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.
