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Plant Hormones

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Plant hormones are specialized chemical substances produced by plants. They are the main internal factors controlling growth and development. Hormones are produced in one part of a plant and transported to others, where they are effective in very small amounts. Depending on the target tissue, a given hormone may have different effects.

Plant hormones play an integral role in controlling the growth and development of plants. A plant hormone is generally described as an organic compound synthesized in one part of the plant and translocated to another part, where in low concentrations elicits a physiological response.

There are five generally recognized classes of plant hormones; some of the classes are represented by only one compound, others by several different compounds. They are all organic compounds, they may resemble molecules which turn up elsewhere in plant structure or function, but they are not directly involved as nutrients or metabolites.

Hormone Source Action

Auxins apical meristem (only moves down),

embryo of seed,

young leaves * Control of cell elongation

* apical dominance (prevents lateral buds)

* prevents abscission

* continued growth of fruit

* cell division in vascular and cork cambium

--formation of lateral roots from pericycle

--formation of adventitious roots from cuttings

Gibberellins Roots and young leaves * Cell (stem) elongation (works in stems and leaves, but not roots)

* breaking seed/bud dormancy

* stimulating fruit set

Cytokinins roots, embryos, fruits

actively growing * Promote cell division

--signal axillary/lateral bud growth

--prevent leaf abscission

* chloroplast development

* breaking dormancy in some seeds

* enhance flowering

* promote fruit development

Abscissic Acid leaves, stems, green fruit * Reduces cell division (helps maintain dormancy of seeds and buds)

* prepare plants for winter

1. decreasing cell division

2. developing protective scales

3. deposition of waterproofing substances

* closes stomata

Ethylene tissues of ripening fruit,

nodes of stems,

senescent leaves, flowers Growth inhibitor

* fruit ripening

* leaf abscission

* initiation of flowering

* apical hook of some dicots

AUXINS: Nature of Auxins

Compounds are generally considered auxins if they can be characterized by their ability to induce cell elongation in stems and otherwise resemble indoleacetic acid (the first auxin isolated) in physiological activity. Auxins usually affect other processes in addition to cell elongation of stem cells but this characteristic is considered critical of all auxins and therefore defines the hormone.


1. Stimulates cell elongation

2. Stimulates cell division in the cambium and, in combination with cytokinins in tissue culture

3. Stimulates differentiation of phloem and xylem

4. Stimulates root initiation on stem cuttings and lateral root development in tissue culture

5. Mediates the tropistic response of bending in response to gravity and light

6. The auxin supply from the apical bud suppresses growth of lateral buds

7. Delays leaf senescence

8. Can inhibit or promote (via ethylene stimulation) leaf and fruit abscission

9. Can induce fruit setting and growth in some plants

10. Involved in assimilate movement toward auxin possibly by an effect on phloem transport

11. Delays fruit ripening

12. Promotes flowering in Bromeliads

13. Stimulates growth of flower parts

14. Promotes (via ethylene production) femaleness in dioecious flowers

15. Stimulates the production of ethylene at high concentrations

Gibberellins: The Nature of Gibberellins

Unlike the classification of auxins which are classified on the basis of function, gibberellins are classified on the basis of structure as well as function. All gibberellins are derived from the ent-gibberellane skeleton. All gibberellins are acidic compounds and are therefore also called gibberellic acids (GA) with a different subscript to distinguish between them. GA's are widespread and so far present in both flowering (angiosperms) and non-flowering (gymnosperms) plants as well as ferns. They have also been isolated from lower plants such as mosses and algae, at least two fungal species and most recently from two bacterial species. There have been over 90 GA's isolated, all of which are most likely not essential to the plant. Instead, these forms are probably inactive precursors or breakdown products of active gibberellins.


Active gibberellins show many physiological effects, each depending on the type of gibberellin present as well as the species of plant. Some of the physiological processes stimulated by gibberellins are:

1. Stimulate stem elongation by stimulating cell division and elongation.



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