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The plant hormones
or phytohormones are involved in all the stages of plant life from pollinisation
- inducing the fecondation and the zygotic embryo development, all the
way through its development into an adult plant - to flowering, development
of the fruit and senecence control.
The same phytohormones not only control the growth and development processes:
for that, they are crucially involved in specific processes of division,
elongation and cellular differentiation, but are also necesary in the
primary and secondary metabolisms.
The
phytohormones are very important in control of in vitro cells,
tissues, organs or whole plants cultures, according to the orientation
chosen: keeping in live, growing, initiation of specific organogenesis
(production of organs like stems with leaves, roots, somatic embryos*),
organ or shoot multiplication, etc… they are also widely used
for the control of the production of secondary metabolites of various
interest.
Here whole, or nearly whole, plants will be under consideration.
*not from the development of a seed.
Some simple experiments to
explain the role of phytohormones in plants
A. the auxins
- First experiment
- This hormone favours roots development. |
The inferior
part of the plant including roots ( or root cap) is cut off and
the plant is put in a medium containing auxin or free of it.
Without auxin, adventitious roots can developed. This is the principle
of cuttings.
However, with auxin, root development is much better.
Example  |
- Second experiment: other effects of auxin |
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- When the
apical bud is cut off a plant, the development of axillary
buds in lateral branches is observed.
- If the apical bud is replaced by cotton impregnated
with auxin, no axillary bud development is observed. |
- So the auxin replaces the apical bud.
- It can be deduced that this hormone is produced in the apical part
of the plant.
It is called apical dominance : the apical part of the plant prevent
the development of the axillary buds in favour of its own development.
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B. The cytokinins |
- The cytokinins favour the adventitious budding.
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If a geranium leaf is put on some soil or on a synthetic medium
without hormone some adventitious
buds appear.
If cytokinins are added to the medium, much more buds are observed.
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C. Auxins and cytokinins
- At the top,
the apical dominance have been annulled by cutting the
apical bud.
- At the centre, the terminal
bud and the root cap have been cut. No futher development of axillary
buds on the explant can be observed.
So the roots are necessary for the development of buds.
- on the other hand (at
the bottom), if one identical explant is introduiced
in a medium containing cytokinins, the development of axillary buds occurs.
So the cytokinins replace the roots and it can be
deduced that these hormones are produced in the roots.
It is observed also that there is less root development
in the presence of cytokinins.
D. Summary
Auxins:
- They are produced in the apical part of the plant.
- They prevent the development of the
axillary buds.
- They favour the rhizogenesis ...(development of roots).
Cytokinins:
- They are produced in the roots.
- They prevent the development of roots.
- They favour the development of the
axillary buds.
Use of hormones in in vitro
culture
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- With large concentrations of cytokinins
and low concentrations of
auxins, the development of
axillary or adventitious buds can be obtain and
in this way the plants are multiplied. |
- With large concentrations of auxins
and low concentrations of cytokinins
or no cytokinins, the rooting
of the shoots can be arrived at.
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- with equal concentrations of the two hormones,
a callus is obtain. The callus is the result
of the anarchic proliferation of cells which
are more or less differentiated but which cannot organised
them and form tissues and distinct organs.
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If the knowledge of the quantity of hormones introduced (or sometimes
secreted by the plants) in the culture medium is often of great
usefulness, it's principally the nature and the quantity of the
phytohormones (and the report between their concentration) naturally
metabolised by the plants that are important for a fondamental approach
of the phenomenons. That permits the checking and modification of
their endogenous content.
(If you are interested by the extraction and/or the measerement
of hormones or other substances (essential oils, secondary metabolites):
see also "Analyses
and Measurements")
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