FLS2, or FLAGELLIN-SENSING 2, is a protein that acts as a sensor on the outer surface of a plant’s cells, providing a foundational layer of defense. It recognizes the presence of many types of bacteria, and when a potential threat is detected, it initiates a cascade of internal signals that activate the plant’s immune system. This system represents the first line of defense, allowing plants to respond to infection before a pathogen can cause widespread damage.
A Plant’s Sentry System
Plants are constantly exposed to various microbes and have evolved systems to distinguish between harmless and harmful organisms. This is achieved by recognizing Pathogen-Associated Molecular Patterns (PAMPs). PAMPs are conserved molecular structures common to many microbes but not found in the plant itself, acting like universal barcodes that signal a microbe’s presence.
To detect these barcodes, plant cells have proteins called Pattern Recognition Receptors (PRRs). FLS2 is a PRR located on the plasma membrane surrounding the plant cell. Its job is to identify a PAMP called flagellin, the main protein that makes up the flagellum, a tail that many bacteria use for movement.
FLS2 does not recognize the entire flagellin protein. Instead, it is highly attuned to a specific 22-amino-acid segment known as flg22. This small piece is sufficient to alert the plant to the presence of bacteria.
The Molecular Handshake
The detection of a threat by FLS2 initiates a sequence of molecular events that transmits a signal from the outside to the inside of the plant cell. This process begins when the flg22 peptide, acting like a key, binds to the FLS2 protein, which functions as a lock. This binding induces a physical change in the FLS2 protein’s shape, an important first step in activating the alarm.
This new shape allows FLS2 to interact with a partner protein, the co-receptor BAK1 (BRI1-ASSOCIATED KINASE 1). Before detection, FLS2 and BAK1 exist separately in the cell membrane. The binding of flg22 causes FLS2 to form a complex with BAK1, a connection that acts as a molecular handshake confirming the bacterial signature.
Once the FLS2-BAK1 complex is formed, it becomes active. Both proteins are kinases, meaning they add phosphate groups to other proteins through phosphorylation. Within the complex, they phosphorylate each other, which turns the receptor fully on. This self-activation transmits the danger signal across the cell membrane to the cell’s interior.
Activating Plant Defenses
The signal from the FLS2-BAK1 complex triggers defense responses known as PAMP-Triggered Immunity (PTI). One of the first responses is a rapid production of reactive oxygen species (ROS). These chemically reactive molecules can directly harm invading microbes and act as secondary signals to amplify the response throughout the plant.
Simultaneously, the plant takes physical measures to block pathogen entry. The signal from FLS2 activation causes the rapid closure of stomata, the small pores on the leaf surface used for gas exchange. Shutting these entry points helps contain the threat at the surface.
A more sustained defense follows the immediate actions. The signal travels from the cell surface to the nucleus, where it activates numerous defense-related genes. This activation leads to the production of antimicrobial compounds to kill pathogens. It also leads to the reinforcement of the plant cell wall through callose deposition, making it harder for microbes to penetrate.
Broader Implications for Plant Science
Understanding how FLS2 and similar receptors function has implications for agriculture. This knowledge allows for the development of crops with enhanced natural defenses. By selecting for plants with more effective FLS2 signaling, breeders can produce varieties resistant to many bacterial diseases. This approach could reduce the reliance on chemical pesticides and lead to more sustainable farming.
FLS2 also serves as a model for research into plant immunity and cell biology. Its mechanisms, including ligand binding and signal transduction through phosphorylation, apply to many other plant receptor systems. Scientists use FLS2 to dissect the signaling networks that govern how plants perceive and respond to their environment. This research provides insights into both plant defense and development, as some signaling components are shared between the two pathways.