Detection of phytopathogenic Xanthomonads via a novel perception mechanism in Arabidopsis thaliana
Plant Genetics and Genomics
Plant Breeding and Plant Pathology
Final Report Abstract
Bacteria of the genus Xanthomonas are destructive pathogens for various cultivated plants. In preliminary work, we found that a protease secreted by xanthomonads triggers immune responses in the model plant Arabidopsis, which lead to increased resistance in these plants. Arabidopsis cells detect this bacterial protein indirectly via its specific protease activity. As part of this project, we were able to elucidate this recognition mechanism and show that the bacterial protease, a subtilase called SLPA-X, cuts out a small polypeptide from a protein secreted by plant cells, which then triggers the typical defense responses in the plant cells. Starting from around 10 kg of plant cells and a few mg of purified SLPA-X protease, it was possible to purify and ultimately identify the active immune signal via a series of chromatography steps. This plant factor, here called SAMP for 'subtilase activated molecular pattern', is a dipeptide held together by a Cys-Cys bridge. SLPA-X releases active SAMP from the Pro-SAMP substrate protein by proteolytical cuts of at least 2 sites. Post-translational modifications, including variable hydroxylations and glycosylations, made identification of SAMP difficult but were found to be not essential for its immunonogenic activity. Synthetic, unmodified dipeptide is immunogenic in the subnanomolar range, but inactive after reduction of the Cys-Cys bridge. The Pro-SAMP protein belongs to a family of small, secreted proteins in Arabidopsis that show sequence differences in the activation sites cut by SLPA-X. These proteins could serve as activatable pro-SAMPs for proteases with other proteolytic specificities. Indeed, secreted proteases from some other phytopathogenic bacteria and fungi showed immunogenic activity like SLPA-X, but these proteases seem to activate pro-SAMPs different from the one activated by SLPA-X. These results lead to the new hypothesis that proteins secreted by Arabidopsis serve as 'decoys' for proteases from various pathogens. The proteolytically released signals betrays the presence of the pathogens to the host plant, enabling the induction of an effective immune response.
