Lecture 6: Plant Hormones and Development

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Questions and Answers

What is one of the functions of auxin related to root development?

It suppresses root initiation on stem cuttings.

It promotes root initiation on stem cuttings. (correct)

It enhances fruit ripening during harvest.

It stimulates leaf growth from seeds.

How does auxin influence lateral bud growth?

By inhibiting apical dominance, allowing lower buds to flourish.

By promoting apical dominance, thus suppressing their growth. (correct)

By enhancing their growth when ethylene is present.

By stimulating their growth under all conditions.

What is the effect of auxin on fruit setting and growth?

It induces fruit growth after fertilization only.

It has no effect on fruit growth.

It promotes fragmentation and loss of fruits.

It can induce fruit setting even without fertilization. (correct)

Which of the following statements about ethylene is true?

Ethylene accelerates leaf senescence. Signup and view all the answers

What role does auxin play in abscission?

It can inhibit abscission when present in high quantities. Signup and view all the answers

Which hormone is primarily associated with the promotion of flowering in Bromeliads?

Auxin Signup and view all the answers

What phenomenon occurs when auxin contributes to the development of fruit without prior fertilization?

Parthenogenesis Signup and view all the answers

Which process is influenced by auxin to enable callus formation in tissue culture?

Promotion of cell division and differentiation. Signup and view all the answers

What role do gibberellins play in the germination of seeds?

They signal germination activities and mobilize food materials. Signup and view all the answers

How do gibberellins affect dwarf plants?

They can make dwarf plants grow tall temporarily. Signup and view all the answers

What effect does gibberellin have on flowering?

It can induce flowering in some biennial plants. Signup and view all the answers

What is the primary function of abscisic acid (ABA) in plants?

It is a natural plant growth retardant. Signup and view all the answers

Which of the following is a primary condition for flowering in many winter crops?

Exposure to cold conditions. Signup and view all the answers

How is abscisic acid (ABA) primarily produced and transported in plants?

In roots and can move through xylem and phloem. Signup and view all the answers

What is the effect of gibberellins in the grape industry?

They are used to grow larger grapes with longer internodes. Signup and view all the answers

What function do gibberellins and auxins serve when combined?

They promote rapid elongation and division of stem tissue. Signup and view all the answers

What is a primary function of cytokinins in plant tissue culture?

Speed up cell division during the cell cycle Signup and view all the answers

How do cytokinins affect the balance between root and shoot growth?

They promote axillary bud growth against auxin's effects Signup and view all the answers

Where is gibberellin primarily biosynthesized in plants?

Proplastids or plastids Signup and view all the answers

Which of the following statements about gibberellins is correct?

Most gibberellins produced are inactive precursors Signup and view all the answers

What is one effect of ethylene on fruit maturation?

Enhances respiration and ripening Signup and view all the answers

What role does auxin play in the growth of plants?

Stimulates root elongation and cell expansion Signup and view all the answers

What effect does cytokinins have on leaf senescence?

They delay senescence by promoting chlorophyll retention Signup and view all the answers

Which of the following describes parthenogenesis in plants?

Development of seeds without fertilization Signup and view all the answers

Study Notes

Lecture 6: Regulation of Developmental and Differentiation Hormones

The lecture covers plant growth regulators (phytohormones)
Students will learn to list these regulators and explain their functions
Students will learn how hormones affect plant development
Plant hormones are chemicals that alter growth and maintenance within a plant
These substances are produced in low concentrations
Hormones are mobile, and act over short and long distances
Plant hormones are regulated in various tissues undergoing different development stages
Two main groups of plant hormones: endogenous and exogenous
Endogenous hormones are produced by the plant, and affected by its environment.
Exogenous hormones are applied to a plant, and affect it in the same way as endogenous hormones.

