FLS2
FLS2 is a transmembrane receptor protein found on plant cells, recognising flagellin-22, a protein commonly associated with bacteria (in other words, a MAMP, a microbe-associated molecular pattern). This pathogen-recognition receptor (PRR) activates signalling pathways within the plant cell, leading to an immune response occurring.
One immune response induced by FLS2 activation is callose synthesis in PMR4. Callose is a polysaccharide that temporarily strengthens the cell wall during stress. Signalling occurs through pathways that are not fully understood, including a MAP kinase cascade.
The extracellular domain consists of a leucine-rich repeat, and has direct interaction with Flg22 (if present). The transmembrane domain is very thermodynamically stable. The intracellular domain includes a serine/threonine kinase, which on activation by the extracellular LRR binding to Flg22, becomes phosphorylated and activates a signalling cascade (including the MAP kinase cascade). The activated cascades impact growth and plant defences (callose synthesis, …).
Flagellin (Flg22) is found across both Gram-positive and Gram-negative bacteria, so it is an ideal MAMP for a PRR to recognise. FLS2 plays an important role in plant innate immunity. As FLS2 is not able to distinguish between virulent and avirulent bacteria, the plant is not capable of adapting the immune response. This leads to stronger responses from the plant against avirulent bacteria than virulent bacteria, which seems counterintuitive. This is because more elicitors are produced by avirulent bacteria, leading to more innate immunity pathways being activated in the plant.
Refs: mostly Wikipedia (https://en.wikipedia.org/wiki/FLS2)