Phytohormones, 1

Phytohormones

HORT 3000

Phytohormones

Naturally occurring organic (C-H-O based) substance that influences physiological processes

Chemical messenger within the plant Provides communication between different areas within the plant Site of action is often different from site of synthesis Six main types in plant

Auxins, Gibberellins, Cytokinins, Abscisic Acid, Ethylene, Brassinosteriods Auxin

Indole-3-acetic acid First hormone to be discovered Ubiquitous throughout the plant Synthesized in embryos and shoots Concentration varies highest in actively growing regions

Stimulates cell elongation Auxin Polar Transport

Preferential movement in one direction Predominately basipetal auxin movement in the shoot auxin moves from morphological

apex towards morphological base

Auxin moves from shoot apex down stem Path of transport not totally clear travels in cells near vascular tissue

Basipetal and acropetal movement occur in roots Basipetal auxin moves away from root tip via epidermal and cortical cells Acropetal auxin moves towards root tip through stele

Auxin Polar Transport

Auxin Polar Transport

Auxin Acid Growth Hypothesis

Auxin-activated proton pump in cell membrane acidifies cell wall space Pumps in protons from cytosol

Phytohormones, 2

Lower pH activates wall-loosening enzymes, extensins Extensins loosen glycan bonds between microfibrils within cell wall

Turgor then causes micofibrils to be displaced, allowing elongation Auxin Acid Growth Hypothesis

Auxin Vascular Differentiation

IAA-induced xylem regeneration Polar auxin transport visible in accumulation above wound Xylem regeneration occurs first above wound Auxin Root Initiation

Auxin Axillary Bud Break

Auxin Tropic Curvature and Growth

Gibberellins

Gibberellic Acid, GA3 Synthesized in seeds, fruits, and young leaves Modified in roots Stem elongation Flower and fruit development Gibberellins Internode Elongation

Gibberellins Seed Germination

Helps to mobilize endosperm reserves Gibberellins Flower and Fruit Development

Cytokinins

Kinetin Benzyladenine Zeatin Synthesized in root Important for cell division

Cytokinesis division of cytoplasm during mitosis Used in tissue culture to encourage shoot proliferation

(auxin used to promote root proliferation) Cytokinin Witchs Broom

Phytohormones, 3

Fungal infection causes overproduction of cytokinin and uncontrolled axillary bud development

Abscisic Acid

ABA Synthesized mostly in mature leaves Not actually directly involved in abscission Important signal for plant water relations, stomatal activity Active during seed germination Ethylene

H2C=CH2 (gas) Synthesized

In all plant organs In tissues experiencing stress In tissues experiencing senescence and/or ripening

MCP = ethylene blocker used to preserve cut flowers Methylcyclopropene

Phytohormones

Brassinosteriods

Brassinolide is active component Newly discovered group of hormones Stimulate shoot growth and inhibit root growth Involved in stem elongation and leaf morphogenesis Play a role in vascular differentiation Phytohormones

HORT 3000