Control of Plant Growth and Development
Control of Plant Growth and Development
Plant Hormones
- Plant hormones play critical roles in regulating growth and development. Key classes include:
- Auxins: Promote growth.
- Gibberellins: Stimulate growth, particularly stem elongation.
- Cytokinins: Enhance growth and delay aging.
- Ethylene: Regulates responses like senescence.
- Brassinosteroids: Regulate growth responses.
- Abscisic acid: Suppresses growth; involved in stress responses.
- Jasmonates and Oligosaccharins: Regulate growth; defense mechanisms.
Seven Classes of Hormones
Overview of Hormones and Effects
| Hormone | Where Synthesized | Tissues Affected | Effects |
|---|
| Auxins | Apical meristems, developing leaves | Growing tissues, buds, roots, leaves, fruits | Promote growth and elongation, encourage lateral roots formation, inhibit leaf abscission |
| Gibberellins | Shoot/root tips | Stems, developing seeds | Promote stem elongation, seed germination, and bud break |
| Cytokinins | Root and shoot tips, young leaves | Mainly in root tips | Stimulate cell division, promote leaf expansion and prevent aging |
| Ethylene | Shoot tips and various plant parts | Fruits, flowers, leaves | Promotes ripening, senescence, and abscission |
| Brassinosteroids | Young seeds, shoots, and leaves | Various plant tissues | Stimulate cell division and elongation |
| Abscisic acid | Leaves | Stomata and various plant parts | Promotes dormancy, stomatal closure under stress conditions |
| Jasmonates | Roots, seeds, various tissues | Damaged tissues | Regulate growth and trigger defensive responses |
| Oligosaccharins | Cell walls | Various tissues | Play a role in defense and growth regulation |
Auxins
- Indoleacetic acid (IAA): Main auxin synthesized in shoot apical meristem and young stems.
- Effects include:
- Promoting cell elongation in stems.
- Governing phototropism and gravitropism responses.
Darwins' Experiments on Phototropism
- Initial observations included responses of seedlings to light.
- Experiments showed that removal of tips or use of caps impacted growth direction.
Went's Experiments
- Cut the tip of an oat seedling, which ceased elongation.
- Placed the excised tip on an agar block, allowing IAA to diffuse.
- Agar was placed on the side of a detipped seedling, causing it to bend due to uneven IAA distribution.
Gibberellins
- Most common gibberellin: GA1.
- Function: Promote stem elongation, break dormancy in seeds and buds.
- Effects demonstrated through experiments showing bolting and fruit enlargement.
Cytokinins
- Zeatin: Most well-known natural cytokinin.
- Stimulate cell division, enhance leaf expansion, and delay senescence.
- Ratios of auxin to cytokinin are important for tissue culture growth.
Ethylene
- A gaseous hormone involved in fruit ripening, leaf abscission, and senescence.
- Influences physiological processes such as the abscission zone formation at leaf bases.
Abscisic Acid
- Promotes stomatal closure and influences seed/bud dormancy.
Jasmonates
- Play signaling roles affecting growth and activating defense mechanisms.
Signal Responses at the Cellular Level
- Signal pathways alter the behavior of target cells through hormone receptor binding.
- Internal second messengers amplify the hormone's effects, leading to changes in cell metabolism and structure.
- An example is IP3, which aids ABA in closing stomata by regulating calcium ion flow.
Plant Chemical Defences
- Involves jasmonates and other compounds that lead to hypersensitive responses and enzyme production for defense.
- Gene-for-gene recognition allows quick responses to specific threats.
Plant Responses to the Environment
Movements
- Phototropisms: Growth responses to light.
- Gravitropism: Response to gravitational pull (roots positive, stems negative).
- Thigmotropism: Growth in response to contact (reaching around obstacles).
- Nastic movements: Non-directional responses, such as those seen in Mimosa pudica.
Biological Clocks
- Regulate daily and seasonal biological rhythms through circadian clocks and photoperiodism.
- Phytochrome: Blue-green pigment that switches between active (Pfr) and inactive (Pr) forms depending on light conditions, influencing growth and flowering.
- Flowering responses depend on day length and night length, affecting various plant types (long-day, short-day, and day-neutral).
Dormancy
- A survival mechanism where certain plants cease growing under unfavorable conditions, often controlled by phytochrome.
- Breaking dormancy requires exposure to specific environmental cues like low temperatures, often indicated by gibberellins and abscisic acid signaling.
Summary
- Understanding plant hormones and their signaling pathways is vital for managing plant growth, development, and responses to environmental stressors. These pathways are crucial for agricultural practices and understanding plant physiology.