FLS2 (Flagellin Sensing 2) is a receptor protein embedded in the surface of plant cells that detects a specific fragment of bacterial flagellin, the protein that makes up the whip-like tail bacteria use to swim. When FLS2 catches that molecular signature, it kicks off a rapid chain of events inside the cell that amounts to an immune alarm: enzymes get activated, toxic molecules are produced, and leaf pores slam shut to block bacterial entry. The system is one of the best-understood examples of how plants, which lack the roving immune cells that animals rely on, defend themselves using sensors hardwired into every cell.
What FLS2 Actually Detects
Flagellin is the structural protein that assembles into the bacterial flagellum. Plants do not respond to the entire flagellin molecule. Instead, FLS2 recognizes a short stretch of 22 amino acids near one end of the protein, known as flg22. This peptide sits in a conserved region of flagellin, meaning many different bacterial species share a similar version of it. That makes flg22 a reliable alarm signal: it is hard for bacteria to change without losing the ability to build a functional flagellum.
The receptor FLS2 itself is a large protein that spans the cell membrane. Its outer portion, which faces the space outside the cell, is built from repeating units called leucine-rich repeats that curl into a curved, spring-like shape. This curved surface is what physically grabs flg22. Crystal structure work showed that two separate contact sites on the inner surface of that curve grip the two ends of the flg22 peptide, locking it in place without requiring the receptor to change its overall shape or cluster together beforehand.1PubMed. Structural basis for flg22-induced activation of the Arabidopsis FLS2-BAK1 immune complex Chemical cross-linking experiments confirmed that flg22 binds directly and specifically to FLS2, and that this binding is what determines which flagellin variants a plant can or cannot sense.2The Plant Cell. The Arabidopsis Receptor Kinase FLS2 Binds flg22 and Determines the Specificity of Flagellin Perception
How the Receptor Assembles Its Signaling Team
Binding flg22 is only the first step. On its own, FLS2 cannot relay a signal to the inside of the cell effectively. It needs a partner called BAK1, a co-receptor that also sits in the cell membrane. Almost the instant flg22 locks onto FLS2, BAK1 joins the complex. This pairing happens so quickly that researchers described it as nearly instantaneous, and it can even occur in isolated membrane preparations in the lab. Importantly, blocking the enzymes that transfer phosphate groups (kinase activity) does not prevent the two receptors from coming together, which means the physical association comes first and the enzymatic signaling follows.3Journal of Biological Chemistry. Rapid Heteromerization and Phosphorylation of Ligand-activated Plant Transmembrane Receptors and Their Associated Kinase BAK1
This partnership is not unique to the model plant Arabidopsis. In rice, the FLS2 equivalent also teams up with a BAK1-related protein (OsSERK2), and the presence of flg22 strongly boosts that interaction.4PubMed Central. Structural and biochemical basis of FLS2-mediated signal activation and transduction in rice The conserved nature of this receptor-plus-co-receptor architecture across species underscores how fundamental the design is to plant immunity.
Passing the Baton Inside the Cell
Once the FLS2-BAK1 complex forms at the membrane, the signal needs to move inward. The key relay runner is a protein called BIK1, a kinase that normally hangs around the inner face of the cell membrane in loose association with FLS2. Upon flg22 perception, BAK1 adds phosphate groups to BIK1, switching it on. BIK1 then phosphorylates both BAK1 and FLS2 in return, creating a rapid feedback loop that amplifies the signal.5PubMed Central. A receptor-like cytoplasmic kinase, BIK1, associates with a flagellin receptor complex to initiate plant innate immunity The timing matters: BIK1 gets phosphorylated first, then passes phosphorylation back to the receptor complex, ensuring the signal propagates in the right order.6PubMed Central. Tyrosine phosphorylation of protein kinase complex BAK1/BIK1 mediates Arabidopsis innate immunity
Plants that lack BIK1 show weakened responses to flagellin across the board, from reduced production of defensive chemicals to impaired resistance against bacterial infection. BIK1 is therefore not just a messenger but an essential hub connecting the surface receptor complex to the cell’s interior defenses.5PubMed Central. A receptor-like cytoplasmic kinase, BIK1, associates with a flagellin receptor complex to initiate plant innate immunity
The Immediate Defensive Burst
One of the fastest visible consequences of FLS2 activation is a burst of reactive oxygen species (ROS), toxic molecules that can damage bacterial cells directly and serve as further alarm signals. BIK1 triggers this by phosphorylating an enzyme called RBOHD, which sits in the membrane and generates superoxide. This phosphorylation does not require calcium ions, which distinguishes it from a separate, calcium-dependent phosphorylation that also targets RBOHD. Both phosphorylation events are needed for the full-blown ROS burst and for effective antibacterial defense.7PubMed. Direct regulation of the NADPH oxidase RBOHD by the PRR-associated kinase BIK1 during plant immunity
Running in parallel is a cascade of enzymes called MAP kinases (MPK3 and MPK6 in Arabidopsis). These are activated by flg22 perception and drive a suite of their own defense outputs, but they operate independently of the ROS burst. Experiments using plants that lack both MPK3 and MPK6 showed that the ROS burst still fires normally, proving these are two separate arms of the immune response branching off from the same receptor.8PubMed Central. A chemical genetic approach demonstrates that MPK3/MPK6 activation and NADPH oxidase-mediated oxidative burst are two independent signaling events in plant immunity The calcium influx that follows flg22 detection also activates calcium-dependent protein kinases, adding yet another independent signaling branch that reinforces immune output.9PubMed Central. Regulation of Plant Immune Signaling by Calcium-Dependent Protein Kinases
This parallel architecture is a smart design. If a pathogen manages to block one branch, the others still function. Redundancy is a recurring theme in plant immunity.
