Lesson Explainer: Plant Defenses against Pathogens Biology

In this explainer, 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.

Plants are phenomenal organisms. They are described as producers, as they synthesize their own food through photosynthesis. The sugar molecules plants produce in photosynthesis provide humans, and many other living things, with the food they eat to survive, forming the base of almost every food chain. Plants are therefore essential to every ecosystem on the planet, and without them, we would struggle to survive as a species!

Just like diseases in animals, plant diseases must be controlled as they are a major cause of species extinction. In 2016, the number of plant species threatened with extinction was , but by 2020, this had nearly doubled to ! As only 15 plant species, such as wheat, provide of the human population’s food intake, we will be left incredibly vulnerable if even one of these few species goes extinct as a result of disease. Plant disease is therefore an important topic to understand, and we will start by looking at how death and disease in plants can be caused.

Many plants are killed through herbivory. Herbivores are organisms that solely consume plant material to obtain their nutrition. They may consume the whole plant or so much of the plant that it is unable to survive. Humans encourage herbivory by breeding more and more animals to eat or from which we obtain other commercial products. Cattle grazing for dairy farming is one example.

Plants rely on the mineral ions they absorb from soil, such as magnesium and nitrates, for many of their key functions. Magnesium, for example, is used to make chlorophyll, and nitrates are needed to make amino acids. If soil is deficient in these minerals, the plant will not be able to photosynthesize or grow, eventually leading to death. Sickly, yellow apple tree leaves with deficiencies in nitrogen, potassium, and iron ions are shown in the picture below.

Gardeners and farmers sometimes add chemicals to soil, such as herbicides. Herbicides are designed to kill weeds and unwanted plant species so that there is less competition with the crop plant. Herbicides can spread, however, and contaminate other sources, such as water, to kill plant species that are not the target.

Environmental pollutants, such as heavy metals from factory effluent, can have a direct toxic effect on the plant or an indirect effect. For example, toxic aluminum salts absorbed from the soil can decrease the energy used in photosynthesis. This causes a decline in the rate of photosynthesis and therefore can lead to a decreased rate of plant growth.

It is important to note that unsuitable environmental conditions can also be a major cause of plant death. Such unsuitable conditions might include changes in the water content of soil and extreme temperatures that are either too hot or too cold for the plant to survive. Because of ongoing climate change, this could spell disaster for many plant species worldwide.

Pathogens are disease-causing microorganisms that are a major cause of plant death. The diseases caused by pathogens such as bacteria, fungi, or viruses are highly risky for plant populations as they are infectious and are likely to spread between individual organisms.

Let’s look at a few examples of plant diseases caused by pathogens, before seeing how the plants are able to respond through a diverse range of physical and chemical defenses.

Tobacco mosaic virus (TMV) is a virus that affects many different species, especially tobacco and tomato plants. It infects chloroplasts, changing the leaves’ color from green to yellow or white, giving the leaves a mosaic-like appearance, as you can see in the picture of an eggplant plant with TMV below.

Another example of a plant disease is potato blight. Potato blight is a disease caused by the spores of a fungus-like protist. This pathogen is adapted to live in wet environments and its spores spread in the wind. An infection can cause a whole field of potatoes to decay, in some cases killing off the population in just 10 days! The photograph below shows the havoc that potato blight can wreak on crops, causing many potato tubers to rot and decay, leading to a far smaller crop yield for farmers.

When such pathogen spreads on a large scale, it can result in a food crisis for the reliant population. For example, the Great Famine caused by potato blight resulted in about one million human deaths in Ireland alone due to starvation and disease.

Plants defend themselves both through structural immunity, with physical defenses and responses, and through biochemical immunity, by releasing chemicals. Humans can create acquired immunity in plants by genetically engineering and selectively breeding plants together that are immune to certain diseases.

First, let’s look at how the preexisting structural defenses of a plant help limit the entry of a pathogen.

The epidermis is the outermost layer of a plant’s leaf, stem, and roots. You can see it located both on the top and the bottom of the leaf in Figure 4.

In some cases, the 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, then water-borne pathogens cannot either. The epidermis can also be covered with hairs or thorns, which helps in deterring herbivory. These factors combined make it less likely that bacteria or fungi will grow and reproduce on the plant’s surface, limiting the risk of their entry.

A major defense present in all plant cells is their cell wall. Plant cell walls are made of a carbohydrate polymer called cellulose, which is made up of thousands of glucose molecules bonded together. You can see how cellulose makes up these cell walls in Figure 5, which magnifies an image of a leaf gradually to reveal the composition of its cells, the cell walls, and cellulose itself. Cellulose is very strong, and the mesh it forms in the cell wall forms a physical barrier between the cell contents and any pathogen that may be attempting to access them from outside the cell wall.

Some pathogens make it past these preexisting structural features. Let’s look at how physical plant structures can be used to help prevent the spread of infection once the pathogen has entered the plant. These are called induced structural defenses.

Plants often experience physical cuts in their surface as they grow and change seasonally or are damaged by other organisms. Cuts provide an access point for pathogens to enter the plant. To isolate cut regions from the rest of the plant, some plants are able to form tough layers of cork to block up these newly formed entranceways. Other plants can secrete gums or resins in the cells that surround the cut. These gums and resins help to block up the cut and to prevent the entry of pathogens by trapping them. You can see a tree producing resin to block up a cut in its surface in the picture below, which would help prevent the entrance of pathogens.

