Plants, despite not having a mobile immune system like animals, have developed highly sophisticated mechanisms to defend themselves against microorganisms. A central aspect of this defense is their ability to recognize specific components in the cell wall of fungi and bacteria. These molecules, such as chitin and peptidoglycan, not only serve as building blocks for microbial cell walls, but also act as “barcodes” that plants can “read” to identify whether a microorganism is a pathogen or a harmless symbiont. Amazing, right? Stay tuned because we are going to tell you all about it.
# # # Plant defense mechanisms # # #
The first thing we are going to do is talk a little about what the plant recognizes and how. The plant does not have an immune system like animals, but it has a large number of sentinels that watch for anything foreign that penetrates the plant. Who are the plant sentinels:
Receptors: they are proteins anchored in the wall of plant cells that are responsible for recognizing fragments of the components of the cell wall of fungi, such as glucan and chitin. One of the most studied receptors is CERK1, responsible for detecting chitin. Once they recognize these fragments, they trigger the plant's defense response.
Enzymes: Among the enzymes we find degradative enzymes such as glucanases and chitinases whose function is to cut fragments of the plant wall so that they are recognized by the receptors. There is also a group of proteins that act like a street sweeper, collecting small pieces of chitin and taking them to the receptors to activate the defenses.
Once the plant's defense mechanisms are activated, a cascade of signals begins, causing compounds to accumulate at the pathogen's entry site that act as cement, trying to stop the entry (such as callose and lignin) and toxic compounds such as reactive oxygen species (ROs). And in the extreme case, apoptosis, which is the programmed death of the cells that are being attacked and the surrounding cells and even large regions of plant tissue.
# # # Chitin in Fungi: Recognizing Friends and Foes # # #
Chitin is present in all fungi. It is a polymer composed of N-acetylglucosamine units and is a key component of the fungal cell wall. In order for plants to identify the presence of a fungus, they must be able to detect chitin fragments that are released when the fungus attempts to invade plant tissues.
But not all chitin fragments are created equal. Deacetylation is a process by which acetyl groups (a group derived from acetic acid) are removed from chitin, converting it into chitosan, a molecule that is much less visible to plant defenses. This process has a direct effect on how the plant perceives a fungus.
High Chitosan Content and Symbiosis: Fungi that form symbiotic relationships with plants, such as mycorrhizae, often have highly deacetylated chitin, i.e. they are high in chitosan. Chitosan is less detectable by chitin receptors in plants, allowing the fungus to coexist with the plant without triggering a strong immune response. In this context, deacetylation of chitin acts as a “barcode” that signals to the plant that the fungus is harmless and beneficial.
Low chitosan content and Pathogenicity: On the other hand, pathogenic fungi usually have chitin with a lower degree of deacetylation, i.e. less chitosan, so that this chitin is more easily recognized by plant receptors, leading to the activation of a defensive immune response. The plant "reads" this amount of chitosan as a danger signal, and responds accordingly to try to stop the invasion of the fungus.
The question is, if the cell wall of all fungi contains chitin, why does the plant not defend itself against symbiotic agents? This is where the chitin-chitosan pattern comes into play.
# # # The Chitin-Chitosan Pattern: A Molecular Barcode # # #
Not only the amount of chitosan is crucial, but also the chitin-chitosan pattern. This pattern, which refers to the specific arrangement of chitosan along the chitin chain, acts as a highly specific "barcode" that plants can "read". Depending on this pattern, the plant can differentiate between a pathogenic fungus and a harmless, symbiotic one.
Complex Patterns and Symbionts: Symbiotic fungi tend to have more complex chitosan patterns distributed in ways that make it difficult for plant receptors to detect. This complex pattern sends a signal that the fungus is harmless or even beneficial, allowing for a successful symbiotic relationship.
Simple and Pathogenic Patterns: In contrast, pathogenic fungi typically have simpler and more predictable chitosan patterns, making them easier to detect by the plant's immune receptors. This triggers an immune response that aims to eliminate the threat.
# # # Pathogens trick the plant. How? # # #
Just as plants recognize a pattern in chitin – chitosan to identify potential pathogens, pathogens have learned from this and disguise themselves as good guys. Yes, you read that right, they disguise themselves as good guys. Pathogens have developed strategies to hide or modify the chitin in their wall, and their survival depends on doing it well.
