Video Transcript
In this video, we will learn how to
describe the plant structures that limit the entry of pathogens and different plant
adaptations that limit the damage caused by infection. We will start by exploring the main
causes of plant death, including via pathogens that cause disease. We will then investigate the
preexisting structural defenses that plants possess to prevent the entry of
pathogens by blocking them with physical barriers and how other induced structural
responses may be employed after infection has occurred, such as by releasing
chemicals. We will look in some detail as to
how biochemical immunity is involved in plant defense against pathogens. Finally, we will look at how mass
genetic resistance can be conferred in plants through genetic engineering and
selective breeding.
All animals depend either directly
or indirectly on producers like plants for our own nutrition and survival. So, producers, like this wheat
plant here, form the base of every food chain, including our human food chain. In fact, plants such as wheat are
one of only 15 plant species that provide a huge 90 percent of the human
population’s food intake. If these species became threatened
with extinction, humans and many, many other living species would be in serious
trouble. So, let’s look at the different
ways that plant death can be caused so we have a better idea of how this can be
prevented.
Many plants are killed by
herbivores, such as this cow here, which are organisms that can seem plant material
to obtain their nutrition in a process called herbivory. Many humans encourage plant death
via herbivory by breeding animals to eat or from whom we obtain other commercial
products, like milk from dairy farming. The more animal products the
growing human population consumes, the more herbivores are needed to produce it, so
more plants are consumed and killed.
Plants rely on the mineral ions
they absorb from soil for many of their key functions. So, deficiency in these minerals
can lead to plant death. Magnesium, for example, is used to
make chlorophyll, without which plants struggle to obtain nutrition through
photosynthesis, and often they will die. Gardeners and farmers sometimes add
chemicals to soil, such as herbicides, which are designed to kill weeds and other
unwanted plants. Herbicides can spread, however, and
contaminate other sources to kill species which are not the target, such as the
plants in the hedgerow surrounding this field.
Environmental pollutants can be
toxic to plants. For example, effluent from
factories can contain toxic heavy metals, which leach into water sources like
rivers. These pollutants are taken up by
plants, and in high concentrations, they kill the plant. Pathogens such as bacteria, fungi,
viruses, or protists are biological agents that cause disease and are a major cause
of plant death. Pathogenic infection is highly
risky for plant populations, as they are likely to spread quickly between individual
organisms and can even lead to species extinction. Let’s look at some examples of
plant diseases caused by pathogens.
Tobacco mosaic virus is a disease
that affects many different species, particularly tobacco and tomato plants. It infects chloroplasts, changing
their color from green to yellow or white, giving the leaves a mosaic-like
appearance as you can see in this drawing. Potato blight is a disease caused
by the spores of a fungus-like protist. This protist pathogen is adapted to
live in wet environments, and its spores, which are shown here in pink, are spread
by the wind. An infection can cause a whole
field of potatoes to decay rapidly, massively decreasing the crop yield for the
farmer and therefore their financial income. When this happens on a large scale,
it can result in a food crisis for the population that relies on them. For example, the great potato
famine caused by potato blight resulted in about one million human deaths in Ireland
alone due to starvation and disease.
Let’s discover how plants can
defend themselves against pathogens, starting with their preexisting structural
defenses that limit the entry of pathogens. This diagram shows the magnified
cross section of a leaf. The epidermis is the outermost
layer of cells in a plant’s leaf. You can see here that there is a
layer of epidermis cells both at the top and at the bottom of the leaf. Sometimes, the epidermis produces a
cuticle to coat it. The cuticle is a waxy and
water-resistant layer that coats the epidermis of the aerial or upper parts of a
plant. If water cannot easily settle on
the surface of the epidermis due to the cuticle, nor can waterborne pathogens. The epidermis can also be covered
with hairs or with sharp thorns. These prickly structures are mainly
helpful in deterring herbivory. The defenses reduce the likelihood
that pathogens will grow and reproduce on the plant surface, limiting the risk of
their entry into the plant’s body.
