How Does a Plant's Immune System Work?

Yes, plants do have immune systems! Unlike mammals they do not have defender cells that are mobile or adaptive cells within their bodies, but the ways in which plants defend themselves from diseases and pests is fascinating to say the least.

Plant Invaders: Dangers To Your Plants

Pathogens are those life forms that attack and damage our plants. Pathogens have different strategies for surviving and thriving within the plant—bacterial pathogens, for instance, proliferate in the spaces between the cell walls. They often get started by entering through a gas pore (stomata) or a water pore (hydathode) and of course they can enter through a wound as well.

Some other plant invaders are aphids and nematodes—which insert their little stylets into plant cells—and fungi, which have many ways to get under the protective skin of your plants. There are many types of pathogens that try to use, feed off of or injure plants; without some system of defending themselves plants would be unable to survive.

Plant Defense Processes

To obtain a basic understanding of these plant defense processes you’ll need to add a few terms to your vocabulary: disease-resistance proteins (R), pathogen-encoded effectors (PE), transmembrane pattern-recognition receptors (PRR), microbial-associated molecular patterns (MAMP) and pathogen-associated molecular patterns (PAMP).

In order for a disease-resistant plant protein to be effective and result in no disease there needs to be a dominant resistance gene in the plant and a corresponding avirulence gene in the pathogen. In this type of match the right protein can trigger responses that can prevent disease from affecting the plant.

A pathogen-encoded effector is a protein secreted by a pathogen and it works to help the pathogen adjust to its new living environment within the host plant—sort of like the Trojan horse.

The plant has a counter move to this attack, however, as the introduction of an effector often triggers the activation of a disease-resistant protein within the plant. The ‘R’ protein recognizes the modified version of the plant caused by the effector in a similar way to the ‘danger signal’ models that kick into action in a mammal. The war goes on.

There are two basic branches of plant immune systems—one uses transmembrane pattern receptors (PRRs) that respond to the slowly evolving changes caused by microbial- or pathogen-associated molecular patterns (MAMP or PAMP). The other acts mostly inside the cell, using protein products activated as the result of some effector.

The thin black arrows indicate the ongoing level of plant immunity. In the first phase, the plant detects the initial effectors from the microbial or pathogen-associated molecular pattern (MAMP or PAMP) and triggers increased resistance and immunity (PTI) for that pathogen. During phase two, certain pathogens deliver effectors that interfere with PTI.

This enables pathogen nutrition and dispersal, resulting in effector-triggered susceptibility (ETS). During phase three, one of the effectors (Avr-R) is recognized by a plant protein, which activates effector-triggered immunity again.

Sometimes the immunity response is so strong that it triggers a hypersensitive cell-death response (HR) in the plant. In phase four, pathogen isolates are selected that have lost the red effector and gained new effectors through gene flow.

These blue PRRs can help the pathogens to suppress the ETI in their attempt to survive the plant’s defense. This selection, however, helps the plant to again recognize the new effector and the ETI response is retriggered.

Hypersensitive Cell Death

Hypersensitive cell death (or HR) is a process of programmed cell death in a plant that is associated with the plant’s reaction to pathogens. It is a kind of suicide reaction by the plant initiated in order to kill or limit the threat of the pathogen that is attempting to invade the plant—and is one of the most fascinating aspects of the plant immune system.

You might have seen whole branches of a tomato plant curl up and die shortly after a bite from an insect—the concept is to burn the bridges in front of the invading pathogen in order to save the plant as a whole.

This process spreads rapidly and is often quite effective. For gardeners tending plants exhibiting this reaction, the plant should be stripped back to the stem or branch just before the dying portion in order to help minimize the amount of ‘HR’ that will occur and the pathogens that might invade.

Other plant reactions are also initiated as a result of ‘HR.’ Local and systemic-acquired resistance is often found very near the site of pathogen attack—or sometimes quite some distance away, indicating the key role of these forms of resistance in the plant. Many studies have examined these plant responses and the results clearly indicate the complexity of the regulation of these responses within the plant and the interplay of the signals between the pathogen, the environment and the host plant itself.

The hypersensitive cell-death response is one of the most powerful mechanisms the plant has to defend itself and as gardeners we need to be able to recognize this response as being something different from plant failure. Some pathogens—such as those carried by an insect—are not transferable in themselves to other plants and therefore the entire plant need not be removed. It is the insect itself in this case that spreads the disease to other plants.

Tag - A Game of Plant Self-Defense

Another very interesting method of plant self-defense is when they ‘tag’ an invading insect with a protein. When digested by the attacking insect this tagging protein converts to another chemical within the insect, a chemical that will then be recognized by a plant when the insect is feeding from it. The chemical gives off a sort of SOS to the other plants to alert them that this particular insect is a bad guy.

They then immediately begin to put into force their defense systems, which will repel the insect before it has had much of a chance to create trouble. In this example the plants might not know that every wasp is a menace, but they would recognize the ones that had already eaten, limiting the damage from insects to just the initial attack and avoiding any further damage.

Quite fascinating. Research has also shown that bacteria and fungi can also trigger a variety of chemical warning signals, causing plants to respond by increasing hormones in order to build up their defense systems.

It is obvious that not only do plants have immune systems, but that these systems are vital to their health and productivity. If we want optimum health, vigor and production from our crops, we need to help ensure that they are given what they need to keep their immune systems strong.

This is why it is so important to examine more than just the NPK of our fertilizers—micronutrients also play a large part in supporting these plant functions. For example, magnesium is one micronutrient that plays a significant role in facilitating photosynthesis. In order for the plant to synthesize chlorophyll, magnesium must be present in sufficient amounts within the plant’s tissues. It is also an enzyme activator.

The plant’s metabolism itself is an interwoven maze of reactions that regulate and promote growth, health and the immune system. Establishing the right protocol for everything from watering and nutrition to temperature and light control is vital for maximum plant vigor and health and will promote the effective functioning of plant immune systems. Keep your plants strong and they will be able to fight off much of the disease and pest damage they face on their own.