Download Chapter 39 (part) –Plant Hormones I. Introduction to Plant Hormones

Randa, Bio103
1
Chapter 39 (part) –Plant Hormones
I. Introduction to Plant Hormones
A. Are chemical signals between cells
1. Effective at very low concentrations
2. Very important to the growth of plants
B. Hormones and Second Messengers
1. Can involve G-protein signaling
2. Operate via second messengers, such as cAMP, within the cell
3. This activates a protein kinase cascade
4. Can affect transcription, physiological activity
II. Auxin—an Introduction
•
•
•
Darwin showed first evidence for this substance
Shaded different portions of the coleoptile of canary grass seedlings
Concluded that a substance was passed from the tip to the lower parts, causing it to bend
A. Darwin and Phototropism
1. Opaque covers on the tip caused a loss of response to light
2. Covers on the lower portion of the stem had no effect on bending toward light
3. If the tip was removed, no bending
4. Concluded: something produced in the tip is responsible for phototropism
B. Went, Avena (Oats), and Auxin
1. Isolated phototropic hormone from oat coleoptiles in the 1920s
2. Placed coleoptiles on agar blocks
3. When agar block placed on decapitated shoot, growth resumed
4. Agar blocks caused increased growth on the side of the growing shoot
C. Effects of Auxin
1. Auxins cause the increase in cell length on the side away from light
2. Causes bending
3. Indole acetic acid (IAA) is one of the most important auxins
4. Causes acidification, and softening, and so elongation with turgor pressure, of the cell
wall
Randa, Bio103
III. Gibberellins—an Introduction
• Kurosawa (Japan) in 1926 studied a disease of rice (Oryza sativa) that caused
•
•
•
•
•
•
exaggerated growth – caused by a fungus called Gibberella fujikuroi
In 1956 MacMillan, in England, first isolated gibberelins from a plant (the bean,
Phaseolus vulgaris)
Cause rapid stem elongation – can be extraordinary
Cause bolting – rapid growth of flower-bearing stem in many plants, prominent in
cabbage-related species
Cells are stimulated to elongate AND divide
Stimulate flowering and germination in many plants
Can bypass requirement for vernalization in many plants
A. Gibberellins in Corn
1. Application of gibberellins to dwarf corn plants caused exceptional growth
2. Left two plants are dwarf
3. Had little effect on normal corn
B. Bolting
1. In the first year Indian blanket (Gaillardia pulchella) grows as a rosette
2. In the second year of growth bolting occurs (right)
3. Is the result of gibberellin production
IV. Cytokinins—an Introduction
•
•
•
•
•
•
•
Cytokinins promote cell division and differentiation
Required if a researcher wishes to grow plant tissues in culture
A balance effect occurs between cytokinin and auxin
High ratio of cytokinin to auxin promotes shoot formation
Low ratio of cytokinin to auxin promotes root formation
In apical dominance, auxin inhibits axillary bud formation; cytokinin promotes it
Cytokinins delay cellular aging, or senescence
Cytokinin and Auxin
1. Control of meristematic tissue activity by auxin and cytokinin
2. Determination of the differentiated tissue type depends on the ratio of auxin to
cytokinin
V. Ethylene—an Introduction
•
•
•
•
•
Ethylene gas is produced by plants
Is critical to many plant functions
Cell elongation
Seed germination
Apical dominance
2
Randa, Bio103
•
•
3
Wound responses
Fruit ripening
1. Ripening fruit produce ethylene, so a ripe fruit promotes ripening of nearby fruit
2. Ripe banana in a bag with other fruit will promote ripening of the second fruit
3. Leaf abscission in deciduous plant promoted as the ethylene: auxin ratio in cells at the
base of the petiole increases during the fall
VI. Abscisic Acid—an Introduction
• Promotes plant changes during water stress
• Causes stomata to close, saving water
• Abscisic acid is present in large amounts in dormant seeds; dormancy is not released
•
until the abscisic acid concentration drops as it is washed out
Gibberellin and cytokinins also stop dormancy
VII. Other Plant Hormones
A. Brassinolides
• Steroids
• Important to plant growth
B. Salicylic acid
• Related to aspirin; similarly, relieves headaches
• Defends plants against insect pests and pathogens
• A derivative, methyl salicylic acid, or wintergreen oil, is released by tobacco plants
to activate antiviral protein production
C. Oligosaccharins
• Short cell wall carbohydrates
• Can trigger production of antifungal phytoalexins
• Can also inhibit flower growth, induce vegetative growth
D. Jasmonates
• Fatty acid derivatives
• Induce many different plant differentiation processes
VIII. Not All Hormonal Responses Are Fully Characterized
• Grafts of different types of tobacco to each other can induce different behaviors
• Here, a day-neutral strain grafted to a short-night strain will induce short-night
behavior, causing both plants to flower
• An unknown substance, referred to as a florigen is thought to be responsible for the
response