A Plant Pathogen Type III Effector Protein Subverts Translational Regulation to Boost Host Polyamine Levels

Pathogenic bacteria inject effector proteins into host cells to manipulate cellular processes and facilitate the infection. Transcription-activator-like effectors (TALEs), an effector class in plant pathogenic bacteria, transcriptionally activate host genes to promote disease. We identify arginine decarboxylase (ADC) genes as the host targets of Brg11, a TALE-like effector from the plant pathogen Ralstonia solanacearum. Brg11 targets a 17-bp sequence that was found to be part of a conserved 50-bp motif, termed the ADC-box, upstream of ADC genes involved in polyamine biosynthesis. The transcribed ADC-box attenuates translation from native ADC mRNAs; however, Brg11 induces truncated ADC mRNAs lacking the ADC-box, thus bypassing this translational control. As a result, Brg11 induces elevated polyamine levels that trigger a defense reaction and likely inhibits bacterial niche competitors but not R. solanacearum. Our findings suggest that Brg11 may give R. solanacearum a competitive advantage and uncover a role for bacterial effectors in regulating ternary microbe-host-microbe interactions.