Plant Hormones Overview

Plant Hormones

• Plant hormones, also known as phytohormones, are signaling molecules produced within plants, controlling various physiological processes.
• They are produced in very small amounts but have profound effects on growth, development, and responses to environmental stresses.
• Several major classes of plant hormones exist, each with diverse roles:
  • Auxins: Promote cell elongation, involved in apical dominance, phototropism, and gravitropism. They are often produced in the shoot tips and transported downwards.
  • Gibberellins: Stimulate stem elongation, flowering, and fruit development. They also play a crucial role in mobilization of food reserves during seed germination.
  • Cytokinins: Promote cell division and differentiation, delay senescence (aging) in leaves and flowers, and are involved in apical dominance regulation. They are often produced in roots and transported upwards.
  • Abscisic acid (ABA): Inhibits growth, regulates stomatal closure during water stress, and promotes seed dormancy. It's also involved in responses to various environmental stresses.
  • Ethylene: Promotes fruit ripening, leaf abscission (falling), and senescence. It also plays a role in stress responses.
• Plant hormones often interact in complex ways, modulating each other's effects. For instance, auxins and cytokinins are often antagonistic, influencing the balance between cell division and elongation.
• Environmental factors significantly affect hormone production and actions, influencing plant growth and development in response to light, temperature, water availability, and nutrient levels.

Animal Hormones

• Animal hormones are chemical messengers produced by endocrine glands, traveling through the bloodstream to target tissues to produce specific responses.
• These hormones regulate a vast array of functions including metabolism, growth, reproduction, and homeostasis.
• The major classes of animal hormones include:
  • Steroid hormones: Derived from cholesterol, typically hydrophobic and can cross cell membranes. Examples include cortisol, estrogen, progesterone, and testosterone. They affect gene expression in target cells.
  • Peptide hormones: Short chains of amino acids, typically hydrophilic and cannot cross cell membranes. They bind to receptors on cell surfaces, triggering intracellular signaling cascades. Examples are insulin, glucagon, and growth hormone.
  • Amine hormones: Derived from amino acids like tyrosine. They can be either hydrophilic or lipophilic, depending on the specific hormone. Examples include epinephrine (adrenaline), norepinephrine, and thyroid hormones.
• Hormone production is highly regulated, usually by negative feedback mechanisms. When a hormone's level rises, it triggers a response that reduces further production, ensuring homeostasis.
• Endocrine glands like the pituitary, thyroid, adrenal, and gonads are crucial for hormone production and regulation.
• Hormone receptors on target cells are highly specific, ensuring that only certain cells respond to particular hormones. Specific receptors determine the targets for hormone action.
• Hormones are involved in complex interactions throughout the body, mediating communication between different organs and systems.
• The actions of some hormones can be affected by other hormones in a synergistic or antagonistic manner; multiple hormones may work together for a full developmental response.
• Hormonal imbalances can lead to various disorders, from metabolic problems to reproductive issues, highlighting the importance of precise hormonal regulation in animals.