Reprogramming Gene Activity
Within minutes of flg22 detection, signals reach the nucleus and trigger sweeping changes in which genes get turned on or off. A family of proteins called WRKY transcription factors plays a major role here. Three prominent members, WRKY18, WRKY40, and WRKY33, each bind to over a thousand gene locations across the genome in response to flg22. They latch onto a specific DNA sequence known as the W-box motif, flipping the switches on genes involved in defense compound production, cell-wall reinforcement, and hormone signaling.10The Plant Cell. Induced Genome-Wide Binding of Three Arabidopsis WRKY Transcription Factors during Early MAMP-Triggered Immunity
The scale of this reprogramming is worth appreciating. Thousands of genes shift their expression levels, diverting the cell’s resources from normal growth and metabolism toward defense. That diversion is part of why activating immunity consistently comes at a growth cost for the plant, a trade-off that matters in agriculture where yield is the bottom line.
Stomatal Closure as a Physical Barrier
Many disease-causing bacteria enter leaves through stomata, the tiny pores that plants open to exchange gases for photosynthesis. FLS2 activation triggers those pores to snap shut. The FLS2-BAK1 receptor complex initiates downstream signaling that causes guard cells, the paired cells flanking each pore, to lose water pressure and go slack, shrinking the opening.11The Plant Cell. No Entry: SIF2 Closes Stomatal “Doors” to Bacteria by Making Guard Cells SLAC(1) Experiments with the bacterium Pseudomonas syringae confirmed that FLS2 plays a prominent role in this stomatal defense, reducing both the number of bacteria that get in through the epidermis and their ability to multiply inside the leaf.12PubMed Central. A Prominent Role of the Flagellin Receptor FLAGELLIN-SENSING2 in Mediating Stomatal Response to Pseudomonas syringae pv tomato DC3000 in Arabidopsis
Stomatal closure is arguably the single most intuitive piece of the FLS2 response: the plant literally locks its doors when it senses intruders. But keeping stomata closed also limits carbon dioxide uptake and water regulation, so the plant cannot maintain this state indefinitely without paying a price in growth.
How Plants Turn the Alarm Off
An immune response that never shuts down would be just as damaging as one that never turns on. Plants have a built-in brake. After FLS2 is activated, BAK1 phosphorylates two enzymes called PUB12 and PUB13, which are E3 ubiquitin ligases. These enzymes tag FLS2 with ubiquitin molecules, marking it for destruction. The tagged receptor gets pulled from the membrane and degraded. Plants lacking PUB12 and PUB13 show exaggerated immune responses to flagellin, confirming that this degradation pathway is genuinely dampening the signal, not just tidying up spent receptors.13PubMed Central. Direct ubiquitination of pattern recognition receptor FLS2 attenuates plant innate immunity
There is a certain elegance to this circuit. BAK1, the same co-receptor that helps start the immune signal, is also the one that activates the machinery to end it. That creates a self-limiting loop: the stronger the initial activation, the more aggressively the braking system engages.
