant Hormones?
- Plant hormones (a.k.a. plant growth regulators) act in very low concentrations.
- They don’t travel through blood like animal hormones and often act locally.
- Each hormone:
- Has multiple effects depending on context.
- Works in combination with other hormones.
- Acts differently at different concentrations.
Overview of Major Plant Hormones
- Auxin (IAA): Produced in shoot tips, moves down via polar transport. Regulates:
- Cell elongation
- Apical dominance
- Phototropism & gravitropism
- Root and fruit development
- Cytokinins: Made in roots, move upward in xylem sap. Promote:
- Cell division
- Lateral bud growth
- Delay of leaf senescence
- Gibberellins (GA): Found in young tissues and seeds. Trigger:
- Stem elongation
- Fruit growth
- Seed germination
- Bolting in some plants
- Abscisic Acid (ABA): Inhibits growth, induces dormancy, and closes stomata during drought.
- Ethylene (gas): Involved in:
- Triple response (short, thick, curved growth)
- Fruit ripening
- Leaf drop and senescence
- Brassinosteroids: Promote cell division and elongation, enhance xylem differentiation, and inhibit root growth at high levels.
- Jasmonates: Defense against herbivores, regulate ripening, flowering, root growth, and pollen production.
- Strigolactones: Derived from carotenoids, they attract mycorrhizae, suppress lateral buds, and promote seed germination.
Hormone Interactions: Synergy and Antagonism
- Hormones interact to fine-tune development:
- Auxin + cytokinin: Equal → undifferentiated; more cytokinin → shoots; more auxin → roots.
- Auxin + strigolactone: Suppress lateral bud growth.
- Cytokinins: Can counteract this suppression to encourage branching.
Practical Applications
- Synthetic auxins: Used as herbicides (e.g., 2,4-D targets broadleaf weeds).
- Gibberellins: Enlarge grapes and induce seedless fruits.
- ABA: Sprayed to enforce dormancy in crops.
- Ethylene control: Manages fruit ripening in storage and transport.
- Genetic engineering: Alters hormone pathways to regulate growth and stress resistance.
In a Nutshell
Plant hormones are essential regulators of growth and adaptation. Each plays many roles, and their interactions determine developmental outcomes. Understanding them offers powerful tools for agriculture, from weed control to fruit production and stress management.