Abstract
Most angiosperm plants recognise the flg22 epitope in bacterial flagellin via homologs of cell surface receptor FLS2 and mount pattern-triggered immune responses. However, flg22 is buried within the flagellin protein indicating that proteases might be required for flg22 release. Here, we demonstrate the extracellular subtilase SBT5.2 not only releases flg22, but also inactivates the immunogenicity of flagellin and flg22 by cleaving within the flg22 epitope, consistent with previous reports that flg22 is unstable in the apoplast. The prolonged lifetime of flg22 in sbt5.2 mutant plants results in increased bacterial immunity in priming assays, indicating that SBT5.2 counterbalances flagellin immunogenicity to provide spatial-temporal control and restrict costly immune responses and that bacteria take advantage of the host proteolytic machinery to avoid detection by flagellin having a protease-sensitive flg22 epitope.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
We have updated the manuscript. Specifically, we added that no ROS response is triggered in the fls2 mutant control (NEW Fig1c); we have measured ROS peak times with flagellin in WT vs sbt mutant (NEW Fig5ef); included GFP-His as negative control in processing quenched peptides (NEW Fig6a) and degrading flagellin (NEW Fig6b); and we monitored the release of flagellin peptides with purified SBT5.2a-His and GFP-His (NEW Fig6d and NEW FigS6). Brian Mooney was added as author since he did some of these experiments. We have also moved the flagellin purification and monomerization to the start of the results (Fig1a); replaced ROS assays with the polymer vs. monomer (NEW Fig1b); included zoom-in images for all MS datasets; replaced flagellin degradation by purified SBT5.2-His (NEW Fig6c), and made edits to the text.
