Biological Foundations 112, Lecture 18


Plant Hormones I

Chapter 35, pages 750-756


I.   Plant Movements and Growth Responses

     A.   Tropisms

            1.   Definition:  Plant growth in response to an external stimulus.

            2.   Phototropism

                  a.   Plant growth in response to the stimulus of light

                  b.   Etiolated stems - stems grown in the dark that are yellowish in color and have
                                                     long internodes.

            3.   Gravitropism - plant growth in response to gravity

            4.   Thigmotropism - plant growth in response to a mechanical stimulus (touch, etc.)

     B.   Turgor Movements

            1.   Movements of plants in response to changes in turgor pressure in the cells of the
                  plant

            2.   Leaves fold up when you touch a sensitive plant, Mimosa pudica

                  a.   An external stimulus triggers the response

                  b.   Loss of turgor pressure causes the leaves to fold quickly

     C.   Circadian Rhythms

            1.   Definition:  A biological clock that approximates a 24-hour cycle

            2.   Phytochrome

                  a.   Probably the photoreceptor chemical involved in resetting the biological clock

                  b.   Plants appear to "anticipate" sunrise and sunset

            3.   Nyctinastic movements are sleep movements of the plant directed by the
                  biological clock

II.  Hormonal Regulation of Plant Growth

      A.  Introduction

            1.   Plants are totipotent

                  a.   Each plant cell contains all the genetic information to form a complete plant from
                        that cell

                  b.   Thus cells need external signals to coordinate growth and development

                  c.   Plant hormones do this coordination

            2.   How hormones work:

                  a.   Act in low concentrations

                  b.   Function as internal signals

                  c.   How plant hormones initiate their effects is not fully understood

                  d.   Effects of hormones overlap

                  e.   Plant hormones coordinate:

                        (1)  Cell division - new cell production as a result of mitosis

                        (2)  Cell growth - increases in cell size and organ size

                        (3)  Differentiation - specialization in structure and function of the cell

            3.   Five classes of hormones:

                  a.   Auxins

                  b.   Gibberellins

                  c.   Cytokinins

                  d.   Ethylene

                  e.   Abscisic Acid

      B.  Auxins

            1.   First detected in the coleoptiles of grass because auxin stimulated elongation of the
                  grass coleoptiles

            2.   Indoacetic Acid

                  a.   Only naturally occurring auxin (IAA)

                  b.   Synthesized from the amino acid tryptophan

                  c.   A number of synthetic auxins have been produced

            3.   Synthesized in the apical meristem of stems

                  a.   Transported polarly

                  b.   Transported in a basipetal (downward) direction [acropetal direction is up]

                  c.   About 10 mm/hr transportation rate (too fast for diffusion)

                  d.   Takes energy to transport auxin

            4.   Function of auxins

                  a.   Causes cell elongation or growth

                        (1)  Does this by breaking the cross bonds of the cellulose polymers so that the
                              cell can expand and then reforms them again

                        (2)  The acid-growth hypothesis suggests that auxin causes hydrogen ions to be
                               pumped into the cell wall, activating enzymes which break the bonds,
                              making the wall flexible for expansion

                        (3)  Turgor pressure causes the cell wall to expand

                  b.   Phototropism

                        (1)  Sunlight causes differential flow of auxin down the stem.  More auxin flows
                              down the dark side of the stem than the on the light side.

                        (2)  Thus more elongation of the cells occurs on the dark side of the stem and  
                              causes the stem to bend toward the light

                         (3)  IAA (natural auxin only) is destroyed by sunlight

                  c.   Apical dominance

                        (1)  Auxin inhibits the development of lateral buds

                        (2)  Does this by inhibiting the formation of a vascular connection between the
                               lateral bud and the stem

                        (3)  Removing the apical bud removes the source of auxin and the lateral buds 
                               begin to grow

                  d.   Fruit development

                        (1)  IAA produced by seeds in the fruit stimulates the pericarp to develop

                        (2)  IAA applied to flowers can cause fruit development without fertilization

                  e.   Synthetic auxins

                        (1)  NAA (naphthaleneacetic acid) is used to stimulate root development on
                              cuttings for asexual propagation

                        (2)  2,4-D

                               (a)  A selective herbicide

                               (b)  Kills dicots but not monocots

      C.  Cytokinins

            1.   Discovery

                  a.   1940's and 1950's researchers were trying to find substances that would induce 
                        plant cells to divide in tissue culture

                  b.   Found that coconut water and autoclaved herring sperm DNA would do the
                        trick

                  c.   In 1956 the active substance was isolated from herring sperm and was called  
                        cytokinin (because it induced cell division or cytokinesis)

                  d.   In 1963 the first natural cytokinin was isolated from corn and named zeatin

            2.   Chemistry

                   a.   Cytokinins are similar to purine and adenine

                   b.   They are very closely related to nucleic acids (particularly t-RNA)

            3.   Produced in actively growing tissues, particularly roots, embryos, and fruits

            4.   Transported in the xylem

            5.   Functions of cytokinin

                  a.   Promotes cell division (mitosis) and differentiation in only some plants

                  b.   Required ingredient in plant tissue culture media for inducing mitosis

                  c.   In tissue culture cytokinins work with auxin to induce organogenesis from
                        callus

                        (1)  High cytokinins to low auxin induces shoot formation in tobacco

                        (2)  Low cytokinins to high auxin induces root formation in tobacco

                  d.   Auxin and cytokinin interact in apical dominance

                        (1)  Auxin inhibits lateral buds from growing while cytokinin stimulates lateral  
                              buds to grow

                        (2)  Reversed in roots:  auxin stimulates branch roots and cytokinin inhibits it

                  e.   Delays senescence (aging) of plant cells

                        (1)  Inhibits senescence in plant parts that are cut (like cut flowers)

                        (2)  Promotes maintenance of normal levels of protein and nucleic acids in cut
                               plant parts

                        (3)  Stimulates RNA and protein synthesis

                        (4)  Cytokinin sprays, in spray cans, are used to keep cut flowers fresh

                        (5)  Cytokinins can increase shelf-life of fruits and vegetables (this, to my  
                               knowledge, has not been approved yet by the FDA)


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