HORMONE PHYSIOLOGY AND SIGNAL TRANSDUCTION Huseyin Tombuloglu, Phd. GBE 304 Spring 2015

Growth and Development Growth: irreversible increase in size (net increase in protoplasm) brought about by increases in cell size and number. Development: which includes… Morphogenesis: creation of pattern and shape resulting in morphological and anatomical development. Differentiation: physiological and biochemical specializations.

Specialization of Cells Coordination/Cooperation between specialized cells for survival. Development involves a specific, logical sequence of events…. Life Cycle (highlights): Fertilization Embryo development Germination Vegetative growth Reproductive Growth Gametogenesis Senescence Plant Death

Plant Hormones Coordinate complex steps in growth and development. Coordinate complex steps in growth and development. Play key roles in signal transduction. Play key roles in signal transduction. Integrate many different plant parts. Integrate many different plant parts. Coordinate “timing” of growth and development. Coordinate “timing” of growth and development. Very potent at low concentrations. Very potent at low concentrations.

Plant Hormones Definition of a Plant Hormone: 1. Naturally occurring organic substance. 2. Affects plant growth and development. 3. Operates in very low concentrations. 4. Actions may involve sites far removed from hormone origin. Hormones elicit a wide range of responses…. from growth to dormancy.

Hormones Classes Commonly grouped into five (5) categories…but they have multiple, overlapping, and interacting functions: 1. AUXINS 2. GIBBERELLINS 3. CYTOKININS 4. ETHYLENE 5. ABSCISIC ACID

AUXINS

Auxins [indoleacetic acid (IAA)] first plant hormone to be discovered. early on it was believed to be the “master” plant hormone. studies date back to the time of Charles Darwin (1880s). very simple chemical structure….dramatic influences on growth. “growth promoting hormone”

Cell elongation is controlled by IAA believed to cause release of H + protons from protoplast into cell wall. acidic protons weaken cell wall structure which softens the wall and allows for expansion from turgor pressure. Thus, auxin promotes cell elongation.

Auxin-induced cell wall expansion.

Functions of Auxins Elongation of stems and roots. Responsible for tropisms….phototropisms. Enlargement of fruits and tubers. Promotion of cell divisions in cambium tissues. Vascular cambium…auxin signal in Spring from buds. Secondary xylem initiation.

Taiz and Zeiger Plant Physiology, 3 rd Ed. Auxin and root formation

Taiz and Zeiger Plant Physiology, 3 rd Ed. Auxin and Geotropism

Functions of Auxins Wound healing. Promotes pith and cortex cells to differentiate into sclerenchymatous xylem to cover wounds. Promotes rooting of plant cuttings. Promotes rooting of soft wood cuttings. Inhibits growth in areas other than root tissues. Shorten stems and retards growth. Apical dominance…function of auxin distribution.

Functions of Auxins Inhibits abscission of leaves and fruit. Prevents leaf and fruit drop. Tissues send continuous auxin signal to prevent abscission layer development. Auxin release and movement to zones of abscission is halted by: Disease Injury…lighting strikes Natural/Seasonal development

Auxins and Leaf Abscission In the Fall, leaves fall from the trees because the leaf tissues STOP producing auxins. Same mechanisms in place for fruit drops (apples). Auxins are in highest concentrations in the stem apex of plants. Such high concentrations inhibit the growth of axillary buds….apical dominance.

Pruning, or removing the auxin signals results in elimination of apical dominance and a flush of later meristematic growth.

Auxins function best at very low concentrations.

Auxins and Horticulture Practical applications in agriculture and horticulture: Useful in propagating/rooting of plant tissues cuttings. As a herbicide. Prevention of fruit drop. Thinning of fruits….depending on the concentration.

Auxins and Horticulture Most often, synthetic auxins are preferred over natural IAA. 2,4-D = 2,-4-dichlorophenoxacetic acid Herbicide which mimics the activity of IAA. Plant Growth Regulators are often synthetic auxins. Remember that plant hormones are most effective in SMALL quantities. Low concentrations…stimulate growth. High concentrations…herbicidal.

Auxins and Horticulture 2,4-D is very effective at controlling broadleaf dicot plants. It has no effect on grass monocots. Differences in control represent metabolic differences between the dicots and monocots. This fact is utilized successfully by agriculture and horticulture professionals.

Agent Orange U.S. ArmyU.S. Army Huey helicopter spraying Agent Orange over Vietnamese agricultural land.Huey helicopter U.S. Air Force's herbicide program in Vietnam

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GIBBERELLINS