Video Transcript
In this video, we will learn how to
describe the importance of the plant hormone auxin in plant growth. We will explore the action of auxin
specifically in a plant’s light-dependent growth pattern. Furthermore, we will have a brief
discussion about some of the other functions of auxin in the plant, such as plant
and flower development and fruit ripening.
Have you ever noticed that if you
place a potted plant near a window, the plant naturally bends toward the window to
absorb more sunlight? Have you ever wondered how a plant
does that? Or how plants can coordinate their
different organ systems to grow, develop, and respond to stimuli? These functions are controlled by
plant hormones. Hormones are chemical messengers
that travel throughout an organism’s body, usually in the blood or another transport
medium. Unlike hormones in animals, which
are produced in specific glands, plant hormones can be produced by cells in most
parts of the plant, often the growing ones.
To reach their target cells, plant
hormones can diffuse locally by gravity or by traveling through the plant’s
transport tissues. As we will learn, some hormones
have transport mechanisms influenced by light. In this video, we will look at one
such group of key plant hormones, auxins.
Auxins are named after the Greek
phrase, auxein, meaning to grow or to increase. They are a group of hormones that,
among their other roles, control cell elongation. They specifically play an important
role in stimulating directional growth, which is a growth movement toward or away
from a stimulus. This type of growth is called a
tropism. Remember that a stimulus is a
detectable change in an organism’s internal or external environment that causes an
effect in that organism. One example of a stimulus that most
plants can respond to is light.
Most plants require light so that
they can perform photosynthesis to produce their own food. A plant will therefore usually try
to grow towards the light source in order to increase its exposure to light. Growing toward or away from light
is a plant response to the stimulus light. This process is summarized by the
term phototropism.
Let’s look at a commonly accepted
mechanism of how phototropism occurs in plant cells. Auxins are usually produced by the
cells at the tip of shoots and roots. We know, for instance, that auxins
are produced in the coleoptile. This is a sheath that surrounds the
shoot tip in the growing regions of plants. The presence of auxin can either
stimulate or inhibit cell elongation depending on where in the plant it’s acting and its
concentration. If the light source is directly
above the plant shoot, auxin is produced in the tip of the plant and diffuses down
both sides of the plant stem equally. This will cause symmetrical
elongation in the cells either side of the stem, causing the stem to grow directly
upward toward the light source.
However, if a light source arrives
from one side, auxin produced in the shoot tip accumulates themselves on the shaded
side of the shoot. This causes these cells that are
not in direct sunlight to elongate comparatively more than the side that is in
direct sunlight, which causes the stem to bend in the direction of light.
You might wonder how is it found
out that auxins are produced in the shoot tip and the role that they play in a
growing plant. Different scientists have performed
various experiments to show this. One of them, Boysen-Jensen,
researched the production of auxin in a shoot tip in 1913. Let’s take a look at his
experiments and understand his conclusions.
In his experiment, Boysen-Jensen
cut the tip of a shoot and placed a thin layer of agar or gelatin on top of one of
the cut shoots. He then replaced the tip of the
shoot above the layer, creating a separation between the tip and the rest of the
plant. He then observed the growth of the
plants with the light source coming from one side of the plant and observed that
without the tip the plants would not grow any further. Even though the tip of the plant
with the agar or gelatin layer had originally been cut off, he observed that the
plant would still grow and bend towards the light source.
With this, his experiments were not
finished. In another plant, Boysen-Jensen
created a similar separation between the tip and the rest of the plant, but this
time he used cocoa butter instead of agar or gelatin. Cocoa butter allows the diffusion
of only lipid-soluble substances. When these plants were exposed to
sunlight, Boysen-Jensen observed that the plant with a layer of cocoa butter showed
no response. He also observed no growth when he
used a sheet of mica instead of a layer of cocoa butter. So what was the difference? Why did we see these different
outcomes?
Both agar and gelatin are water
permeable, which means that they allow water-soluble substances to diffuse through
them. Both cocoa butter and the mica
sheet are both water-impermeable barriers. As we know, cocoa butter only
allows the diffusion of lipid-soluble substances. This experiment showed that the
substance responsible for a plant’s directional growth response to a light stimulus
is a water-soluble molecule produced by the tip of the shoot that needs to move down
the plant, auxin.
Let’s observe how auxin works a
little closer by examining a specific example of an auxin. Indole-3-acetic acid or IAA is a
type of auxin produced mainly in developing leaves and in the top bud on a plant,
which is often called the apical bud. IAA is responsible for regulating
the necessary cell division and elongation that’s needed for a plant to grow. Have you ever noticed that most
plants tend to show more upward growth than lateral or sideways growth? This helps them to grow taller to
be more exposed to light. For example, if trees in a dense
forest did not do this, they would be shaded by other trees, therefore receiving
less light, doing less photosynthesis, and therefore producing less food and would
have a more difficult time surviving.
