A Pathogen Effector Subverts Translational Regulation to Boost Host Polyamine Levels

Pathogenic bacteria inject effector proteins into host cells to manipulate cellular processes promoting infection. Transcription-activator-like effectors (TALEs), an effector class found in some plant-pathogenic bacteria, transcriptionally activate host genes to promote disease. We have identified arginine decarboxylase (ADC) genes as the sole host targets of Brg11, a TALE-like effector from the bacterial broad-host-range plant pathogen Ralstonia solanacearum. Brg11 targets the ADC-box, a 50-bp motif upstream of ADC genes broadly conserved across land plants. The transcribed ADC-box attenuates translation from native ADC mRNAs. Yet, Brg11 induces transcription of truncated ADC mRNA lacking the 5’ ADC-box, thus bypassing translational control. ADCs are rate-limiting enzymes of polyamine biosynthesis, and we show that the Brg11-induced elevation of polyamine levels triggers a plant defense reaction that likely inhibits bacterial niche-competitors, but not R. solanacearum. Our findings illustrate a novel concept in effector biology whereby bacterial effectors act as mediators of tripartite microbe-host-microbe interactions.