Phytohormones as Signalling Chemicals Controlling Growth, Development, and Response to Stimuli in Plants
- Plants, unlike animals, lack a nervous system.
- However, they rely on phytohormones, chemical messengers, to regulate growth, development, and responses to environmental stimuli.
- These hormones are produced in one part of the plant and transported to target cells, where they trigger specific physiological responses.
- Imagine a seedling growing in a forest.
- It needs to find light, anchor its roots, and respond to changes in its environment—all without a nervous system or muscles.
- How does it achieve this?
- The answer lies in phytohormones, the chemical messengers that coordinate nearly every aspect of a plant’s life.
What Are Phytohormones?
Definition
Phytohormones
Phytohormones are signalling chemicals that regulate growth, development, and responses to environmental stimuli in plants.
- Phytohormones are chemical messengers that influence plant processes.
- Unlike animal hormones, which are produced in glands, phytohormones are synthesized in various plant tissues and can act locally or be transported to distant sites.
- Auxin promotes growth in shoots but inhibits growth in roots.
- This is why roots grow downward while shoots grow upward.
Types of Phytohormones
- There are five major types of phytohormones that are essential in the regulation of plant activities.
- Each type of hormone has specific roles, and they often interact with each other to produce the overall response of the plant.
Auxins
- Role: Promote cell elongation, root initiation, and development of lateral buds.
- Key Function: Auxins control phototropism (growth towards light) and gravitropism (growth in response to gravity).
Cytokinins
- Role: Promote cell division and delay leaf senescence.
- Key Function: Cytokinins stimulate the growth of lateral buds (acting as anti-apical dominance hormones) and regulate leaf development.
Gibberellins
- Role: Stimulate stem elongation, seed germination, and flowering.
- Key Function: Gibberellins play a role in breaking dormancy in seeds and promoting growth by elongating the stem. They also regulate flowering in some plants.
- Don’t confuse auxin and gibberellins.
- While both promote growth, auxin is primarily involved in cell elongation, whereas gibberellins focus on stem elongation and seed germination.
Abscisic Acid (ABA)
- Role: Inhibits growth and promotes dormancy.
- Key Function: ABA is involved in stress responses such as drought tolerance, by promoting stomatal closure to prevent water loss. It also regulates seed dormancy and helps plants adapt to adverse environmental conditions.
Ethylene
- Role: Regulates fruit ripening, leaf abscission, and response to stress.
- Key Function: Ethylene is a gaseous hormone that triggers the ripening of fruits and the aging of leaves (senescence). It also plays a role in plant stress responses and wounding.
Think of phytohormones as the government of a country, regulating all aspects of plant life:
- Auxins are like city planners, directing how the plant grows and ensuring taller growth in the right direction.
- Cytokinins are like business investors, encouraging new developments (cell division) and promoting lateral expansion.
- Gibberellins act like sports coaches, pushing plants to grow taller and break seed dormancy (like an athlete breaking out of rest mode).
- Abscisic Acid (ABA) is the disaster management team, responding to drought, stress, and closing stomata to prevent water loss.
- Ethylene is like the news media, spreading signals quickly to trigger fruit ripening and aging processes.
How Phytohormones Control Development
1. Apical Dominance
- Auxin produced in the shoot tip inhibits the growth of lateral buds, ensuring the plant grows taller rather than bushier.
- When the shoot tip is removed (e.g., by pruning), cytokinins in the lower parts of the plant stimulate lateral bud growth, leading to a bushier shape.
Think of auxin as a manager who focuses on upward growth, while cytokinins are like team members who encourage branching out when the manager is away.
2. Fruit Ripening
- Ethylene is a gaseous hormone that triggers fruit ripening.
- It softens fruit by breaking down cell walls and converts starches into sugars, making the fruit sweeter.
- Ethylene also acts in a positive feedback loop: ripening fruit produces more ethylene, which accelerates the process.
Placing a ripe banana in a bag with unripe tomatoes speeds up their ripening due to ethylene released by the banana.
How Phytohormones Control Responses to Stimuli
Tropisms
- Tropisms are directional growth responses to external stimuli, such as light (phototropism) or gravity (gravitropism).
- Auxin plays a central role in these processes by redistributing itself in response to the stimulus, causing differential growth.
Shoots are typically positively phototropic(grow toward light) and negatively gravitropic(grow against gravity), while roots are positively gravitropic(grow with gravity).
Stress Responses
- Abscisic Acid (ABA) helps plants survive stress conditions.
- During drought, ABA signals stomata (tiny openings on leaves) to close, reducing water loss.
- It also inhibits growth to conserve energy.
How do plants "decide" which hormones to produce in response to environmental changes? Consider the role of natural selection in shaping these mechanisms.
Key Concepts and Roles of Phytohormones
- Auxins and Cytokinins often counterbalance each other: auxins promote growth in one direction, while cytokinins promote lateral growth. Together, they help maintain balance in plant growth.
- Gibberellins and Abscisic acid play opposing roles in seed germination. While gibberellins promote germination, ABA inhibits it until favorable conditions are met.
- Ethylene is often associated with aging and stress responses. It accelerates processes like fruit ripening and leaf abscission, often in response to environmental changes.
- How do auxins and cytokinins work together in the plant?
- Describe the role of gibberellins in seed germination.
- How does ethylene affect fruit ripening?
