What comes to mind when you hear ‘evolutionary arms race’? Maybe you think of the growing problem of bacteria becoming more resistant to human-made antibiotics. Or you might be reminded of the principles of natural selection, in which prey and predator develop new characteristics based on their long-term interactions. What about the hidden and long-standing warfare between plants and pathogens?
Why plants have developed ‘immune systems’
Like humans, plants have their own immune systems that have developed over a time period longer than we as a species have been alive on this Earth. There are predators and pests that plants need to fend off; pathogens are especially creative in the ways they infect plants.
Some directly enter the plant’s tissue, while others use toxins to weaken the plant. Others are even transported into the plant with the help of disease vectors or living organisms that can carry a pathogen, spreading it to a healthy organism. For example, aphids are prominent disease vectors for plants as they can unintentionally spread pathogens and viruses from infected plants to healthy ones while they feed on plant tissue. Overall, the goal sounds simple: gain a new home and a long-term food source.
In many cases, a pathogen is biotrophic, meaning it keeps the host alive to provide a continuous source of food. You can tell when a plant’s been infected by a biotrophic pathogen because there are common symptoms a plant shows, such as leaf rust, appearing as uneven brown spots on the leaves, and white mildew powder dusting. There are also species of necrophilic pathogens, which kill the plant first before feeding on it, and even pathogens called hemibiotrophs, who do a combination of both by keeping the plant alive for some time before pulling the plug.
The First Lines of Defense
The plant immune system’s first line of protection against these pathogens and other enemies is its epidermal cells. These cells create the cuticle, that shiny, waxy layer on a leaf. For one, it’s thick and tough. It also contains ‘cuticular wax’ that makes the leaf waterproof, ensuring pathogen spores, essentially dried-up pathogen cells, don’t have a moist environment to thrive.
Epidermal cells are also lined with various antimicrobial chemicals such as phytoanticipins, steroids, and terpenoids. Oftentimes, these chemicals target a pathogen’s metabolic pathways in which they produce their energy. The plant’s defense chemicals can also damage the pathogen’s cell membranes or prevent the production of important molecules, such as DNA and RNA among other functions.
Later Lines of Defense
If a pathogen makes it past these barriers, plants utilize proteins known as Pattern Recognition Receptors, or PRRs, that can be located on cell surfaces and within cells. PRRs detect and recognize foreign molecules on or released by invading pathogens called PAMPs / MAMPs (Pathogen / Microbe-Associated Molecular Patterns). Many microorganisms share the same structures that PRRs recognize like a human differentiating the shadow of a pigeon from that of a bear and making the necessary changes to themselves with that information. Some of the most well-known PAMPs and MAMPs include chitin, a material used by many fungal cell walls, and flagellin, a protein often seen in bacterial flagella that helps the bacterium move.
There are several types of PRRs, but the most common are kinases. Kinases are proteins that add phosphate groups (PO43-) to a molecule and are usually located on the surface of a plant cell. By tagging the passing molecule with a phosphate group, it’s essentially telling the cell, “Hey! Intruder over here!”. From there, a series of chemical interactions occur until either 1) the cell has procured other immune cells to kill the invading microorganism or 2) the signal reaches the nucleus of the cell, ‘telling’ the cell to start replicating DNA that specifically codes for genes that better fortify the cell from the microorganism.
Other proteins that perform a similar function within the plant cell are called nucleotide-binding leucine-rich repeat proteins, or NLRs, and give the plant an additional layer of protection. Together, NLRs and PRRs send out signals to downstream immune responses to their individual cells, starting a chain reaction that preps the plant to retaliate against the pathogen. As stated earlier, pathogens and other microorganisms have many ways to invade a host, hence the reason for all the different proteins, pathways, and other molecules crafted over generations for a successful response that protects the plant.
Recent Discoveries in Plant-Microbe Interactions
A recent discovery found the pepper plant Capsicum chinense has an NLR recognizing a protein called NSs. This protein is produced by the RNA virus Tomato spotted wilt virus (TSWV) and plays a vital role in the spread of the virus. Despite the name, this virus infects a range of hosts, all of which are plants in temperate, subtropical regions of the world. TSWV has a vector, an insect called a thrip, that carries and transmits the virus to other plants. Plants infected with TSWV display symptoms such as major leaf wilting and the appearance of yellow or brown ring spots on fruits and leaves until the virus has sucked up all of the plant’s nutrients.
Researchers discovered that TSWV was so difficult to treat partly because NSs proteins blocked hormone receptors used by plants to respond to pathogens. This is an effective way the virus inhibits the plant’s immune system while slowly killing it. Over time, Capsicum chinense started producing an abnormally large NLR immune receptor. This blocks the NS protein from blocking that plant’s hormone receptors, so the plant’s immune system is not suppressed.
But it’s not all terrible: there are a plethora of microbes that help plants too; it all depends on context. One of the most common is mycorrhizae, a bacteria or fungi that has formed a mutually beneficial relationship with a plant’s root system over the course of hundreds of millions of years. Mycorrhizae play a vital role in increasing nutrient uptake for the host, protecting it from other outside pathogens, and increasing the production of plant growth hormones, among a whole host of other benefits.
But even with all of that, we’ve barely scratched the surface of these relationships between plants and microbes. According to Nature, less than 1% of all microbial species on Earth have been identified by humans, and we’ve been doing this since the 1600s! There’s still a whole world out there to discover, and you might just find it on your neighborhood tree!
Sources:
https://www.sciencedirect.com/science/article/abs/pii/S136013
https://www.cell.com/molecular-plant/fulltext/S1674-2052(19)30171-6
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3131
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC45331/
https://academic.oup.com/pcp/article/55/11/1864/2756060?login
https://www.sciencedirect.com/science/article/pii/S0944711321001690
https://www.nature.com/articles/nrmicro2644
https://www.sciencedirect.com/science/article/pii/S1097276514002615
https://www.nature.com/articles/d41586-022-04218-x
https://www.cabidigitallibrary.org/doi/10.1079/cabicompendium.54086