This process of growing high rather
than wide is called apical dominance. The apical bud at the top of the
stem grows vertically upwards, while the lateral buds at the side of the stem are
indirectly suppressed. The apical dominance is maintained
by IAA. We’ve already discussed that auxins
produced by the shoot tip will diffuse down the stem, causing the cells to elongate
and therefore the plant to grow.
Researchers have examined the
special auxin IAA further to see if IAA does anything else than signaling shoot
cells to elongate. By taking a closer look at just the
top of this plant, we can see what they found more clearly. They found that when IAA diffuses
down the shoot, it accumulates in the nodes between the lateral buds. The accumulation of IAA is believed
to inhibit lateral growth by diverting the sugars or carbohydrates that are
necessary for growth away from the lateral buds and towards the apical bud. This diversion of food results in
apical dominance. As the lateral buds are supplied
with insufficient sugars to grow, the growth of an apical bud is supported by the
extra supply of food.
A simple experiment can prove that
the apical bud is responsible for producing apical dominance. When the tip of the shoot is
removed and the plant is allowed to grow, lateral growth is no longer inhibited, and
the lateral buds begin to grow. You can easily try this at home if
you have a plant which you do not mind stopping growing upwards but gaining some
width.
Let’s discover exactly what happens
in the plant when you remove the shoot tip. As we explained previously, auxin
is produced in the shoot tip and diffuses down the shoot, where it diverts plant
sugars away from the lateral buds and toward the apical bud. By removing the tip, the cells that
produce IAA disappear. No IAA means that the sugars are no
longer diverted to the apical bud, so the lateral buds receive the sugars they need
to grow. This results in the plant growing
wider rather than higher.
Now we’ve talked a lot about how
auxin works in the plant shoot, but we also talked very briefly about auxin being
produced in the root tip. Does auxin work in the same way in
the root tip as it does in the shoot tip? An interesting fact about auxins is
that different concentrations of the same hormones can have different effects in
different plant parts. For example, while high
concentrations of auxins in plant shoots can promote cell elongation, the opposite
is true in the roots of some plants. A high concentration of auxins can
in fact inhibit the growth of root cells. Let’s take a closer look at
this.
In roots, auxins accumulate on the
lower side of the root tip. At this high concentration, auxins
inhibit the growth of the cells on the lower side of the root, while the cells on
the upper side of the root are allowed to elongate normally. The asymmetrical growth enables the
plant roots to grow away from sunlight and deeper into the soil and therefore is a
phototropic effect. While the plant shoots show
positive phototropism, growing towards a light stimulus, the plant roots show
negative phototropism, growing away from a light stimulus.
Note that light is not the only
stimulus that can cause a root to grow downwards. Two further stimuli that influence
the direction of root growth are gravity and water content. In fact, since the primary function
of the roots is to burrow into the soil to absorb water and minerals, they always
grow away from the light source and deeper into the ground where water and minerals
can be found.
We have now thoroughly discussed
the role of auxins in controlling plant growth, acting as a signal for different
plants tropisms, and in maintaining apical dominance. However, auxins have a broad
variety of other functions. These hormones are also essential
for the growth and development of the plant as a whole, the development of flowers,
and the formation of fruits. They even work with other hormones
to promote fruit ripening and to help leaf drop in fall, which is sometimes known as
autumn.
Auxin employment begins in the
embryo and in the first stages of life of a plant as a seedling. It helps to coordinate the proper
development of the arising organs, such as roots, cotyledons, and leaves. And it mediates long-distance
signals between them, thereby contributing to the overall architecture of the
plant. In flower development, auxin plays
a role in the initiation of flowering and development of the plant’s reproductive
organs. In low concentrations, it can delay
wilting of the flowers. Auxin is required for fruit growth
and development, and it delays the death of the fruit cells.
To demonstrate the role of auxin in
fruit growth, let’s have a look at a strawberry. Strawberries carry seeds on their
skin. When the seeds are removed from
strawberries, fruit growth is stopped, while strawberries that do still contain
seeds will continue to ripen and become larger. This is because the seeds produce
auxin, which is part of the signaling that’s required for the fruit to grow. By removing the seeds, the source
of auxin is also removed, and the fruit stops growing.
Let’s apply what we’ve learned
about the plant hormone auxin to a couple of practice questions.
