bio 250 asgmnt uitm
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BIO 250
PLANTS HORMONES AND RESPONSE TO
INTERNAL AND EXTERNAL SIGNALS
NAME : MUHAMMAD HASIF BIN MOHAMAD
SUZAINI
ID NUMBER : 2011890782
CLASS : AS1204G2
LECTURE’S NAME : AHMAD ZAIMI BIN MOHDZAAI
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PLANTS HORMONES
In general, plant hormones take control in plant growth and development. Plants cells use
hormones to communicate with one another. Plants hormonesare signaling molecules that can
stimulate or inhibit plant development, including growth. Environmental cues such as the
availability of water,temperature and gravity influence plants by triggering the production and
dispersal of hormones. When a plants hormone binds to a target cell, it may modify gene
expression, solute concentration, enzyme activity or activity another molecule in the cytoplasm.
ome ma!or classes of plant hormones are auxin "I##$, cytokinins, gibberellins, abscisic acid"#%#$ and ethylene.
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Auxin (Indoleacetic acid, IAA)
&he term auxin is derived from the 'reek word, auxein which mean to grow. &herefore, any
chemical substance that have ability and promotes cell elongation can be considered as auxin.
&he natural auxin in plants is indoleacetic acid, or I##. #uxins are plants hormones that promote
or inhibit cell division and elongation, depending on the target tissue.
#uxins that are produced in apical meristems result in elongation of shoots. &hey also induce cell
division and differentiation in vascular cambium, fruit development in ovaries and lateral root
formation in roots. #uxins also have inhibitory effect. (or example, auxins produced in shoot tip
prevents the growth of lateral buds along a lengthening stem, an effect called apical dominance.
• )hemical structure of #uxins.
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Cytokinins
'enerally, cytokinins take control in the cell division and differentiation. )ytokinins are
compounds with a structure resembling adenine which promote cell division and have other
similar functions to kinetin. *inetin was the first cytokinin discovered and so named because of
the compounds ability to promote cytokinesis "cell division$. )ytokinins have been found in
almost all higher plants as well as mosses, fungi, bacteria, and also in t+# of many prokaryotes
and eukaryotes.
)ytokinins are produced in actively growing tissues, particularly in roots. )ytokinins produced
in the root reach their target tissues by moving up the plant in the xylem sap.
)ytokinins interact with auxins to stimulate cell division and differentiation. In the absence of cytokinins, a piece of parenchyma tissue grows large, but the cells do not divide. In the presence
of cytokinins and auxins, the cells divide, while cytokinins alone have no effect.
If the ratio of cytokinins and auxins is at a specific level, then the mass of growing cells, called a
callus, remains undifferentiated. If cytokinin levels are raised, shoot buds form from the callus. If
auxin levels are raised, roots form.
)ytokinins, auxins, and other factors interact in the control of apical dominance, the ability of the
terminal bud to suppress the development of axillary buds.
)ytokinins retard the aging of some plant organs. &hey inhibit protein breakdown by stimulating+# and protein synthesis and by mobilizing nutrients from surrounding tissues. -eaves
removed from a plant and dipped in a cytokinin solution stay green much longer than otherwise.
• )hemical structure of cytokinins.
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Ethylene
Ethylene the only gaseous hormone. It produced by damaged cells in response such as drought,
flooding, in!ury, and infection. It also produced in autumn in deciduous plants or near the end of the life cycle as part of a plant/s normal process of aging.
Ethylenes initiate a seedling to perform a growth called the triple response that enables a
seedling to avoid an obstacle as it grows through soil. In the triple response, stem elongation
slows, the stem thickens, and curvature causes the stem to start growing horizontally. It is
ethylene and not the physical obstruction that induces the stem to grow horizontally. In simple
words, the response of ethylene is mediates fruit ripening.
&he uses of ethylene in commercial is allows shipping of green, still0hard fruit. )arbon dioxide
application stops ripening of fruit in transit to market, then ethylene is applied to ripen
distributed fruit 1uickly.
)hemical structure of ethylene
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Abscisic acid (ABA)
#bscisic acid is a hormone that was inhibits growth, and has little to do with abscission. #%# is
part of a stress response that causes stomata to close. It also diverts photosynthetic products from
leaves to seeds, an effect that overrides growth0stimulating effects of other hormones as the
growing season ends. #%# inhibits seed germination in some species, such as apple. uch seeds
do not germinate before most of the #%# they contain has been broken down, for example by a
long periods of cold and wet conditions.
ome cormmercial uses of #%# is induces nursery stock to enter dormancy before shipment to
minimize damage during handling.
C!"#$%&' ()*+%)+*" ,- #bscisic acid
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ormones Primary souce Effect ite of effects
#uxins tem tip, young
leaves
timulates cell
elongation
'rowing tissues
Initiate formation of lateral roots
+oots
Inhibits growth
"apical dominance$
#xillary buds
timulate
differentiation of
xylem
)ambium
Inhibit abscission -eaves, fruits
2eveloping embryos timulates fruit
development
ovary
'ibberellins tem tip, youngleaves
timulates celldivision,elongation
tem inertnode
Embryo timulates
germination
eed
Embryo "grass$ timulates starch
hydrolysis
Endosperms
#bscisis acid leaves )loses stomata 'uard cells
timulate formation
of dormant buds
tem tip
ovule Inhibits germination eed coat
)ytokinins +oot tip timulate cell division tem tip, axillary buds
Inhibit senescence
"aging$
-eaves
Ethylene 2amaged or aged
tissue
Inhibits cell
elongation
tem
timulate senescence
"aging$
-eaves
timulate ripening (ruits
Major plants hormones and some o their eects!
