FLS genes have been discovered to be involved in flagellin reception of bacteria. FLS1 was the original gene discovered shown to correspond with a specific ecotype within Arabidopsis thaliana. Even so, further studies have shown a second FLS gene known as FLS2 that is also associated with flagellin reception. FLS2 and FLS1 are different genes with different responsibilities, but are related genetically. FLS2 has a specific focus in plant defense and is involved in promoting the MAP kinase cascade. Mutations in the FLS2 gene can cause bacterial infection by lack of response to flg22. Therefore,FLS2’s primary focus is association with flg22 while its secondary focus is the involvement of promoting the MAP kinase cascade in plant defense.
FLS2 and its similarities to receptor-like kinases HOLAKHEASEN
FLS2 is considered a receptor like kinase (RLK). RLKs are very similar to tyrosine kinases but different in the fact that tyrosine kinases are present in mammals and RLKs are plant specific. RLKs are transmembrane proteins that consists of inner, outer, and central membrane regions. Being proteins, they also have an amino-terminus domain and a carboxyl-terminal kinase domain. RLKs play an important role in plant reception which will be shown through FLS2. BRASSINOSTEROID INSENSITIVE 1 (BRI1) is another important RLK with a lot of similarities to FLS2. Despite share many of the same signalling components, FLS2 and BRI1 have very different effects on the cell.
FLS2 Structure
FLS2 consists of 3 domains: an extracellular, a transmembrane, and an intracellular. The extracellular domain is known as the Leucine-rich repeat (LRR) domain. It is in this region, which is the amino-terminus, where it is said to have direct interaction with flagellin initiating the response of FLS2 to flg22. The transmembrane domain is where proteins transition from extracellular to intracellular. This region is usually very thermodynamically stable and occurs only in the phospholipid membrane between cells. The intracellular domain is the serine/threonine kinase domain. In this domain, phosphorylation catalyzes a protein kinase cascade leading to a response. In FLS2, this response elicits changes in growth and plant defenses.
Ectopic Expression of FLS2
One experiment performed to show involvement of FLS2 in flagellin perception was an ectopic expression of FLS2 using the CaMV 35S gene promoter in one wild type Arabidopsis thaliana plant and two supposed mutant Arabidopsis thaliana plants. The wild type Arabidopsis plant was indicated by Col-0 and the two mutants were indicated by 35S::FLS2-col2 and 35S::FLS2-col4. In this experiment, callose deposition and production of active oxygen species were the variables considered to show flagellin perception after treatment of flg22 as indicated by previous research (Gomez-Gomez, et al. 1999). Results for this study showed a few things. The first thing it showed was a response in callose deposition 12 hours after treatment in only one plant type which was the mutant 35S::FLS2-col2. In the wild type and mutant 35S::FLS2-col4, no callose deposition was detected in plants that exhibited reduced levels of FLS2. In direct correlation to the callose deposition response, the second thing observed was a luminol-based assay to show oxidative burst in leaf tissues of the three plants types. The luminol-based assay showed a rapid and strong oxidative response in mutant 35S::FLS2-col2 with treatment of 10nM flg22, whereas a 10nM treatment of flg22 in the mutant 35S::FLS2-col4 was unable to induce any oxidative response that would occur naturally in the wild type Arabidopsis plant. Even so, higher treatment levels of flg22 would elicit an oxidative response but not significant compared to the wild type. The last thing observed in this experiment was treatment of flg22 in regards to growth inhibition to T2 seedlings. Growth inhibition in 35S::FLS2-col2 was greater than the wild type under flg22 treatment, whereas 35S::FLS2-col4 showed a reduced growth inhibition compared to wild type plants under flg22 treatment which is inverse of the oxidative response in these two mutants relative to the wild type.