Plant Responses and Hormones

Plant Responses

Overview of Plant Responses to the Environment

  • Plants are capable of sensing and integrating information from their environment.

  • They alter growth and development based on environmental cues.

Key Concepts

Etiolation and De-Etiolation

  • Etiolation: A growth response observed in plants left growing in darkness, characterized by:
      - Pale stems
      - Unexpanded leaves
      - Short roots

  • De-Etiolation (Greening): The process triggered by exposure to light where:
      - Shoots and roots grow normally.

Example of Signal Transduction

  • Phytochrome: A light-detecting receptor that plays a critical role in plant signaling.

  • Second Messengers are important for amplifying responses and include:
      - Opening of Ca2+ channels
      - Production of cGMP
      - Results in increased enzymatic activity.

  • Transcription Factors (TFs) control the transcription of genes, either activating or repressing them depending on the signal received.

De-Etiolation Process

  • Proteins Activated by De-Etiolation:
      - Enzymes functioning in photosynthesis.
      - Supply chemical precursors for chlorophyll production.
      - Affect levels of plant hormones that regulate growth.

Chemical Communication in Plants

  • Plants utilize chemicals to communicate with their external environment.

  • Communication occurs between different parts of the plant through:
      - Plasmodesmata: Microscopic channels that transport macromolecules cell-to-cell.

Plant Hormones/Growth Regulators (Phytohormones)

Definitions and Functions

  • A hormone is a signaling molecule produced in low concentrations in one part of the plant and transported to other locations.

  • These hormones control specific physiological processes and bind to specific receptors in target cells and tissues.

  • Some signaling molecules act locally, while others exist in higher concentrations than typical hormones.

Major Classes of Plant Hormones

  1. Auxin (IAA)

  2. Cytokinins

  3. Gibberellins (GA)

  4. Abscisic Acid (ABA)

  5. Ethylene

  6. Brassinosteroids

  7. Jasmonates

  8. Strigolactones

Overview of Plant Hormones (Table 39.1)

Auxin (IAA)

  • Location: Produced in shoot apical meristems and young leaves. Also in root apical meristems and developing seeds/fruits.

  • Functions:
      - Stimulates stem elongation.
      - Promotes formation of lateral and adventitious roots.
      - Regulates fruit development.
      - Enhances apical dominance and functions in phototropism and gravitropism.
      - Retards leaf abscission.

Cytokinins

  • Location: Synthesized primarily in roots and transported to other organs.

  • Functions:
      - Regulate cell division in shoots and roots.
      - Promote lateral bud growth.
      - Stimulate seed germination.
      - Delay leaf senescence.

Gibberellins (GA)

  • Location: Produced in meristems of apical buds and roots, young leaves, and developing seeds.

  • Functions:
      - Stimulate stem elongation, pollen development, and fruit growth.
      - Regulate sex determination and the transition from juvenile to adult phases.

Abscisic Acid (ABA)

  • Location: Synthesized in almost all plant cells.

  • Functions:
      - Inhibits growth and promotes stomatal closure.
      - Promotes seed dormancy and desiccation tolerance.

Ethylene

  • Location: A gaseous hormone produced in various parts of the plant.

  • Functions:
      - Promotes fruit ripening and leaf abscission.
      - Involved in the triple response of seedlings (slowing stem elongation, thickening stem, promoting horizontal growth).

Brassinosteroids

  • Functions:
      - Promote cell elongation and division.
      - Slow leaf abscission and promote xylem differentiation.

Jasmonates

  • Functions:
      - Involved in plant defense against herbivory and pathogens.
      - Regulate various physiological processes including nectar secretion, fruit ripening, and seed germination.

Strigolactones

  • Functions:
      - Stimulate seed germination.
      - Suppress adventitious root formation and aid in mycorrhizal associations.

Detailed Functions of Auxin

  • Production: Synthesized at shoot apical meristem (SAM) and transported down the stem.

  • Effects on Buds:
      - Prevents lateral bud development when intact.
      - Promotes adventitious root formation when applied as a rooting hormone.

  • Fruit Development:
      - Developing seeds produce auxin essential for fruit growth.
      - Delays fruit senescence (aging process).
      - Auxin application can promote parthenocarpy (fruit development without fertilization).

  • Leaf Abscission Prevention:
      - Normally synthesized at the leaf tip, influencing senescence and the sensitivity to ethylene.

Cytokinins: Influencing Cell Division

  • Produced: In roots, embryos, and fruits, influencing cell division and differentiation.

  • Work in conjunction with auxin to regulate development based on their ratio (A:C ratio) which determines differentiation pathways:
      - A = C: habitat of undifferentiated cells.
      - A < C: induces shoot buds.
      - A > C: induces root formation.

  • Anti-Aging Effects: They inhibit protein breakdown. Adding cytokinins can keep leaves green longer post-removal.

Gibberellins: Modulation of Growth Processes

  • Effects on Growth:
      - Stimulate stem elongation and promote fruit growth and seed germination.
      - Important for bolting (rapid floral stalk growth) in plants like lettuce.

Abscisic Acid: Stress Responses

  • Effects:
      - Inhibitory effects on growth and maintaining seed dormancy.
      - Acts as a primary signal of drought stress by inducing rapid stomatal closure.

Ethylene: The Gaseous Hormone

  • Production: In response to stresses such as drought and flooding.

  • Functions:
      - Regulates leaf abscission and fruit ripening.
      - Induces a triple response in seedlings facing obstacles, characterized by stem thickening and horizontal growth.

  • Fruit Ripening:
      - Climacteric Fruits: Experience rapid ripening with increased ethylene production (e.g., bananas).
      - Non-Climacteric Fruits: Ripen gradually without major ethylene bursts (e.g., citrus fruits).

Recently Discovered Phytohormones

Brassinosteroids

  • Chemically similar to cholesterol and animal sex hormones. Induces cell elongation at low concentrations and promotes xylem differentiation.

Jasmonates

  • Produced in response to wounding, involved in defense, as well as various growth and reproductive processes.

Strigolactones

  • Stimulates seed germination and helps establish mycorrhizal relationships, while suppressing adventitious root formation.