Imagine you went to the doctor with an infected cut on your finger. It is bad but not that bad. But instead of giving you an antibiotic or recommending a tetanus shot or a few stitches, what if the doctor recommended cutting off your finger so you could grow a new one! Ok, maybe that is a bit dramatic, but it highlights fundamental differences between plant and animal immunity.
There are two fundamental differences between plants and animals that help explain different strategies for immunity. First, plants don’t move! While this seems obvious, it extends down to the cellular level, too. In other words, plants do not have specialized cells (macrophages) roaming around able to recognize and attack pathogens. This means that basically every plant cell must be able to respond to and attack pathogens. Second, plants are continually developing. In other words, plants keep making new ‘fingers’ (leaves) and ‘toes’ (roots) after they are ‘born’. This means plants are able to ‘cut off’ their ‘fingers’ and ‘toes’ and grow new ones.
Now let’s go through a pathogen attacking a plant say your favorite houseplant or rosebush. First, the pathogen has to get past the plant ‘skin’ called the plant cuticle. This waxy covering physically prevents pathogens from getting into the plant much like your skin does. When pathogens do get through, the plant cells are able to recognize the pathogen. The plant recognizes something about the pathogen that is vital to the pathogen and highly conserved among pathogens of that type i.e. all bacteria or all fungi. For example, legs are very important for getting around. Bacteria have a ‘leg’ called flagella which lets them to move. The components of the flagella are highly conserved among different bacteria. Why reinvent the wheel, right! So one thing plants are able to recognize is a vital, highly conserved piece of the flagella. Interestingly, you are also able to recognize bacteria through their flagella but using a different vital, highly conserved piece of it.
Once the plant recognizes a pathogen, it mounts an initial defense response. This includes things like reactive oxygen species (ROS). In other words, the plant uses hydrogen peroxide (a ROS) to ‘disinfect’ wounds just like you would. Other responses include reinforcing the physical barrier between the pathogen and plant (thicker skin!) and releasing antimicrobial compounds (plant antibiotics!). But some pathogens have found ways to overcome this initial defense response.
Such pathogens have proteins, called effectors, that are able to stop the plant from recognizing the pathogen. For example, some effectors are able to keep the plant from ‘seeing’ the bacteria flagella. It isn’t looking good for your favorite houseplant, but don’t worry plants have another level of defense!
Plants have a second level of defense. Specific plant genes, called resistance or R-genes, are able to recognize the effectors. So using our flagella example, while the plant still cannot ‘see’ the flagella because of the effector, it can ‘see’ the effector with a specific R-gene. When this happens, the plant stops playing around! The plant cells immediately surrounding the pathogen kill themselves! This is called the hypersensitive or HR response and is very effective at stopping certain pathogens. While extreme, remember the plant is able to grow new ‘fingers’ (leaves) so losing some cells or a whole leaf is acceptable if it gets rid of the pathogen. If you used the same strategy as plants, the doctor really might suggest cutting off your finger!
Your favorite houseplant or rosebush is safe now right? Sorry to say but the pathogen and plant are in a constant battle. See, the pathogen evolves new effectors that are not recognized by the R-genes or stop the R-genes themselves. This leads to a zigzag model where the plant and the pathogen are constantly battling it out -the pathogen trying to ‘hide’ from the plant and the plant constantly trying to ‘see’ the pathogen. Since the plants ‘see’ the pathogen using R-genes this battle happens between populations of plants. In a whole field of houseplants or rosebushes a few will have slight changes that allow their R-genes to ‘see’ the pathogen better than the rest. Those few are selected for and produce more seeds than the rest. This is one thing that plant breeders do looking to make sure your favorite houseplants or rosebushes are from those special few plants that are able to ‘see’ the pathogen better. At least until the pathogen evolves new effectors!
