You have now learned the major metabolic pathways: photosynthesis (making sugars), respiration (breaking sugars for energy), nitrogen assimilation, and lipid synthesis. But pathways don't run constantly at full speed—plants must constantly adjust them based on:
Light, temperature, water availability, CO₂ levels
Germination, vegetative growth, flowering, fruiting, senescence
ATP/ADP ratio, sugar levels, redox state
Drought, salinity, pathogen attack, wounding
Key concept: Regulation ensures that the right pathways are active at the right time, in the right place, and at the right rate—optimizing growth, survival, and reproduction.
Three levels of regulation: Plants control metabolism at multiple levels, from rapid adjustments to long-term adaptations.
Plant hormones (phytohormones) are signaling molecules produced in small amounts that coordinate metabolic activities across tissues and organs. They are the "command signals" that tell cells when to switch pathways on or off.
| Hormone | Main Metabolic Functions | Horticultural Application |
|---|---|---|
| Auxin (IAA) | Cell elongation, vascular differentiation, apical dominance, promotes root growth | Rooting powders for cuttings; fruit set in tomatoes |
| Gibberellins (GA) | Seed germination (mobilizes stored reserves), stem elongation, fruit growth | Inducing seed germination; increasing fruit size in grapes; malting barley |
| Cytokinins (CK) | Cell division, shoot growth, delay senescence, nutrient mobilization | Micropropagation (tissue culture); delaying leaf yellowing |
| Abscisic Acid (ABA) | Stress hormone: Stomatal closure, drought tolerance, seed dormancy, stress gene activation | Inducing drought tolerance; controlling transpiration |
| Ethylene (C₂H₄) | Fruit ripening, senescence, abscission, stress responses (flooding, wounding) | Controlled ripening of bananas/tomatoes; promoting flower senescence (undesirable in ornamentals) |
| Brassinosteroids | Cell expansion, stress tolerance, promoting photosynthesis | Potential for improving yield under stress |
| Jasmonates (JA) | Defense hormone: Anti-herbivore responses, secondary metabolite production | Inducing defense compounds; stress tolerance |
| Salicylic Acid (SA) | Pathogen defense: Systemic acquired resistance (SAR), heat tolerance | Inducing disease resistance; post-harvest protection |
Hormones interact in complex ways—they can synergize, antagonize, or regulate each other's production. This creates a sophisticated regulatory network.
ABA and GA oppose each other in seed germination (ABA promotes dormancy, GA promotes germination)
Auxin and cytokinin together promote cell division in tissue culture
Ethylene production is triggered by auxin during fruit ripening
The ratio of hormones often determines the outcome (e.g., auxin:cytokinin ratio controls root/shoot formation)
Enzymes are proteins. To change the amount of an enzyme, the plant must regulate the expression of genes—the process of reading DNA to make mRNA (transcription) and translating mRNA to make protein (translation).
When a seedling emerges from soil, light activates phytochrome (a light receptor). Phytochrome triggers a cascade that turns on genes for:
Result: The seedling switches from heterotrophic (eating stored reserves) to autotrophic (photosynthesis) metabolism.
For immediate adjustments, plants use faster mechanisms than making new proteins.
Some enzymes have allosteric sites where regulatory molecules bind, changing the enzyme's shape and activity.
Enzymes can be chemically modified, usually by adding or removing phosphate groups (phosphorylation/dephosphorylation).
Pathway rate is often limited by how much substrate is available. If CO₂ is low, the Calvin cycle slows regardless of enzyme amounts.
In cold storage, apple cells still respire, but slowly. Why? Cold temperatures slow all enzyme reactions (kinetic effect). But also, high sugar levels from the fruit can feedback inhibit glycolytic enzymes. This dual regulation (temperature + allostery) preserves sugars and extends storage life—a key principle for post-harvest management.
Plants don't use just one regulatory mechanism—they integrate multiple signals at multiple levels. Here's how a single environmental signal (drought) triggers a coordinated metabolic response:
This integrated response operates across all three timescales and involves hormones, gene expression, and metabolic adjustments—all working together.
Using ethylene to synchronize ripening in bananas; using ethylene inhibitors (1-MCP) to delay ripening in apples
Applying gibberellins to germinate seeds or break tuber dormancy in potatoes
Understanding ABA signaling helps schedule irrigation to avoid stress
Using silver thiosulfate (STS) to block ethylene receptors in cut flowers, delaying senescence
Manipulating auxin:cytokinin ratios to control root vs. shoot formation
Many herbicides disrupt specific enzymes or hormone pathways (e.g., auxin mimics)
Answers will be discussed in the next section. Write down your thoughts!
➡️ Next up: 2.4.7 Checkpoint Quiz – Test your understanding of assimilation, secondary products, and metabolic regulation.