Xylem vessels are found within the vascular system of plants. Xylem is responsible for transporting water and mineral ions from the plant roots to the rest of the plant organs that require them. 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 and blocking up the xylem. This is helpful, as the infection is less likely to spread through the vascular system to other organs. These outgrowths by the parenchyma cells surrounding the xylem are called tyloses.

Some cells, particularly epidermis cells and those beneath them, are capable of swelling their cell walls to respond to an infection. This makes the cells larger and more challenging for the pathogen to penetrate. As a result, it is harder for the pathogen to gain access to the plant’s inner tissues. Plants can also strengthen their cell walls by adding tougher substances such as lignin and callose to them. In the cases where the pathogen is fungal, the swelling of plant cell walls can completely cover and insulate the fungus to prevent it transmitting from one cell to its neighbor.

The final induced structural defense a plant may exhibit in response to infection that we will discuss in this explainer is the hypersensitive response. This involves destroying the plant’s own infected tissues, which restricts the movement of the pathogen. This slows the spread of infection and can 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 cell membrane of all plant cells. Receptors can detect the difference between “self,” meaning it belongs to the plant, and a cell that is “nonself,” indicating the presence of another organism that could be pathogenic.

The entrance of a pathogen is detected by plant receptors that recognize the pathogen as a nonself organism. Molecules on the surface of the pathogen bind to the receptors, as you can see in Figure 7. This activates the receptors, and as a result, the concentration of receptors increases. When these receptors are activated, chemicals such as salicylic acid are released to alert the plant’s innate immune system that a potentially dangerous organism has entered and needs to be dealt with.

Let’s look at some of the biochemical defenses these receptors induce.

Antimicrobial chemicals often increase in concentration following an infection. For example, phenols, glycosides, and amino acids like canavanine are toxic to pathogens such as bacteria. They can either kill the pathogen directly or inhibit its reproduction and growth to prevent the infection spreading.

Some plants can produce specific proteins with antitoxin properties following an infection. These proteins can convert toxins produced by the pathogen into less toxic products that do not harm the plant. Other proteins such as detoxifying enzymes can be produced. These enzymes can break down toxins produced by the pathogen to limit the damage they cause.

During a response to infection, the plant will aim to decrease the use of energy on nonessential processes. Therefore, structures such as food storage organs will not be formed, as the majority of energy will be expended on inducing structural immunity and biochemical immunity.

If the infection subsides, plants can promote and strengthen their defenses. A plant that has already been infected can induce their responses far faster upon reinfection. Amazingly, plants can even warn each other about a disease! They do this by releasing volatile organic compounds into the air. This helps their neighboring plants to prepare their physical and chemical defenses for the imminent onslaught of pathogens, to aid the overall survival of the plant population.

Example 4: Describing the Role of Salicylic Acid as a Chemical Defense Method

Many species of willow, shown in the figure, make and distribute salicylic acid to all parts of the plant following an infection.

What is the role of salicylic acid in defending the willow against disease?

1. It increases the formation of food storage organs.
2. It alerts the whole plant that an infection has occurred.
3. It promotes widespread cell death to limit the infection.
4. It kills the infecting pathogen directly.

Answer

Infections in plants are caused by disease-causing pathogens. They can spread quickly to other parts of the plant, and even to other organisms. Plants have many structural and biochemical adaptations that allow them to respond to an infection by a pathogen appropriately to slow or stop its spread.

During a response to infection, the plant will aim to decrease the use of energy on nonessential processes. Therefore, food storage organs will not be formed, as the majority of energy will be expended on inducing structural immunity and biochemical immunity.

The entrance of a pathogen is detected by plant receptors that recognize the pathogen as a nonself organism. Molecules on the surface of the pathogen bind to the receptors, which activates them. When these receptors are activated, they release chemicals, such as salicylic acid, to alert the plant’s innate immune system that a potentially dangerous organism has entered and needs to be dealt with. This allows the plant to respond appropriately.

Though one of these responses is localized cell death to limit the spread of infection, this is not caused directly by salicylic acid. Other chemicals that may be released to combat the infection will be toxins or enzymes that can break down the pathogen, but, again, this is not a function of salicylic acid itself.

The role of salicylic acid is therefore that it alerts the whole plant that an infection has occurred.

Let’s discuss how farmers can bring about immunity in their crop plants.

Some actions to prevent the spread of disease, such as spraying chemicals toxic to a pathogen upon plants, can be harmful to other species. There are other methods that can reduce the plant damage caused by pathogens without having such negative effects. Genetic engineering and selective breeding are such methods, which can be very effective at leading to an increase in immunity of crop plants against disease.

Some plants have a genetic disposition to disease resistance. They may have a version of a gene that allows them to exhibit the structural and chemical defenses we have looked into, thereby conferring disease resistance.

Selective breeding is a process by which humans choose two individuals with a certain desired characteristic and breed them together. In this context, an example could be selecting two plants that possess a version of a gene that helps them resist a certain disease-causing pathogen. The plants would be artificially bred together. Their offspring would be screened to see if they also possess the trait for disease resistance. Those that do are bred together, before repeating the process over several generations. This can lead to a large proportion of plants resistant to a disease.

While many organisms have a degree of innate immunity from birth, some organisms are able to gain immunity to certain diseases across their lifetime. This is called acquired immunity.

Acquired immunity might either result from exposure to a disease or immunity that is gained from another organism. An example of how acquired immunity can be gained in organisms is through genetic engineering.

Genetic engineering physically edits the DNA of an organism. A gene that codes for resistance to a certain disease in another organism can be incorporated into the DNA of the plant. As plants grow quickly and are easy to clone, a large proportion of the population will be resistant to this specific disease.

Let’s recap some of the key points we have covered in this explainer.