How Bacteria Fight Back
Pathogens have not sat idle while plants evolved FLS2. Successful bacteria inject proteins called effectors directly into plant cells that sabotage the immune system at various points. Two well-studied effectors from Pseudomonas syringae, called AvrPto and AvrPtoB, target BAK1 specifically. By binding BAK1, they prevent it from partnering with FLS2 when flagellin is detected, effectively cutting the alarm wire at the co-receptor level. Because BAK1 is a shared partner for multiple immune receptors, not just FLS2, this single sabotage move weakens the plant’s ability to detect a range of different microbial signatures.14PubMed Central. Bacterial effectors target the common signaling partner BAK1 to disrupt multiple MAMP receptor-signaling complexes and impede plant immunity
Plants counter this with a second layer of immunity that uses intracellular receptors to detect the effectors themselves. When these intracellular sensors catch an effector in the act, they trigger a stronger, often lethal response in the infected cell. Research has shown that this intracellular branch and the FLS2-mediated surface detection work together synergistically, each amplifying the other’s output to build a more robust defense than either could mount alone.15PubMed Central. PTI-ETI synergistic signal mechanisms in plant immunity
Where FLS2 Sits on the Membrane Matters
FLS2 does not float freely across the cell surface. It concentrates in tiny patches of the membrane called nanodomains, and its positioning within those patches turns out to be critical for signaling. A chemical modification called S-acylation (the attachment of a fatty acid chain) helps anchor FLS2 in the right nanodomains and controls which other proteins it can interact with.16bioRxiv. S-Acylation of plant immune receptors mediates immune signaling in plasma membrane nanodomains
The story gets more specific with BIK1, the cytoplasmic kinase described earlier. High-resolution imaging revealed that a motor protein called myosin XIK physically drags BIK1 into the same nanodomains where FLS2 resides. It does this by linking up with another membrane protein called remorin, which acts as a scaffold. The net effect is that BIK1 is pre-positioned near FLS2 before any flagellin arrives, so the moment the receptor fires, its relay partner is already in arm’s reach. Plants lacking myosin XIK show weaker BIK1 activation and impaired defenses, confirming that this spatial choreography is not optional.17PubMed Central. Myosin XI-mediated BIK1 recruitment to nanodomains facilitates FLS2-BIK1 complex formation during innate immunity in Arabidopsis
Plants Versus Animals in Flagellin Detection
Animals also detect flagellin, using a receptor called TLR5 (Toll-like receptor 5) that, like FLS2, has leucine-rich repeats on its outer surface. But the two systems evolved independently and recognize completely different parts of the flagellin molecule. TLR5 binds a broad surface across the D1 domain of flagellin, making extensive contact with the protein’s three-dimensional structure. FLS2, by contrast, recognizes a short linear peptide from the junction between the D0 and D1 domains near the N-terminus.18PubMed Central. Flagellin sensing, signaling, and immune responses in plants The practical consequence is that a bacterium could mutate the FLS2-binding region to evade plant detection while still being recognized by animal TLR5, and vice versa. Each kingdom has its own blind spots.
Evolutionary analysis suggests that FLS2 belongs to a broader family of leucine-rich repeat receptor kinases that originated in early land plants and diversified alongside the colonization of terrestrial habitats. Some members of this family regulate growth and development rather than immunity, and the immune receptors likely share a common ancestor with those growth regulators.19Nature Communications. Evolutionary trajectory of pattern recognition receptors in plants
Engineering FLS2 for Crop Protection
Because FLS2 recognition is essentially determined by the receptor’s structure, researchers have explored whether transplanting an FLS2 gene from one plant species into another can confer new disease resistance. The results are encouraging. Citrus trees, which are naturally susceptible to citrus canker caused by the bacterium Xanthomonas citri, showed reduced disease symptoms when engineered to express the FLS2 receptor from Nicotiana benthamiana (a tobacco relative). The transgenic trees mounted stronger defense gene expression when challenged with the pathogen, suggesting that their native flagellin-sensing system was simply not strong enough to respond to X. citri’s flagellin.20PubMed. Reduced Susceptibility to Xanthomonas citri in Transgenic Citrus Expressing the FLS2 Receptor From Nicotiana benthamiana
The approach also works in the other direction. When Arabidopsis FLS2 was expressed in N. benthamiana, it enabled the plant to recognize flagellin variants from Ralstonia, a devastating soil-borne pathogen, that the plant could not previously detect. The transplanted receptor successfully partnered with the host plant’s own BAK1-related co-receptors, activated downstream immune signaling, and restricted Ralstonia growth.21PubMed Central. Heterologous expression of Arabidopsis FLS2 in Nicotiana benthamiana confers responsiveness to Ralstonia flg22 and activation of anti-bacterial immunity The compatibility of receptors and co-receptors across species opens the possibility of mixing and matching immune components from different plants to build broader pathogen resistance in crops.
The catch, as with any constitutive immune boost, is the growth trade-off. Plants that chronically run their defenses at elevated levels tend to divert energy away from reproduction and biomass accumulation. Any agricultural application of engineered FLS2 would need to balance stronger pathogen detection against acceptable yield, whether through promoters that activate the receptor only where and when needed, or through careful selection of receptor variants that provide defense without continuous metabolic drain.
FLS2 Pre-Assembles Before Infection
One underappreciated detail is that FLS2 is not purely a solitary molecule waiting for its signal. Evidence from co-immunoprecipitation experiments shows that FLS2 molecules can associate with each other at the membrane even before flagellin arrives. When plants expressing two differently tagged versions of FLS2 were examined, the two tags copurified, indicating that FLS2-FLS2 complexes exist at rest. After flg22 treatment, the abundance of these complexes showed a slight increase within the first few minutes.22PubMed Central. Probing the Arabidopsis Flagellin Receptor: FLS2-FLS2 Association and the Contributions of Specific Domains to Signaling Function Combined with the nanodomain positioning of BIK1 described earlier, this paints a picture of a surveillance system that is pre-loaded and ready to fire. The cell does not build the alarm from scratch each time; it keeps the components loosely assembled and tightens the connections the moment danger is detected.
That readiness matters because speed is everything in the first minutes of infection. Bacteria can double their numbers quickly once inside a leaf, so every second of delay in mounting a defense translates to a larger population the plant has to fight. Pre-assembled receptor complexes, pre-positioned kinases, and parallel signaling branches all serve the same goal: getting the response going before the invader gets established.