Cell Death Triggered by the YUCCA-like Bs3 Protein Coincides with Accumulation of Salicylic Acid and Pipecolic Acid But Not of Indole-3-Acetic Acid.

The pepper (Capsicum annuum) resistance gene bacterial spot3 (Bs3) is transcriptionally activated by the matching Xanthomonas euvesicatoria transcription-activator-like effector (TALE) AvrBs3. AvrBs3-induced Bs3 expression triggers a rapid and local cell death reaction, the hypersensitive response (HR). Bs3 is most closely related to plant flavin monooxygenases of the YUCCA (YUC) family, which catalyze the final step in auxin biosynthesis. Targeted mutagenesis of predicted NADPH- and FAD-cofactor sites resulted in Bs3 derivatives that no longer trigger HR, thereby suggesting that the enzymatic activity of Bs3 is crucial to Bs3-triggered HR. Domain swap experiments between pepper Bs3 and Arabidopsis (Arabidopsis thaliana) YUC8 uncovered functionally exchangeable and functionally distinct regions in both proteins, which is in agreement with a model whereby Bs3 evolved from an ancestral YUC gene. Mass spectrometric measurements revealed that expression of YUCs, but not expression of Bs3, coincides with an increase in auxin levels, suggesting that Bs3 and YUCs, despite their sequence similarity, catalyze distinct enzymatic reactions. Finally, we found that expression of Bs3 coincides with increased levels of the salicylic acid and pipecolic acid, two compounds that are involved in systemic acquired resistance.