Lecture 12 Notes Diversity

Introduction to Plant Hormones

• Objective: Understanding the role of plant hormones and their impact on plant growth and development.

Plant Nutrition Recap

• Discussion of a pitcher plant, a carnivorous plant that adapts to nutrient-poor soils.

  • Structure and Function: The plant’s leaves form a trap, capturing insects for nutrients.

  • Process: Water collects in the trap, attracting insects, which then get trapped due to slippery surfaces and inward-curving structures.

  • Nutrient Acquisition: Insects decompose, providing essential minerals to the plant, compensating for nutrient deficiency in soil.

Historical Context of Agriculture in Iowa

• Agricultural practices dating back to European settlement.

  • Crop Rotation: Early methods involved rotation of wheat, corn, and oats alongside pasture lands.

  • Introduction of legumes (e.g., soybeans) enhanced soil nitrogen content via modulation.

  • Importance of legumes in providing nitrogen through symbiosis with nitrogen-fixing bacteria (Rhizobium) forming root nodules.

Communication and Symbiosis

• Chemical Signals: Roots release flavonoids to attract Rhizobium.

• Infection and Nodule Formation: Rhizobium infects roots, leading to the formation of nodules that fix nitrogen into ammonia for plant use.

Limitations of Nitrogen in Plant Growth

• Nitrogen is crucial for plant development and is often a limiting nutritional factor.

Plant Hormones Overview

• Hormones signal changes in gene expression leading to cellular adaptations.

• Germination Process: Water uptake activates the embryo, leading to sprouting.

• Gibberellic Acid (GA): Produced in response to water, stimulates seed germination and other growth processes.

Germination Mechanism

• Water activates seeds, leading to production of GA in embryos.

• Aleurone Layer: In response to GA, aleurone produces alpha-amylase that breaks down starch in endosperm into sugars for the embryo.

Signal Transduction Pathways

• Hormonal Action: Hormones like GA bind to receptors in target cells, entering the nucleus and initiating gene expression changes (releasing transcription from repressors).

• GA promotes physiological changes such as enzymatic activity and nutrient mobilization.

Overview of Key Plant Hormones

1. Gibberellic Acid (GA)

   • Functions: Seed germination, stem elongation, fruit enlargement.

2. Abscisic Acid (ABA)

   • Functions: Seed dormancy, stomatal closure under water stress.

3. Auxins

   • Functions: Stimulates growth, mediates phototropism (growth towards light), promotes apical dominance.

4. Cytokinins

   • Functions: Promote cell division, branch outgrowth, delay leaf senescence.

5. Ethylene

   • Functions: Involved in fruit ripening and leaf abscission.

Auxin Discovery and Functionality

• Historical Overview: Discovered by Darwin through experiments on plant responses to light.

• Phototropism: Auxin distribution causes unequal growth, leading plants to bend towards light.

• Chemical Nature: Auxin (Indole-3-acetic acid) is derived from tryptophan and promotes growth in targeted areas.

Auxin and Apical Dominance

• Apical dominance keeps axillary buds from growing unless the main shoot is removed.

• Auxin is produced at the shoot tip and inhibits lateral bud growth, while cytokinins promote it under specific circumstances.

Polar Transport of Auxin

• Auxin moves with directionality allowing growth at different parts of plants based on its concentration.

  • Mechanism: Charged/uncharged forms dictate transport through cell membranes; specific efflux carriers facilitate movement.

Summary

• Plant hormones are essential for regulating various processes crucial for plant health and adaptation.

• Understanding the roles of different plant hormones gives insight into plant growth, development, and responses to environmental stimuli.