This image is of a leaf cell
magnified from the drawing on the left. And you can see a major defense
present in all plant cells, the cell wall. Cell walls are rigid structural
layers that are found around the exterior of plant cells. Plant cell walls are made of a
carbohydrate polymer called cellulose, which you can see magnified in the diagram of
the cell wall here. Cellulose is very strong as it is
made up of thousands of glucose molecules joined together. This mesh cellulose forms in the
cell wall is a physical barrier between the cell contents and any pathogen that may
be attempting to access them.
Some pathogens make it past these
preexisting structural features. So, let’s look at how physical
plant structures can be implemented after a pathogen has breached them to help
prevent the spread of infection. These are called induced structural
defenses. One example occurs when a plant
experiences a physical cut or tear in their surface. Cuts provide an easy access point
for pathogens to enter the plant’s internal tissues, so the plant needs to block up
these entry routes. They can do this by forming tough
layers of cork at the cut region. Alternatively, plants can secrete
gums and resins from the cells surrounding the cut. They block it up and trap any
pathogens trying to enter the plant in the sticky material.
The diagram on the right shows the
upper cells in a magnified leaf that has a cut in its surface. Some cells, particularly epidermal
cells and those beneath them, are capable of swelling to respond to an infection and
thicken their cell walls with other structural polymers. This makes the cells larger and
more challenging for pathogens to penetrate, so they cannot gain access to the
plants in a tissue so easily. If these pathogens are fungi, the
plant cells can swell so much that they completely cover and insulate the fungal
intruder to prevent it from spreading from one plant cell to its neighbor.
As we mentioned, plants can also
strengthen certain cell walls by adding tougher structural polymers to them, for
example, callose or a substance called lignin. Xylem tissue contains cells with a
large proportion of lignin in their cell walls. Xylem vessels are responsible for
transporting water and mineral ions around the plant. The lignin in the walls of these
vessels strengthens them and waterproofs them, increasing the efficiency of this
water transport.
Xylem vessels are surrounded by
simple living tissues called parenchyma. When the plant is under stress,
such as by infection, these parenchyma cells respond by protruding into the xylem
vessel, forming outgrowths, called tyloses. Tyloses close and block up the
vascular issues, which are transport vessels such as xylem. This is helpful as the infection is
less likely to spread through the transport vessels to the other organs in the
plant.
Plants can also exhibit an extreme
structural response to infection that is called the hypersensitive response. This response involves the plant
destroying its own infected cells and tissues, which may seem drastic. But destroying infected cells can
restrict the movement of the pathogens. By disabling the pathogens, the
spread of infection to other tissues slows drastically, which may potentially save
the plant’s life. Let’s look at biochemical immunity
next, which is how plants release chemicals to limit the damage caused by
pathogens.
Receptors are present on the
surface of all cells, and here you can see some on the surface of a plant cell. Receptors allow a plant to
distinguish between a self-cell that belongs to the plant and a non-self-cell or a
structure, indicating the presence of something that might be a pathogen. When a pathogen such as this pink
bacterium enters the plant, molecules on the surface of the bacterial cell bind to
receptors on the plant cell surface, which recognizes bacteria as a
non-self-cell. This binding activates the
receptors and increases their production within the cell. The activation of these receptors
also triggers the release of chemicals, such as salicylic acid from this plant
cell. Salicylic acid alerts the plant’s
innate immune system that a potentially dangerous organism has entered and needs to
be dealt with.
Let’s have a look at some of the
biochemical responses that the plant’s immune system may exhibit as a result of this
alert. Antimicrobial chemicals such as
phenols, glycosides, and some amino acids like canavanine and cephalosporin often
increase in concentration following an infection. Antimicrobial chemicals are toxic
to pathogens. They can either kill pathogens
directly or inhibit their reproduction and growth to prevent the infection from
spreading.