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PLANTS RESPONSE TO INTERNAL AND
EXTERNAL SIGNALS
Plants respond to environment stimuli by ad!usting the growth of roots and shoots. &hese
response are called tropisms and they are mediated bythe plants hormones. (or example, a root
or shoot bends because of differences in auxin concentration. #uxin that accumulates in cells on
side of a shoot causes the cells to elongate more than the cells on the other side. &he results is
that the shoot bends away from the side with more auxin. #uxin has the opposite effect in roots.
It inhibits elongation of root cells. &hus, a root will bend towards the side with more auxin.
"hototropism
-ight streaming in from one direction causes stem to curve towards its surce. &his response is
called phototropism. In this response, phototropism, orients certain parts of the plants in the
direction that will maximize the amount of light intercepted by its photosynthetic cells.
Phototropism in plants occurs in response to blue light. onphotosynthetic pigment called
phototropins absorb blue light and translate its energy into a cascade of intercellular signals. &he
ultimate effect of this cascade is that auxin is redistributed to the shaded side elongate of a shoot
or coleoptile. #s a result, cell on the shaded side elongate faster than cells on the illuminated
side. 2ifferences in growth rates between cells on opposite sides of a shoot or coleoptiles cause
the entire structure to bend toward the light.
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Gra#itropism
o matter how aseed is positioned in the soil when it germinates, the radical always grows down,
and the primary shoot always grows up. Even if a seedling is turned upside down !ust after
germination, the primary root and shot will curve so the root grows down and the shoot grows
up. # growth response to gravity is called gravitropism.
&he plant know which direction up by gravity0sensing mechanismsof many organisms are based
on statoliths. In plants, statoliths are starch0grainstuffed amyloplasts that occur in root cap cells,
and also in specialized cells at the periphery of vascular tissue in the stem.
tarch grains are heavier than cytoplasm, so statoliths tend to sink to the lowest region of the
cell, wherever that is. When statoliths move, they put tension on actin microfilamens of the cell,s
cytoskeleton. &he filaments are connected to the cell,s membrane, and the change in tension is
thought to stimulate certain ion channels in the membranes. &he result is that the cell,s auxin
efflux carriers move to the new bottom of the cell within minutes of a change in orientation.&hus, auxin is always transported to down0facing side of roots and shoots.
$hi%motropism
# plant/s contact with a solid ob!ect may result in a change in the direction of its growth, a
response called thigmotropism. &he mechanism that gives rise to the response is not wellunderstood, but it involves the products of calcium ions and at least five genes called touch.
We can see thigmotropism after a plant/s tendril touches an ob!ect. &he cells near the area of
contact stop elongating, and the cells on the opposite sideof the shoot keep elongating. &he result
of une1ual growth rates of cells on opposite sides of the shoot is cause to curl around the ob!ect.
# similar mechanismcauses roots togrow away from contact, which allows them to feel their way
around rock and other impassable ob!ects in the soil.
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"hototaxis
Phototaxis is the ability of organisms to move directionally in response to a light source. 3any
cyanobacteria exhibit phototaxis, both towards and away from a light source. In the
environment, the ability to move into optimal light conditions for photosynthesis is likely to be
an advantage. We are particularly interested in how cells perceive light of different wavelengths4the photoreceptors involved and the signal transduction cascade involved in this process.
Chemotaxis
)hemotaxis, movement toward or away from chemicals, is a universal attribute of motile cells
and organisms. &he cells swim toward amino acids "serine and aspartic acid$, sugars "maltose,
ribose, galactose, glucose$, dipeptides, pyrimidines and electron acceptors "oxygen, nitrate,
fumarate$. (igure 5 shows two simple methods for assessing attractant responses. &he capillary
assay relies on diffusion0generated gradients and is more 1uantitative, but also more laborious.
oft agar assays involve metabolism0generated chemoeffector gradients and provide a more
1ualitative, but expedient, measure of chemotactic ability. )ell also swims away from potentially
noxious chemicals, such as alcohols and fatty acids, but repellent responses haven6t been as
extensively studied.
(igure 57 &he test chemical diffuses from the capillary mouth, establishing a steep gradient that
attracts bacteria to the entrance. &he cells enter the capillary and are subse1uently documented by colony counts.
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&erences
• &he unity and 2iversity of life, 58 th edition.
• )ampbell %iology, 9th edition.
• http7::chemotaxis.biology.utah.edu:Parkinson;-ab:pro!ects:ecolichemotaxis:ecolichemot
axis.html• http7::dpb.carnegiescience.edu:labs:bhaya0lab:pro!ects:phototaxis
• http7::plantsinaction.science.u1.edu.au:edition5:<1=content:>08050gravitropism
• http7::dpb.carnegiescience.edu:article:lighting0plant0hormone0?E8?>@?9)command0
system?E8?>@?92