Molecular characterization of differences between the tomato immune receptors Fls3 and Fls2

Abstract

Plants mount defense responses by recognizing indications of pathogen invasion, including microbe-associated molecular patterns (MAMPs). Flagellin from the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) contains two MAMPs, flg22 and flgII-28, that are recognized by tomato receptors Flagellin sensing 2 (Fls2) and Flagellin sensing 3 (Fls3), respectively. It is unknown to what degree each receptor contributes to immunity and if they promote immune responses using the same molecular mechanisms. Characterization of CRISPR/Cas9-generated Fls2 and Fls3 tomato mutants revealed that the two receptors contribute equally to disease resistance both on the leaf surface and in the apoplast. However, striking differences were observed in certain host responses mediated by the two receptors.Compared to Fls2, Fls3 mediated a more sustained production of reactive oxygen species (ROS) and an increase in transcript abundance of 44 tomato genes, with two genes serving as reporters for Fls3. Fls3 had greater in vitro kinase activity and interacted differently with the Pst effector AvrPtoB as compared to Fls2. Using chimeric Fls2/Fls3 proteins, we found that no receptor domain was solely responsible for the Fls3 sustained ROS, suggesting involvement of multiple structural features. This work reveals differences in the immunity outputs between Fls2 and Fls3, suggesting they use distinct molecular mechanisms to activate pattern-triggered immunity in response to flagellin-derived MAMPs.

Tomato wall-associated kinase SlWak1 acts in an Fls2- and Fls3-dependent manner to promote apoplastic immune responses toPseudomonas syringae

“…To investigate possible co-dependence of Wak1 and Fls3 gene expression, we measured the Wak1 transcript abundance in tomato plants that have mutations in the two Fls2 genes and Fls3 (Δfls2.1/2.2/3; (Roberts et al ., 2020)) and the Fls3 transcript abundance in Δwak1 plants after treatment with flgII-28. The abundance of Wak1 transcripts was greatly reduced in the Δfls2.1/2.2/3 plants compared to wild-type plants, whereas Fls3 abundance was not significantly different in Δwak1 or wild-type plants ( Fig.…”