Autumn.wmf (12088 bytes) Concepts of Biology (BIOL116) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321;;

Plant Growth and Development

I.  Definitions & Life Cycle Revisited

II. Meristems

A.  General

A.  Apical (or Primary) Meristems

B.  Buds

C.  Pericycle

D.  Vascular Cambium

E.  Cork cambium

III.  Seed Structure. (a baby in a suitcase carrying its lunch)

IV.  Germination

V.  Growth is controlled by hormones

  1. Definition:  small, organic molecules, synthesized by the plant, active in low concentration (<10-6 M), typically there is a separation of site of synthesis and site of action; promote or inhibit growth and developmental responses.  Based on these criteria, would the following substances be considered hormones? Ca2+, sucrose, 2 4 - D, glycine, or K+

  2. Evidence for existence
  3. A comparison of plant vs. animal hormones



    number of hormones few many
    specificity of action non-specific specific
    work together yes no
    site action/production separation no yes

        Thus, there are comparatively few plant hormones, each elicits a variety of responses and often works together with other hormones. In contrast, animals have numerous different kinds of hormones, each with a specific function, and it works alone to induce a response.

        Why the differences? Good question. It may be partly a function of our ignorance; in other words, there are likely to be many more plant hormones that simply haven�t yet been identified. Nevertheless, I suspect  that at least part of the reason for the differences is related to body design. Recall that plants have an architectural with a limited number of parts that are repeated. It follows that only a limited number of hormones would be necessary to induce growth/developmental responses. With a mechanical design and numerous separate parts, animals required unique chemical messengers to interact with each one.

  4. Types
  1. Auxin (indole acetic acid, IAA) - cell elongation, adventitious root formation, tropisms, fruit development, apical dominance, leaf abscission

  2. Gibberellin - stem elongation, bolting, flowering, germination (beer making)

  3. Cytokinin - cell division & differentiation, retard senescence (aging)

  4. Ethylene - gas, fruit ripening, senescence, abscission

  5. Abscisic acid (ABA) - stress (i.e., water), dormancy

  6. Others (polyamines - gene expression: systemin - defense: oligosaccharins - defense: salicylic acid - defense; jasmonates & systemin - defense; brassinolides - stem elongation, pollen tube growth, vascular tissue development; "florigen" - flowering)

VI.  Tropisms & other Growth Responses

  1. General Response System:  environmental signal/stimulus receptor transducing mechanism response

  2. Gravitropism:  signal - gravity; receptor - root cap; statoliths (starch grains); Mechanism - auxin transport/synthesis; differential sensitivity; Responses - cells on top of root or bottom of shoot grow more

  3. Phototropism:  signal - light (blue); receptor - pigment (flavin); Mechanism - auxin transport, shaded side elongates

  4. Apical Dominance - apical meristem inhibits development of nearby lateral buds.   Gardeners have long known that you pinch off the tip of the plant and it stimulates it to branch.  Also responsible for the xmas-tree shape of many plants, especially conifers.  Functions:  (a) to prevent plant from getting too top-heavy; (b) minimizes shading of lower leaves; (c) allow another shoot to replace one that is damaged.  Response due to hormones, especially IAA, ethylene and cytokinin.  In the simplest hypothesis - IAA produced at the tip moves toward the base of the plant.  The concentration decreases from shoot tip toward root.  Lateral buds are inhibited by high concentrations of IAA and therefore don't develop until the tip has grown sufficiently far away that the IAA concentration is lower.  Cytokinin and ethylene are also important.

  5. Etiolation - response of growth to dark.  Dark-grown (or light-limited) plants:  (a) lack chlorophyll; (b) have elongated internodes; or in other words grow faster as they attempt to "find" light; (c) apical hook; (d) smaller leaves or unopened cotyledons.  These are all mechanisms to allow the plant to conserve as much energy as possible until it once again "finds" light.

VII.  Flowering

A.  General Sequence of Events:  juvenile vegetative adult vegetative adult reproductive flowering

B.  Photoperiod

  1. Stimulus -daylength (short day = long night; long day = short night)
  2. Receptor - leaves, phytochrome (exists in two forms - Pfr, Pr; converted by light)
  3. Transducing Mechanism - hormone involved - florigen? GA?  Simplistic view - Pfr stimulates flowering in LDP, inhibits in SDP.

C.  Vernalization - cold treatment required to flower; stimulus - cold; receptor - imbibed seeds, seedlings; transducing mechanism - hormone? GA? vernalin?

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Last updated: January 05, 2004        � Copyright by SG Saupe