KINGDOOM OF SAUDI ARABIA King Saud University College

KINGDOOM OF SAUDI ARABIA King Saud University College of Sciences - Department of Botany and Microbiology Plant Physiology BIO 312 Dr. Abdulrahman Al-hash imi

Lecture 12 Plant hormones (Phytohormones) 1. Auxins 2. Gibberellins 3. Cytokinins 4. Ethylene 5. Abscisic acid

Introduction Ø Plant hormones (also known as Phytohormones or plant growth regulators) are chemical substances produced by the plants, that influence the growth and differentiation of plant cells, tissues and organs, effective at very low concentrations. Ø Plant hormones function as chemical messengers for intercellular communication, and work together for coordinating the growth and development of the cells. Ø Five major groups of plant hormones: Auxins - Gibberellins - Cytokinins - Ethylene - Abscisic acid

Auxin: The Growth Hormone Ø The most common natural auxin is indole acetic acid (IAA). Ø Apical meristem is the major site of the auxin production. Ø Physiological effects of Auxin: 1. Promotes apical dominance in higher plants, the growing apical bud inhibits the growth of lateral (auxiliary) buds. 2. Stimulates the stem elongation. 3. Delays the starting of leaf abscission. 4. Promotes fruit development. 5. At higher concentrations, induces differentiation of xylem. 6. At higher concentrations, acts as a root growth inhibitor.

Auxin: The Growth Hormone

Gibberellins: Regulators of Plant Height Ø The gibberellins are a large group of related compounds (more than 125 are known). Ø Physiological effects of Gibberellins: 1. Promote cell elongation and cell division. 2. Stimulate stem growth in dwarf and rosette plants. 3. Induce α-amylase biosynthesis. 4. Promote seed germination. 5. Promote fruit growth and fruit set. 6. Inhibit growth of the root.

Gibberellins: Regulators of Plant Height

Cytokinins: Regulators of Cell Division Ø Cytokinins are synthesized in roots, developing embryos, young leaves and fruits. Ø Physiological effects of Cytokinins: 1. Promote cell division and differentiation. 2. Regulate apical dominance. 3. Stimulate protein synthesis. 4. Modify apical dominance and promote lateral bud growth. 5. Delay leaf senescence. 6. Promote movement of nutrients.

Ethylene: The Gaseous Hormone Ø Ethylene is formed in most organs of higher plants. Ø Senescing tissues and ripening fruits produce more ethylene. Ø Physiological effects of Ethylene: 1. Promotes the ripening of some fruits. 2. Induces lateral cell expansion. 3. Induces the formation of roots and root hairs. 4. Breaks seed and bud dormancy in some species. 5. Enhances the rate of leaf senescence. 6. Regulates the expression of various genes.

Abscisic Acid: Antistress Signal Ø Physiological effects of Ethylene: 1. Inhibits shoot growth 2. Inhibits cell division in the vascular cambium. 3. Regulation of seed maturation and dormancy. 4. Promotes accumulation of seed storage protein during Embryogenesis 5. Promotes root growth and inhibits shoot growth. 6. Closes Stomata in response to water stress. 7. Promotes leaf senescence.

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