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LEC 33
Complement of auxin and other types of hormones
*The Role for auxin
1.stimulate stem elongation (the major function ) .
2. promotethe formation of lateral and adventitious roots (Plant
development) .
3.plays major role in establishing phyllotaxy.
How ??
Arrangement of leaves on the stem : the leading model
propose that polar auxin transport in the shoot apex generates
local peaks in auxin concentration that determine the site of leaf
primordium ( place where leaf grow) formation and thereby the
different phyllotaxies found in nature .
4. it plays role in the direction of the patterns of leaves veins .
When scientists inhibits the polar auxin transport the plant loses
the organized veins and loses the vascular continuity.
5. Control the activity of vascularcambium : the meristem
produces woody tissue is also under the control of auxin transport
so there is a lack of producing auxin .
6. in addition it plays a role in organization of female
gametophyte.
*Practical uses for auxin
The neutral auxin
1. Indolebutyricacid(IBA) : is used in vegetative propagation of plants
by cuttings . using IBA powder causes adventitious roots to form near
the cut surface .
2.Developing seeds produce auxin : plants such as tomatoes that lives in
green houses lack insects that make the pollination , so spraying
synthetic auxin on greenhouse induces normal fruit development.
3.synthitic auxin
[2,4-dichlorophenoxyacetic acid ] widely used as herbicides and because
Monocots can inactivates the hormonal overdose and the euidicot
cannot ,spraying dicot with [2,4-dichlorophenoxyacetic acid ]
eliminates them weeds .
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*Cytokinins
How are they discovered it?
Researchers stimulated the growth of the plant embryo in culture by
adding coconut milk and they found that they can enhance the tobacco
cells to divide by adding DNA samples.
Both of them have modified forms of adenine: a component of nucleic
acids.
*They named as this because they stimulate cytokinesis or cell division.
*The most common natural cytokine (zeatin) named as this because it
was discovered first in (zea mays).
*Control of cell division &differentiation :
*cytokinins are produced in actively growing tissues particularly in
roots, embryo, and fruits it reach to the target cells in the xylem
sap.
*cytokinins stimulate cell division and influence the pathway of
differentiation.
*when apiece of parenchyma tissue from a stem is cultured in the
absence of cytokinins, the cell will large but won't divide .
* Cytokinin alone has no effect, the ratio of cytokinins to auxin
control the cell differentiation, but it remains a cluster of
dedifferentiation cells called callus.
*cytokinin
*auxin
….. Shoot buds developed from callus.
….. Roots form
* Control of apicaldominance :
* the hormone which control this process are :
1. Cytokinin
2. Auxin
3. Strigolactones
*Apical dominance: the ability of apical bud to suppress the
development of axillary buds.
*auxin and cytokinin act antagonistically in regulating axillary
bud growth.
*auxin transported down the shoot from the apical bud inhibits
axillary buds from growing .
*cytokinin entering shoot system from root, signaling the
axillary buds from growing.
*if the apical buds (the primary source of auxin) are removed,
the inhibition of axillary buds is removed.
*Mutants that overproduce cytokinins or plant treated with
cytokinins tend to be bushier than normal .
*The polar flow of auxin down the shoot trigger the synthesis
of Strigolactones which repress buds growth.
*Anti-aging-effects:
Opotosis: type of programmed cell death .
*Cytokinins slow the aging of certain plant organs by:
They stimulatingRNA, stimulating protein synthesis and
mobilizing nutrients from surrounding tissue
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* Gibberellins
How the discovered it?
Farmers in Asia noticed that some rice seedlings grew so tall and
spindly they toppled over before they could mature. it was discovered
that afungus genus Gibberella causes this "Foolish seeding disease " by
secreting gibberellin
Also the researchers discovered that plants produce gibberellins and the
identified more than 100 different gibberellins.
*stem elongation :
The major sites of gibberellins production are growing roots and leaves
 enhancing cell elongation and cell division.
*it works with auxin to stimulate stem elongation.
*the evidence:
Certain (dwarf) (mutant) varieties of plants are treated with gibberellins
The researchers found that they grow tall if they treat them with
gibberellins, but if the gibberellins applied to wild-type plants there is
not effects, because these plant already produce an optimal dose of the
hormone (gibberellin).
*Bolting: rapid growth of the floral stalk.
*Fruit growth :
Auxin +gibberellins
*spraying of Thompsons seedless grapes:
1. They make the individual grapes grow larger.
2. Make the internodes of the grape branch elongate, allowing more
spaces for individual grapes  enhancing air circulation between grapes
and this prevent from microbes.
*Germination:
The embryo of seed is a rich source of gibberellins.
After water is imbibed, release of gibberellins from the embryo
signals seeds to germinate
Refer to figure 39.11 and the steps of the process.
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Brassinosteroids:
*
Steroids similar to cholesterol and the sex hormones of animal.
They induced cell elongation and division in stem segments and
seedlings at concentration as low as
M.
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