Auxins are a group of hormones
found in plants. Which of the following best defines
hormones? (A) Hormones are chemical
messengers that travel throughout an organism to act on target cells or organs. (B) Hormones are electrical signals
that travel throughout an organism via the nervous system to act on target cells or
organs. (C) Hormones are soluble
carbohydrates that communicate changes in stimuli between the external and internal
environments of an organism. Or (D) hormones are biological
catalysts that speed up the rates of chemical reactions without being used up.
To answer this question, let’s look
at what a hormone is and what it does. And to do this, we’re going to
remove the answer options for now. Hormones are chemical
messengers. And, as the question tells us, they
can be found in plants. Plants, like this one, do not have
nervous systems, so instead they need some kind of chemical messenger to send
signals around the body. These hormones can cause changes in
the organism’s body and can communicate between organs and tissues and allow them to
respond to their external environment.
Let’s take a look at a few of these
responses in plants. Hormones are responsible for
triggering blossoming of flowers, are responsible for pores on the leaves called
stomata to open and close and allow the exchange of gases. Hormones can even promote root
growth and the growth of the other parts of the plant too. In fact, some hormones can have
multiple effects in different areas of the plant. For this reason, hormones need to
be able to travel around the body of an organism to act on target cells where
they’re needed.
Let’s bring back all our answer
options so we can eliminate those that we know are not correct and find our correct
answer. As we now know, plants do not
possess a nervous system, but plants do possess hormones. Therefore, we can eliminate option
(B), as if plants don’t have a nervous system, hormones can’t be electrical signals
that travel via a nervous system in a plant. The term biological catalyst is
used to describe a structure called an enzyme rather than a hormone. So option (D) is also
incorrect.
While option (C) does describe the
fact that hormones can communicate changes between an external and internal
environment of an organism, hormones are usually not carbohydrates. In fact, they’re most often derived
from proteins or lipids. So option (C) is also
incorrect. This leaves us with option (A),
which accurately describes hormones as chemical messengers that travel throughout an
organism to act on target cells or organs.
Let’s have a go at another question
together.
Which of the following statements
about auxins is not true? (A) The primary role of auxins in a
plant is to encourage growth by stimulating cell elongation. (B) Indoleacetic acid, IAA, is an
example of an auxin. (C) Auxins are not produced by a
plant during the summer months. Or (D) auxins are important in
regulating phototropic responses.
Auxins are a group of hormones or
chemical messengers in plants that play an important role in cell elongation. An example of an auxin is
indoleacetic acid, otherwise known as IAA. IAA is mostly produced in the
apical or topmost bud, and it’s also produced in very young developing leaves of
plants. In these regions, it stimulates and
encourages cell elongation, which allows the plant to grow upwards.
As the question is asking us to
work out which statement about auxins is not true, we can eliminate both option (B),
as we know that IAA is an example of an auxin, and option (A), as we know that IAA
can encourage growth by stimulating cell elongation.
Auxins play a role in stimulating
directional growth responses. This is the growth of an organism
like a plant towards or away from a stimulus in its environment and is otherwise
known as a tropism. An example of a tropism is
phototropism. The prefix photo- means light, and
a phototropic response is the movement towards or away from a light source.
In the shoot of a plant, auxin
produced in the shoot tip will diffuse down the stem and accumulate on the shaded
side of the shoot. When auxin accumulates in the cells
on the shaded side of the shoot, it stimulates cell elongation in those cells. This causes the cells on the shaded
side to elongate comparatively more than the cells that are lit by the light source,
causing the shoot to bend towards the light source in response to light. This is an example of a phototropic
response that’s being regulated by auxin.
Remember, we’re looking for a
statement about auxins that’s not true, so we can also eliminate option (D). This is because auxins are
important in regulating phototropic responses as we’ve just observed in the
shoot. The production of auxin is not
seasonal, which means that they’re produced throughout the whole year. Therefore, we can deduce that the
incorrect statement is option (C), auxins are not produced by a plant during the
summer months, as in fact they’re produced all year round.
Let’s summarize the key points that
we’ve learned in this video. Auxins are plant hormones involved
in cell elongation and maintaining apical dominance. Phototropism is the growth of a
plant in response to a light stimulus and is also controlled by auxins. The experiments of the scientist
Boysen-Jensen show that auxin is a water-soluble molecule that’s produced by the tip
of the shoot and needs to move down the plant in order to allow the plant to
grow. IAA is an example of an auxin, and
it’s responsible for maintaining apical dominance. Auxins are also involved in flower
and fruit development, fruit ripening, and the falling of leaves.
