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Plant Responses
Plants must respond to the environment in order to avoid stress, avoid
being eaten and to survive long enough to reproduce. Plants must
respond to both biotic factors (living components of the environment)
and abiotic factors (non-living components of the environment such as
high temperatures and drought).
Nastic movements are plant
movements that occur due to
environmental stimuli. Unlike tropic
movements, the direction of the
response is not dependent upon the
direction of the stimulus. Nastic
movements are generally caused by
changes in the turgidity of cells. In the
venus flytrap plant, action potentials
triggered by contact between insects
and the hairs of their leaves cause
cells at the hinge of the leaf
to take up water and swell,
causing the leaf to close.
Tropisms are directional growth
responses in which the direction of
the response is determined by the
direction of the external stimulus.
Phototropism – shoots are positively
phototrophic – they grow towards light.
Geotropism – roots are positively geotropic
– they grow towards the pull of gravity.
Chemotropism – on a flower, pollen tubes
grow down the style, attracted by
chemicals, towards the ovary where
fertilisation can take place.
Thigmotropism – shoots of climbing plants
such as ivy, wind around other plants or
solid structures and gain support.
Auxin: Phototropism
Phototropism is the directional growth
response towards or away from light.
Most plant shoots grow towards light,
whereas roots grow away from it. The
distribution of auxin in a plant explains
how response occurs:
Proteins called phototropins sit in the
membranes of shoot cells. When they
are hit by blue light, they become
phosphorylated. If the light is coming
from one side only, the phototropin on
one side becomes phosphorylated while
that on the shady side does not.
The phosphorylation of phototropin
brings about a sideways movement of
auxin. More auxin accumulates in the
shady side of the shoot.
The presence of auxin stimulates cell
elongation , so where there is more
auxin, there is more growth.
How do Auxins work?
Auxins stimulate growth by cell
elongation . Auxin increases the
stretchiness of the cell wall by promoting
the active transport of hydrogen ions
into the cell wall. The resulting low pH
provides the optimum conditions for wall
loosening enzymes, expansins, to work.
They break bonds within the cellulose so
the walls so they become less rigid and
can expand and become longer as the
cell takes in water. This is a permanent
effect.
Leaf Abscission
Leaf abscission is when a tress drops its leaves. Many trees drop their leaves in autumn,
which helps plants to survive through the winter by reducing water loss, reducing frost
damage and avoiding fungal infections through damp leaves.
Cytokinins stop the leaves of deciduous trees from senescing (ageing – turning brown and
dying). They do this by making sure that the leaf acts as a sink for phloem transport,
meaning that the leaf is guaranteed a good supply of nutrients.
Usually, auxins inhibit abscission: as long
as a leaf is producing plenty of auxin, it will
not fall off the tree. As autumn approaches
and leaves age, the rate of auxin
production declines. A drop in auxin levels
makes the leaf more sensitive to ethene
levels. More ethene is then produced and
this in turn inhibits auxin production.
Ethene is an unusual
hormone as it is a gas:
it diffuses through the
air spaces between
cells. It tends to be
produced by maturing
or ageing fruits or
leaves.
There has therefore been a change in the balance of auxin and ethene – less auxin
and more ethene. As a result, an abscission layer grows at the base of the leaf
stalk (petiole). This is made of thin walled cells which are then weakened by the
enzyme cellulase, which digests the walls, separating the petiole form the stem.
The leaf has also grown a layer of protective tissue, the cells of which have suberin
in their walls to prevent the entry of pathogens.
Apical Dominance
Plants grow length ways by the division and elongation
of cells. A region near the tip of the shoot called the
apical meristem contains small, undifferentiated cells
that are constantly dividing. Lateral buds are on either
side of plant shoots. They also contain meristems, but
they don’t become active until as long as the apical
meristem is in position and functioning.
The presence and growth of the apex of the shoot inhibits sideways
growth from the lateral buds.
The mechanism of apical dominance was
thought to be related to auxin that is
produced in the apical meristem: normal
auxin concentrations of auxin in the
lateral buds inhibit growth and low
auxin concentrations promote growth.
Evidence for this mechanism of apical
dominance:
If we cut off the tip of a shoot and
apply synthetic auxin, the shoot will
continue to show apical dominance
If auxin transport inhibitor is applied
below the apex of the shoot, the lateral
buds will grow.
However, a scientist called Gocal disproved this direct
causative link, and now, 2 other hormones are thought to
be involved:
Abscisic acid inhibits bud growth: High concentrations
of auxins in the shoot may keep abscisic levels high in the
bud. When the tip is removed (the source of auxin), the
abscisic acid levels fall and the bud starts to grow.
Cytokinins promote bud growth: high concentrations of
auxin make the shoot apex a sink for cytokinins produced
int he roots, so most of the cytokinins will go to the shoot
apex. When the apex is removed, cytokinin spreads more
evenly around the plant, promoting growth in the buds.
Gibberellins and Stem Elongation
Gibberellin causes the rapid growth of stems in some kinds of plant. If gibberellin is
applied to the stems of dwarf bean plants, the stems begin to grow rapidly. The stems
get longer as the lengths of the internodes increase.
Experimental Evidence
So it’s been found that gibberellin can cause stem
elongation, but how do we know it does so in nature?
Scientists firstly investigated gibberellin concentrations
in small (homozygous recessive for ‘le’ allele) and tall pea
plants (homozygous dominant for ‘Le’ allele), that were
otherwise genetically identical. The plants with higher
gibberellin concentrations were taller.
In order to synthesise gibberellins, plants must convert
a substance known as GA20 to GA1. Scientists took a
plant that had a mutation that meant GA20 couldn’t be
produced. They grafted its shoot onto a homozygous
recessive plant which could not convert GA20 to GA1. The
new plant produced after the graft grew tall: the grafted
shoot has provided the enzyme that allows GA20 to be
converted to GA1, and all of the GA20 present in the
plant can now be converted.
This confirmed that gibberellins directly causes stem
elongation.
Gibberellins have been shown to
cause growth in the internodes
by stimulating cell elongation
(by loosening cell walls) and cell
division (by stimulating
production of a protein that
controls the cell cycle)
GA20
Enzyme produced by
Le allele
GA1
Commercial Uses of Plant Hormones
Auxin
Sprayed onto developing fruits to
prevent abscission – fruits stay on
toe plant for longer
Sprayed onto unpollinated flowers
to promote the growth of seedless
fruits (parthenocarpy)
Artificial auxins used as selective
herbicides . Synthetic auxins are
transported in the phloem to all
parts of the plant and they can act
within the plant for longer as they
are not a close fit for the enzymes
that break them down. It is also
thought that they may be able to
enter cells through transporters in
the plasma membrane, but cannot
leave through the transporters
that would usually allow natural
auxins out by facilitated diffusion.
The build up of the artificial auxin
may be what kills the plant.
Cytokinins can delay leaf senescence and so are
sometimes used to prevent the yellowing of lettuce
leaves after they have been picked.
Cytokinins are also used in tissue culture to help
mass produce plants.
Ethene promotes fruit ripening and promotes
fruit drop in cotton, cherry and walnut.
Gibberellins
Delay senescence in citrus fruits, extending
the time fruits can be left unpicked and making
them available for longer in shops
Increasing yield of sucrose from sugar cane :
gibberellin makes the stems of sugar cane
elongate, and as sucrose is stored in the
internodes of the stems, more sucrose is present.
Brewing: gibberellin stimulates germination in
barley grains . Germinated barley grains produce
amylase enzymes that break down starch to
maltose, and the action of yeast on the maltose
then produced alcohol.