An example can be found in several scientific articles where they have verified the role that the modification of chitin to chitosan plays in the development of infection in certain pathogens. This is the case of powdery mildew, a group of fungi that only live on living plant tissue and their survival depends on their ability to hide from the plant's defenses. In a recent work, the fungus's ability to convert chitin into chitosan was blocked. And SURPRISE! The result is incredible, the plant goes from not detecting the pathogen to detecting it, activating its defenses and stopping the infection completely. Only the chitin-chitosan pattern has been altered, and in this way the pathogen's disguise has been removed.
If you are interested in learning more about how pathogens hide from plant defenses, here is this article from our blog where we tell you about the different strategies that pathogens use. Blog: The Microbial Trojan Horse
# # # As the unlocking of Mobile # # #
The concept of how plants “read” these deacetylation patterns can be compared to unlocking a phone using a fingerprint. If the phone doesn’t recognize the fingerprint, it locks itself, assuming that a potential intruder is trying to gain access. However, when it detects your fingerprint, it lets you in without a problem. Similarly, if a plant detects a chitin-chitosan pattern that it doesn’t recognize (such as a simple pattern on a pathogenic fungus), it activates its defenses to block the intruder. On the other hand, if it recognizes a known, benign pattern (such as the complex pattern of a symbiont), it allows the interaction without activating its defenses.
# # # Peptidoglycan in Bacteria: A Similar Mechanism # # #
Peptidoglycan is a fundamental component of the cell wall of bacteria, especially in gram-positive bacteria. It is made up of chains of amino sugars and peptides that provide rigidity and protection to the bacterial cell. Similar to chitin in fungi, peptidoglycan can also be chemically modified, and these modifications have a direct impact on how the plant perceives the bacteria.
Modifications to Peptidoglycan: Bacteria can alter peptidoglycan through processes such as deacetylation and methylation, which change the chemical structure of the molecule. These modifications can make peptidoglycan less visible to immune receptors in the plant, allowing the bacteria to avoid a strong immune response.
Recognition and Immune Response: When peptidoglycan is unmodified or less modified, it is more likely to be recognized by plant receptors, triggering a defensive immune response. In contrast, modifications that mask peptidoglycan can trick the plant into not detecting the bacteria as a threat, facilitating infection or allowing for peaceful coexistence.
# # # The Molecular "Barcode": An Evolutionary Strategy # # #
The concept of a molecular "barcode" is a useful metaphor to describe how plants distinguish between pathogenic and symbiotic microorganisms. The deacetylation pattern in the chitin of fungi and the modifications in the peptidoglycan of bacteria are evolutionary strategies that these microorganisms have developed to interact with their plant hosts.
Evolution of Modifications: Both fungi and bacteria have evolved to modify their cell walls in ways that influence how they are perceived by plants. Symbiotic fungi, for example, have evolved high levels of deacetylation in their chitin as a way to avoid detection as pathogens. Similarly, symbiotic bacteria may modify their peptidoglycan to avoid detection or to minimize the plant's immune response.
Diversity of Responses in Plants: Plants, for their part, have evolved a wide range of receptors that can detect different molecular patterns in microorganisms. This allows them to adjust their immune response according to the type of microorganism they encounter, either by activating strong defenses against pathogens or by promoting tolerance and symbiosis with beneficial microorganisms.
# # # Implications for Agriculture and Biotechnology # # #
Understanding how plants “read” these molecular codes has enormous implications for agriculture. If biotechnology can manipulate these mechanisms, it could lead to developing crops that are more resistant to disease or that benefit more from symbiotic relationships with microorganisms.
Disease-Resistant Crops: By selecting or genetically modifying plants that can more efficiently recognize chitin deacetylation patterns and peptidoglycan modifications, we could develop crops that are more resistant to infections by pathogenic bacteria and fungi.
Harnessing Beneficial Symbionts: On the other hand, by promoting interaction with fungi and bacteria that have beneficial "barcodes", we could improve nutrient uptake and overall plant health, reducing the need for fertilizers and other chemical inputs.
The study of how plants recognise and respond to modifications in microorganisms' chitin and peptidoglycan, and ultimately the cell wall, offers us a fascinating window into the complex interaction between plants and microorganisms. These molecular "barcodes" are key to plant survival, and understanding them could revolutionise agriculture, allowing us to grow more resilient and efficient plants and develop more targeted and environmentally friendly treatments.
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