Essential Plant Hormones

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Why This Matters

Plant hormones—also called phytohormones—are the molecular messengers that coordinate virtually every aspect of plant life, from the moment a seed breaks dormancy to the final stages of fruit ripening and senescence. You're being tested on your ability to understand how plants regulate growth, respond to environmental signals, and defend themselves without a nervous system. These hormones don't work in isolation; they form complex signaling networks where ratios and interactions between hormones matter as much as individual hormone levels.

When you encounter questions about tropisms, seed germination, stress responses, or agricultural applications, hormone signaling is almost always the underlying mechanism. Don't just memorize which hormone does what—know why each hormone exists in the plant's regulatory toolkit and how hormones with seemingly opposite functions (like auxins and cytokinins, or gibberellins and abscisic acid) work together to fine-tune plant responses. Understanding these relationships will help you tackle both multiple-choice comparisons and FRQ scenarios that ask you to predict plant behavior under specific conditions.

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Growth-Promoting Hormones

These hormones drive the fundamental processes of cell division, elongation, and differentiation that allow plants to increase in size and develop specialized structures. They work synergistically and antagonistically to balance vertical growth, branching, and overall plant architecture.

Auxins

• Primary role in cell elongation—auxins acidify cell walls, loosening them to allow turgor-driven expansion, particularly in stems and coleoptiles
• Control phototropism and gravitropism through asymmetric distribution; higher concentrations on the shaded or lower side cause differential growth
• Maintain apical dominance by suppressing lateral bud development, ensuring the plant prioritizes vertical growth before branching

Cytokinins

• Stimulate cell division (cytokinesis)—produced primarily in root tips and transported upward to promote shoot growth and development
• Delay senescence by maintaining chloroplast function and protein synthesis in aging leaves, keeping them photosynthetically active longer
• Ratio with auxins determines organogenesis—high cytokinin:auxin promotes shoot formation; low ratio promotes root formation in tissue culture

Gibberellins

• Break seed dormancy by triggering the synthesis of $\alpha$-amylase, which mobilizes starch reserves in the endosperm during germination
• Promote stem elongation through cell division and elongation in internodes; dwarf plant varieties often have gibberellin synthesis mutations
• Regulate flowering timing in long-day plants and bolting in rosette plants like lettuce and spinach

Brassinosteroids

• Promote cell expansion and elongation—structurally similar to animal steroid hormones and essential for normal plant stature
• Regulate vascular differentiation by promoting xylem development and overall vascular tissue organization
• Enhance stress tolerance against drought, salinity, and temperature extremes by modulating gene expression for protective proteins

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Stress and Defense Hormones

These hormones help plants survive environmental challenges and biological attacks. Rather than promoting growth, they often redirect resources toward protection, dormancy, or defense compound synthesis.

Abscisic Acid (ABA)

• Primary drought-response hormone—triggers rapid stomatal closure by signaling guard cells to release potassium ions and lose turgor
• Maintains seed dormancy under unfavorable conditions, preventing germination until environmental signals indicate suitable growing conditions
• Antagonizes gibberellins in germination control; the ABA:GA ratio determines whether a seed remains dormant or germinates

Salicylic Acid

• Triggers systemic acquired resistance (SAR)—when one leaf is infected, salicylic acid signals the entire plant to upregulate defense genes
• Induces pathogenesis-related (PR) proteins that provide broad-spectrum resistance against bacterial, viral, and fungal pathogens
• Regulates thermogenesis in some species like skunk cabbage, generating heat to volatilize attractant compounds for pollinators

Jasmonates

• Coordinate wound responses—when herbivores damage tissue, jasmonates trigger production of protease inhibitors and toxic secondary metabolites
• Activate volatile organic compound (VOC) release that can attract predators of herbivores, providing indirect defense
• Regulate reproductive development including pollen maturation and fruit development alongside their defense functions

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Ripening and Senescence Hormones

These hormones regulate the final stages of plant organ development, including fruit maturation, leaf drop, and programmed cell death. They ensure that resources are reallocated efficiently and that reproductive structures mature at the appropriate time.

Ethylene

• Gaseous hormone controlling fruit ripening—triggers cell wall degradation, chlorophyll breakdown, and sugar accumulation in climacteric fruits
• Promotes abscission of leaves, flowers, and fruit by stimulating the formation of the abscission zone at the base of petioles
• Mediates the triple response in seedlings: inhibited stem elongation, stem thickening, and horizontal growth when encountering mechanical obstacles

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Quick Reference Table

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Self-Check Questions

1. Which two hormones have antagonistic effects on seed germination, and how does their ratio determine whether a seed germinates or remains dormant?

2. A plant is exposed to unilateral light. Which hormone redistributes asymmetrically, and how does this redistribution cause the plant to bend toward the light source?

3. Compare and contrast the defense responses triggered by salicylic acid versus jasmonates. Against what types of threats is each most effective?

4. In tissue culture, how would you manipulate the auxin:cytokinin ratio to induce shoot formation versus root formation? Explain the underlying principle.

5. A farmer wants to delay fruit ripening during transport. Based on your knowledge of ethylene, propose two strategies that could achieve this goal and explain why each would work.