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Plants and pathogens
Microbes are tiny organisms that can be found nearly everywhere – from ocean floors to the soil under your feet. They are even found inside you! They do a range of tasks that make life possible. They help recycle nutrients and dead matter, breakdown rocks, and can keep your gut health in check too. While microbes play important roles within our ecosystems, some have gained a reputation for causing diseases. These agents of illness are called pathogens and include bacteria, viruses, and fungi. They affect all living organisms – including plants.
It might seem like plants are easy targets for pathogens. For starters, they can’t really move away. Also, they don’t have antibodies to fight off infections as animals do. Yet, they have engineered a system of ways to protect themselves from diseases! It all starts with the help of cells called stomata.
What are stomata?
The plants we see today began in the ocean as algae and moved to land nearly 400 million years ago. A key step in this journey was the innovation of structures called stomata. These kidney-bean-shaped pores are spread out all over the plant surface. They are most commonly found on the lower surface of the leaf.
Stomata help plants breathe, as they open up to take in gases from the atmosphere and close off to reduce how much water the plant loses. They also respond to a bunch of different signals from the environment like light, humidity, and gases like oxygen and carbon dioxide. This helps to keep the plant safe. Since stomata are seated right between the plant and the environment, they make perfect homes for pathogens.
Here’s the catch! In order to multiply and cause infection, pathogens have to actually enter the plant. Most bacterial pathogens can’t do this by themselves. They need to use openings that are already present on the leaf to make their way to the tissue. Scientists at first thought that stomata would simply be passive points of entry. Since there are thousands of them on every leaf, wouldn’t this be easy? Well, not so much! Turns out, stomata act like guards and can even sense an invader!
How do stomata identify pathogens?
Just like you have a set of unique fingerprints, microbes have their own patterns made up of proteins called microbe-associated molecular patterns (or MAMPS for short). These MAMPs are characteristic of closely related species and remain unchanged throughout their evolution. For example, bacteria have whip-like hairs called flagella that help them move about during search for resources like food. A tiny portion of one of these flagella acts like a MAMP, as it is unique to bacteria alone. Outer surfaces of the plant have structures called receptors that recognize and bind to these MAMPs. They then also let the rest of the plant know that there are foreigners in the body and that it is time to put your defenses up!
What happens when the plant knows there are pathogens around? Scientists guessed that stomata would close up. So, they tested it out. In an experiment, they had two set-ups. In the first set-up, a plant called Arabidopsis thaliana was sprayed with a suspension of the bacteria Pseudomonas syringae. In the second set-up, the same plant was sprayed with water.
After a few hours, they looked at the leaves of the plants under a microscope to see how the stomata behaved. When the bacteria were around, nearly 70% of the stomata were closed, as an increase in a chemical called abscisic acid (ABA) had caused them to close. They also saw more bacteria were around the open stomata. This was not the case when the plant was sprayed with water. It turns out that the scientists were right! Stomata don’t just help plants breathe, but actively protect the plant from invaders. This is one of the first steps of the plant’s immune response and it’s called stomatal defence.
Stomata and pathogens
Pathogens and plants have been at war for millions of years. Over time, some bacteria have found ways to sneak past the stomatal defense. In the same study, scientists found that bacteria mostly clustered around stomata that were open rather than the closed ones. They also saw that when live bacteria were kept on the plant for longer, they released a chemical called coratine. This chemical forces stomata to stay open so pathogens can enter the plant tissue! So, what now? What is the plant’s next move?
As it turns out, plants are quite chatty. Instead of words, plants use chemicals to talk. When a pathogen sticks around for long, plants send out a series of chemicals that make it hard for the pathogen to multiply. Eventually, there are not enough pathogens to cause illness. Along with this process, older plant leaves send signals instructing new leaves to reduce the number of stomata. The message is simple: the fewer the stomata, the less chance a pathogen has to infect the plant.
Plants do a great job at staying healthy! Despite being rooted firmly in the ground, plants have evolved to recognize and fight off pathogens, and they’ve recruited their best cells to do the job!
Antibodies: Proteins that stick to the surface of microbes
Flagella: A long whip-like structure that helps bacteria move
Pathogens: Microscopic organisms (like viruses and bacteria) that can cause diseases
Proteins: Building blocks of the body. They are made up of amino acids
Stomata: pores, enclosed by kidney-shaped cells, that help plants breathe
Flesch Kincaid Grade Level: 7.1
Flesch Kincaid Reading Ease: 68.5
David, L., Harmon, A. C., & Chen, S. (2019). Plant immune responses—From guard cells and local responses to systemic defense against bacterial pathogens. Plant Signaling & Behavior, 14(5), e1588667. https://doi.org/10.1080/15592324.2019.1588667
de Vries, J., & Archibald, J. M. (2018). Plant evolution: Landmarks on the path to terrestrial life. New Phytologist, 217(4), 1428–1434. https://doi.org/10.1111/nph.14975
Dutton, C., Hõrak, H., Hepworth, C., Mitchell, A., Ton, J., Hunt, L., & Gray, J. E. (2019). Bacterial infection systemically suppresses stomatal density. Plant, Cell & Environment, 42(8), 2411–2421. https://doi.org/10.1111/pce.13570
Melotto, M., Underwood, W., Koczan, J., Nomura, K., & He, S. Y. (2006). Plant Stomata Function in Innate Immunity against Bacterial Invasion. Cell, 126(5), 969–980. https://doi.org/10.1016/j.cell.2006.06.054