Flagellin is the main structural protein of the bacterial flagella and acts as both a virulence factor helping bacteria invade host cells and is also recognised by the host to induce a potent immune response. Flagellin 22 (flg22) is the stretch of flagellin amino acids most conserved across bacterial species and is the minimal ligand recognised by the plant Toll-like receptor 5 (TLR5) homologue FLAGELLIN SENSITIVE 2 (FLS2). FLS2 is a membrane bound leucine-rich repeat-receptor kinase which activates downstream signalling resulting in pattern-triggered immunity (PTI). Flg22 is therefore an extremely useful peptide, widely used in plant defence studies, for example to elicit an immune response in order to discover new plant immunity genes or to investigate different immune responses in mutant backgrounds; as the team led by Jozef Šamaj did (Takáč et al., 2021).
Recognition of flg22 by FLS2 results in receptor heterodimerization with BAK1 and the cytoplasmic-kinase BIK1 and the binding of the SMALL UBIQUITIN-LIKE MODIFIER (SUMO) protein to FLS2. Binding of SUMO to the receptor complex culminates in the activation of downstream signalling leading to PTI. In addition to offering highly active flg22 peptides, we also offer two highly specific antibodies to Arabidopsis SUMO available in our discovery antibodies catalogue as part of our growing plant pathology range.
Following heterodimerisation and receptor internalisation there are a plethora of rapid responses from the plant including: reactive oxygen species (ROS) generation; mitogen‐activated protein kinase (MPK3, MPK6, MPK4, and MPK11) activation; WRKY transcription factor activation, expression of PATHOGENESIS RELATED (PR) genes, upregulation of peroxidases, chitinases and glutathione S-transferases (GSTs), reorganization of the actin cytoskeleton and deposition of callose at the plant cell wall; ultimately resulting in the inhibition of plant growth and increased resistance to pathogens.
Bacterial flagellin is also recognized by the FLS3 receptor in certain solanaceous plants, such as tomato, potato and pepper. Tomato also contains an active FLS2 receptor which is activated by a truncated flagellin 22 (flg22-Δ2) which represents amino acids 1-20 of flg22 and is also available from our discovery peptides catalogue.
Chinchilla et al., (2006). The Arabidopsis Receptor Kinase FLS2 Binds flg22 and Determines the Specificity of Flagellin Perception. Plant Cell. 18(2): 465. PMID: 16377758
Felix et al., (1999). Plants have a sensitive perception system for the most conserved domain of bacterial Flagellin. Plant J. 18(3): 265. PMID: 10377992
Orosa et al., (2018). SUMO conjugation to the pattern recognition receptor FLS2 triggers intracellular signalling in plant innate immunity Nature Communications. 9,: 5185.
Takáč et al., (2021). TALEN-Based HvMPK3 Knock-Out Attenuates Proteome and Root Hair Phenotypic Responses to flg22 in Barley. Front Plant Sci. 12: 666229. PMID: 33995462