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Lesson Video: Plant Defenses Against Pathogens Biology

In this video, we will learn how to describe plant structures that limit the entry of pathogens and different plant adaptations that limit the damage caused by infection.

15:15

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.

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