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Questions and Answers
What is the primary function of auxins in plant growth?
Which statement correctly describes the difference between plant hormones and animal hormones?
What is one effect of gibberellins on plants?
What is the role of abscisic acid (ABA) in plants?
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Which of the following is NOT a type of plant growth regulator?
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Which of the following is a characteristic of plant hormones?
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How does excessive auxin concentration affect plant growth?
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What function of cytokinins is commonly recognized?
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What role does auxin play in fruit development?
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Which hypothesis explains how auxin promotes cell elongation?
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What was the primary observation made by Arpad Paál regarding auxin?
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How does auxin affect vascular tissue activity?
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Which type of auxin is commonly used in plant tissue culture?
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A low concentration of auxin primarily induces which of the following?
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What effect does high auxin concentration have in tissue culture?
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What is one function of synthetic auxins like 2,4-D in plant tissue culture?
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Study Notes
Plant Growth Regulators/Plant Hormones
- Plant growth regulators are hormones produced by plants.
- They are organic compounds active at very low concentrations.
- They mediate internal and external signals that regulate plant growth.
- Other names are Plant Growth Regulators/Phytohormone
- The Greek word for hormones is hormaein, meaning "to excite".
Difference of plant hormone to animal hormone
- Plant hormones' fundamental actions differ from animal hormones
- Plant hormones are not produced in specialized tissues.
- Plant hormones do not have specific target areas like animal hormones.
Each has a Multiplicity of Effects
- The effects of plant hormones depend on site of action.
- The effects of plant hormones depend on the developmental stage of the plant.
- The effects of plant hormones depend on the concentration of the hormone.
- The effects of plant hormones depend on the tissue specificity.
- Naturally occurring plant growth regulators are hormones.
- Synthetic growth regulators are not considered plant hormones.
Type of Plant Growth Regulator
- Auxin (indoleacetic acid): cell elongation
- Cytokinins (zeatin, zeatin riboside, isopentenyl adenine): cell division and inhibits senescence
- Gibberellins (GAx... 125): cell elongation and cell division, translated into growth
- Abscisic acid (ABA): abscission of leaves and fruits, dormancy induction of buds and seeds
- Ethylene: promotes senescence, epinasty, and fruit ripening
- Others (real and fabled): jasmonic acid, brassinolide, florigen, juvenone
Auxin
- Stimulates shoot cell elongation
- Produced in apical and root meristems, young leaves, seeds in developing fruits
- Stimulates growth but too much inhibits growth
- Examples: IAA, IBA, NAA
Auxin Functions
- Root initiation, stem elongation
- Retard abscission (loss) of leaves and fruits
- Stimulate cell differentiation
- Apical dominance
- Inhibit bud formation
- Embryogenesis
Auxin Discovery
- Auxin increases the plasticity of plant cell walls, involved in stem elongation
- Arpad Paál (1919): Asymmetrical placement of cut tips on coleoptiles resulted in bending away from the side the tips were placed.
- Frits Went (1926): determined auxin enhanced cell elongation
Auxin Acid Hypothesis
- Auxins work by causing responsive cells to actively transport hydrogen ions from the cytoplasm into the cell wall space.
- This causes the cell wall to loosen, allowing for cell elongation.
Biosynthesis of auxin
- Auxin is synthesized from tryptophan.
Type of auxins in plant tissue culture
- Indolebutyric acid (IBA)
- Indoleacetic acid (IAA)
- 2,4 dichlorphenoxyacetic acid (2,4D)
- 2,4,5 trichlorophenoxyacetic acid (2,4,5T)
- picloram
Application of auxin in tissue culture
- Low auxin results in adventitious root formation.
- High auxin results in callus formation.
- 2,4D induces mutations and inhibits photosynthesis
Cytokinin
- In 1941, Johannes van Overbeek discovered cytokinin in coconut milk.
- Coconut milk contained potent growth factors.
- Factors greatly accelerated plant embryo and cell development in vitro.
- Crucial for isolated plant tissue studies
- Facilitated research for another major group of growth regulators
Production of cytokinin
- Produced in root meristems, young leaves, fruits, and seeds
- Functions: cell division, stimulates adventitious bud formation, delays senescence, promotes some stages of root development
Cytokinins and Auxins
- In combination, cytokinins and auxins stimulate cell division and differentiation.
