Plants recognize bacterial pathogens to mount effective defence responses. In our new study, published in Science, we uncovered an important step in the release of a fragment of bacteria that nearly all plants can recognize.
Bacteria use a sweeping tail, called flagellum, to swim and invade host tissues. Flagella consist of flagellin proteins, covered with a sugary glycan layer. Plants can recognise a protein fragment from flagellin that is deeply buried inside this flagellin tail and this recognition triggers immune responses in the plant that suppress bacterial infection.
This flagellin fragment is released by breakdown of the flagella at the bacteria-plant cell interface inside the plant tissue. However, plant proteins that breakdown the flagella have not yet been identified. We hypothesised that adapted bacterial plant pathogens will suppress the plant proteins that mediate the breakdown of flagella.
When looking for suppressed plant proteins, we discovered a glycan-degrading enzyme called BGAL1 that is suppressed during infection. We found that genome-edited plants lacking BGAL1 are more susceptible to bacterial infection, consistent with a role for BGAL1 in immunity. Treatment of flagella with BGAL1 promotes the release of flagellin peptides that are recognised by the plant. BGAL1 is not required to release these immunogenic peptides from mutant bacteria that produce the flagellin lacking the glycan cover. Other bacterial strains use different, BGAL1-insensitive glycans to avoid the breakdown and recognition by the host.
What we discovered probably holds true for more bacterial plant pathogens as most plants recognise most bacterial pathogens via these conserved immunogenic flagellin fragments. The importance of this new layer in pathogen recognition is testified by the fact that bacteria try to avoid recognition using BGAL1 inhibitors and BGAL1-insensitive glycans. This host-pathogen arms-race probably extends beyond the Plant Kingdom, because also animals recognise flagellin fragments. Variation in the glycans used by human bacterial pathogens indicates that this arms-race exists also in ourselves. So the sugary tails of bacterial pathogens still have lots of stories to tell. Many more sweet tales.