Abstract
Plants mount defense responses by recognizing indicators 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 (Solanum lycopersicum) receptors Flagellin sensing2 (Fls2) and Fls3, respectively, but to what degree each receptor contributes to immunity and whether they promote immune responses using the same molecular mechanisms are unknown. Here, we characterized CRISPR/Cas9-generated Fls2 and Fls3 tomato mutants and found that the two receptors contribute equally to disease resistance both on the leaf surface and in the apoplast. However, we observed striking differences in certain host responses mediated by the two receptors. Compared to Fls2, Fls3 mediated a more sustained production of reactive oxygen species and an increase in transcript abundance of 44 tomato genes, with two genes serving as specific reporters for the Fls3 pathway. Fls3 had greater in vitro kinase activity than Fls2 and could transphosphorylate a substrate. Using chimeric Fls2/Fls3 proteins, we found no evidence that a single receptor domain is responsible for the Fls3-sustained reactive oxygen species, suggesting involvement of multiple structural features or a nullified function of the chimeric construct. This work reveals differences in certain immunity outputs between Fls2 and Fls3, suggesting that they might use distinct molecular mechanisms to activate pattern-triggered immunity in response to flagellin-derived MAMPs.
Highlights
Plants mount defense responses by recognizing indicators of pathogen invasion, including microbe-associated molecular patterns (MAMPs)
For Pst, the motilityassociated protein flagellin contains two MAMPs, flg22 and flgII-28, that are recognized by the pattern recognition receptor (PRR) Flagellin sensing2 (Fls2) and Fls3, respectively (Gómez-Gómez and Boller, 2000; Robatzek et al, 2007; Hind et al, 2016)
Chimeric constructs combining the ectodomain of Arabidopsis receptor-like kinase (RLK) EF-Tu receptor (EFR), which detects the bacterial MAMP Ef-Tu, with the intracellular domain of cell wall-associated kinase AtWAK1, which recognizes oligogalacturonides released from the cell wall, results in a functional chimeric protein that recognizes Ef-Tu and activates AtWAK1-specific plant defenses
Summary
Plants mount defense responses by recognizing indicators of pathogen invasion, including microbe-associated molecular patterns (MAMPs). For Pst, the motilityassociated protein flagellin contains two MAMPs, flg and flgII-28, that are recognized by the PRRs Flagellin sensing (Fls2) and Fls, respectively (Gómez-Gómez and Boller, 2000; Robatzek et al, 2007; Hind et al, 2016) Upon recognition of these and other MAMPs, a suite of molecular events occurs to promote defense, including the production of reactive oxygen species (ROS), activation of a mitogen-activated protein kinase (MAPK) cascade, transcriptional reprogramming, callose deposition at the cell wall, stomatal closure, and. N. benthamiana plants silenced for Bak have a reduced flgII-28 ROS response compared to control plants, and Fls and Arabidopsis BAK1 interact upon flgII-28 treatment when they are coexpressed in N. benthamiana leaves (Hind et al, 2016) Together, these observations suggest that there may be some similar factors involved in Arabidopsis FLS2 and tomato Fls signaling, but further analysis is needed to determine the molecular mechanisms of Fls. Chimeric constructs swapping Arabidopsis FLS2 and tomato Fls2.1 LRRs aided the authors in finding specific LRR repeats in tomato that are responsible for recognizing the flg peptide, which is not recognized by Arabidopsis FLS2 (Robatzek et al, 2007; Mueller et al, 2012)
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