Structure of cytokinin
- Simple structure
- Zeatin is the most active naturally occurring cytokinin
- High cytokinin/auxin ratios favor shoot formation
- Low cytokinin/auxin ratios favor root formation
Cytokinin/Auxin Ratio
- The cytokinin/auxin ratio regulates the production of roots and shoots in tissue cultures.
- Undifferentiated plant cells can enlarge, divide, and continue undifferentiated or elongate and differentiate without undergoing cell division.
Commercial use of Cytokinins
- Applied as kinetin, benzyladenine or zeatin conjugates
- Used for axillary bud growth in orchids, daylilies
- Used as an antioxidant (browning preventor) in cut salads.
- Used with GAs for fruit size stimulation.
Application use of cytokinins
- High concentrations of kinetin, BA, 2-ip, and PBA induce adventitious shoots, but root formation is inhibited. In tissue culture or tobacco, IAA induces rapid cell expansion- leading to giant cells. Kinetin alone has little or no effect.
PGR and regeneration of Himalayan rice
- Involve different stages of regenerated shoots (e.g., embryogenic calli).
- The stages are related to the presence of specific concentrations of various PGRs such as NAA, BAP.
Other plant hormones
- Brassinosteroids: A class of polyhydroxylated steroids and are plant growth regulators.
- Salicylic acid: Activates genes to produce chemicals against pathogens.
- Karrikins: Plant growth regulators found in the smoke of burning plant material and promote seed germination.
- Systemin: A polypeptide that functions as a long-distance signal to activate chemical defenses against herbivores.
- Nitric oxide (NO): A signal in hormonal defense responses
Jasmonic acid
- Produced from fatty acids.
- Promotes the production of defense proteins against invading organisms.
- Believed to have roles in seed germination, effects on seed protein storage and root growth.
Plant peptide hormones
- Encompasses small secreted peptides involved in cell-to-cell signaling
- These hormones play crucial roles in plant growth, defense mechanisms, cell division, cell expansion, and pollen self-incompatibility
Abscisic acid (ABA)
- General growth inhibitor; causes stomatal closure
- Produced in response to stress
- Natural plant growth retardant
- Produced mainly in mature green leaves and fruits
- Suppresses bud growth and promotes leaf senescence
- Important roles in controlling stomatal opening/closing
- High levels found in dormant seeds and buds
- Drought resistance causes stomatal closure
- Levels increase during early seed development
- Stimulates seed storage protein production
- Prevents premature seed germination
ABA in tissue culture
- Inhibits callus induction.
- Can have various effects on adventitious shoot and root formation.
- Helpful for the normal growth of somatic embryos.
Gibberellins
- Role in many functions associated with tissue elongation and phytochrome-mediated responses
- Cell elongation and expansion
- Stimulation of a-amylase activity
- Elongation of flower stalks of chilled bulbs
- Reversal of "physiological dwarfism"
- Found in immature seeds, highest concentration
- Stimulates cell division and cell elongation
- Application to dwarf mutants causes them to grow tall
- Plays a role in breaking seed dormancy and germination
Commercial use of gibberellins
- Use as GA3 or GA4+7
- Increase flower size on ornamentals
- Increase berry separation and size in bunch grapes
- Overcome shallow dormancies in vegetative buds
- Stimulate seed germination
Ethylene
- Gaseous hormone produced in many plant tissues.
- Autocatalytic (stimulates its own production).
- Volatile gas.
- Production stimulated during ripening, flooding, stress, senescence, mechanical damage, and infection.
Function of Ethylene
- Gaseous form, rapidly diffusing
- Gas produced by one plant affects nearby plants
- Fruit ripening
- Epinasty, downward curvature of leaves
- Encourages senescence and abscission
- Initiation of stem elongation and bud development
- Flowering inhibition in most species, but promotion in some plants.
Ethylene Pathway
- Ethylene is produced from the precursor ACC.
Climacteric Changes of Ethylene Gas in Ripening of Pear
- Respiration rate increases during ripening, with peak ethylene production coinciding with significant cell breakdown stages.
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Description
Explore the fascinating world of plant growth regulators, also known as plant hormones or phytohormones. This quiz delves into their roles, differences from animal hormones, and the various factors that influence their effects on plant growth. Test your knowledge of these essential organic compounds that regulate development in plants.
