Subtilase SBT5.2 counterbalances flagellin immunogenicity in the plant apoplast

Abstract

ABSTRACTMost angiosperms 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 and logic and preliminary experiments have indicated that proteases might be required for flg22 release. Here, we demonstrate that the extracellular subtilase SBT5.2 processes flg22 from its precursor in the apoplast ofNicotiana benthamiana. Interestingly, however, SBT5.2 also inactivates the immunogenicity of flagellin and flg22 by preferentially cleaving within the flg22 epitope, consistent with previous reports that flg22 is unstable in the apoplast. The prolonged lifetime of flg22 insbt5.2mutants 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 to the infection site. Since flagellin can bind to FLS2 and flg22 is stabilised when bound to FLS2, we propose that SBT5.2 might process flagellin when bound to FLS2 to produce a flg22-FLS2 complex that can recruit BAK1 and trigger immune signalling. Counterbalancing flagellin immunogenicity is conserved across the plant kingdom, given the conservation of both SBT5.2 and the cleavage site within flagellin and the observation that very similar processing events occur in the apoplast of tomato and Arabidopsis.