Plant Hormones

Plant hormones (or phytohormones) have multiple effects
Send chemical signals to cells to regulate activity
Activate or deactivate genes that encode specific enzymes
Regulate cell division, elongation, and differentiation
Respond to signals from the plant and its environment

Auxin

Chemically, auxin is indoleacetic acid (IAA) derived from indole or tryptophan
Synthetic auxins also exist, such as NAA and 2,4-D
Auxins primarily move from the apical (top) part of a plant, downwards
Influences plant growth and development
Functions:
- Stimulate cell elongation, cell division in cambium, phloem differentiation, and xylem development.
- Roots can be stimulated to develop from stem cuttings in tissue culture
- Delays leaf senescence and suppression of lateral bud development

Callus Formation

Plant growth regulators in tissue culture medium can induce callus formation (+ 2,4-D)
Stages in callus formation include cell separation, cell suspension, cell clumping, cell divisions, heart stage embryo, torpedo shape embryo, mature embryo, and germination.
2,4-D can be substituted to induce embryos from callus.

Auxin (Further functions)

Suppresses lateral bud growth: apical dominance
Inhibits or promotes fruit and leaf abscission
Promotes flowering in Bromeliads
Influences phototropism (plant bending toward light)

Cytokinin

Derivatives of purine adenine
Found in actively dividing tissues of seeds, fruits, leaves, and root tips, wound tissue sites
Promotes cell division and callus formation
Roots supply cytokinins upwards to the shoots
Cytokinins migrate through xylem tissue
Promotes cell division by speeding up progression from G2 phase to mitosis phase
Acts with auxin to promote elongation and expansion
Effects:
- Promotes axillary (lateral) bud growth; over-rides auxin's inhibiting effect
- Delays (retards) leaf senescence
- Promotes germination of some seeds in the absence of light

Gibberellin

Named after fungus Gibberella fujikuroi
Family of compounds
Many are inactive precursors to active forms
Biosynthesis occurs in proplastids, chloroplasts, and plastids
Accelerates metabolic pathways (respiration, protein synthesis)
Functions:
- Signals germination, breaks seed dormancy
- Promotes mobilization of food materials (e.g. starch in cereal grains) in the embryo
- Plays a role in metabolizing lipids to glucose and then to sucrose

Gibberellin (Further functions)

Works with auxins to promote rapid elongation and stem tissue division.
Can overcome genetic dwarfism
Can stimulate parthenocarpic fruit development in some species
Induces flowering in some plants (especially biennial plants and winter crops).
Vernalization in plants is the cold period required for flowering

Abscisic Acid (ABA)

Natural plant growth retardant
Often acts as an antagonist to gibberellins and auxin
Biosynthesis occurs primarily in leaves
Transported through xylem and phloem
Functions:
- Stimulates stomata closure (water stress response)
- Enhances drought resistance
- Promotes stomata closure during water deficit by activating potassium ion transport
Has important role in seed dormancy
- Maintaining high ABA levels in mature seeds and inhibiting germination
- Degraded by environmental factors such as light and enzymes triggering germination

Abscisic Acid (ABA) (Further functions)

Inhibits shoot growth, but may promote growth of roots
Induces seeds to synthesize storage proteins, and inhibits the effects of gibberellins on stimulating a-amylase
Has a role in seed dormancy
Low levels of ABA in maturing seeds can promote premature germination.
Induces gene transcription, especially for proteinase inhibitors in response to plant wounding and pathogens, which explains a defense role.

Ethylene

Sole plant growth regulator that is a gas (at room temperature)
Produced in many plant organs, including wounded and bruised tissues, and commonly studied in fruits
Synthesized from the amino acid methionine
High auxin concentrations promote the conversion of intermediate substances into ethylene
Toxic substances like air pollutants can trigger the conversion to ethylene
Functions:
- Stimulation of fruit ripening (bananas, honeydew, melon, tomatoes)
- Causes softening of fruit through degradation of cell walls
- Causes leaf and fruit abscission
Affects sex expression, promotes female flower production in some species, while high gibberellins promote male flower formation
Ethylene gas breaks down chlorophyll, causing the degreening of fruits like citrus

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Description

This quiz covers the regulation of developmental and differentiation hormones in plants, focusing on phytohormones. Students will learn to identify and explain the functions of various plant growth regulators and their impact on plant development. Understanding the roles of endogenous and exogenous hormones is crucial in grasping how plants respond to their environment.