Many pathogens release toxins that
damage host cells. Some plant cells are able to
produce specific antitoxin proteins following an infection. Antitoxin proteins can bind to the
toxins produced by the pathogen and convert them into less toxic products, which do
not harm the plant. Detoxifying enzymes can also be
produced by some plant cells. These enzymes break down toxins
produced by the pathogen to limit the damage they cause. A plant which has already been
infected can induce responses such as these far faster upon reinfection. Let’s discuss how farmers can bring
about acquired immunity in their crop plants.
Acquired immunity is gained by an
organism like a plant across its lifetime, as opposed to the immunity it has
innately from birth. Acquired immunity includes any
immunity a plant gains after exposure to a disease or that it might acquire from
another organism. Selective breeding and genetic
engineering are methods that can be effective at leading to an increase in disease
immunity in crop plants that have fewer knock-on effects than methods such as
spraying herbicides. Let’s look at selective breeding
first.
Some plants, such as this one, have
a genetic disposition to disease resistance. Disease resistance tends to arise
when the plant’s DNA contains a version of a gene, which allows them to exhibit some
or all of the structural and chemical defenses that we have discussed. Selective breeding is a process by
which humans choose two individuals with a certain desired characteristic and breed
them together. In this context, humans would
select two plants which both possess a version of the gene which helps them resist a
certain disease-causing pathogen, for example, a gene that codes for the production
of antitoxin proteins.
The two disease-resistant plants
would be artificially bred together. The offspring produced by this
cross would be screened to see if they also possess this trait for antitoxin protein
production. As we can see, three of these
offspring do have the antitoxin protein-coding gene, so they are bred together
before repeating the process over several generations. This can lead to a large proportion
of plants that are resistant to a disease, therefore increasing the survival of
these plants and the farmer’s crop yield.
Genetic engineering artificially
edits the DNA of an organism, in this case a plant. A gene that codes for these
proteins that provide disease resistance in another organism, such as a bacterial
cell, can be extracted, and this gene can be incorporated into the plant’s own
DNA. As plants grow quickly and are easy
to clone, a large proportion of the population can become resistant to a specific
disease through genetic engineering. Let’s see how much we’ve learned
about plant defenses against pathogens by having a go at a practice question.
Which plant structural defense is
not preexisting and is only formed as a result of infection by a pathogen? A waxy cuticle, hairs, thorns,
cellulose cell walls, or tyloses.
Let’s look at some of the key terms
that have been used in the question so we can identify which of the structural
defenses is not preexisting. The preexisting structural defenses
of a plant help to limit the entry of a pathogen. The diagram here shows some of the
key cells in a leaf. In some cases, the plant epidermis
produces a waxy cuticle to coat it. The waxy cuticle is a
water-resistant layer. And if water cannot easily settle
on the surface of the epidermis, nor can waterborne pathogens. The epidermis can also be covered
with hairs or thorns. These features mostly function to
deter herbivores, like this rabbit, from consuming the whole or part of the plant
which would damage it and make it vulnerable to pathogens.
In this diagram, we have magnified
one of the cells in the leaf so we can see their contents more clearly. A major defense present in the cell
and all other plant cells is a cell wall. Plant cell walls are made of a
carbohydrate polymer called cellulose. By magnifying this image of the
plant cell wall further, we can see the cellulose forms a strong mesh and a physical
barrier between the cell’s contents and its external environment. This restricts the entry of
pathogens attempting to access the cell from outside the cell wall.
All of these features that we have
discussed so far are preexisting, which means they’re always present. But some pathogens can make it past
these preexisting structural features. If this occurs, physical plant
structures can be induced to help prevent the spread of infection once the pathogen
has entered the plant. One example of an induced
structural defense is the development of tyloses. Tyloses are outgrowths of the
tissues surroundings xylem vessels, which are the tissues responsible for
transporting water around a plant. So, let’s have a closer look at the
xylem to see what this would look like.
You can see here that tyloses grow
into and block up the xylem vessels. This is really helpful as it occurs
after the pathogen has already been detected by the plant and means that the
infection is less likely to spread through the xylem to damage other cells. The structural defense feature that
is not preexisting but is induced by an infection is, therefore, tyloses.
Here are some of the key points
that we’ve covered in this video.