Elucidating the molecular basis of effector-triggered host susceptibility mediated by the TAL-like effector Brg11 from the bacterial pathogen Ralstonia solanacearum

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Plant pathogens pose a permanent thread to global food security. To invade plants, phytopathogenic microbes rely on effector proteins that they inject into host cells to promote host susceptibility. Knowledge of effector targets inside host cells provides a possibility for knowledge-based breeding of pest-resistant crops. Thus, elucidation of the molecular basis of effector-triggered host susceptibility remains a major aim of fundamental and applied plant pathology.We study Ralstonia solanacearum a devastating bacterial pathogen that triggers bacterial wilt disease on numerous crop species. We have characterized Brg11, a R. solanacearum effector with high similarity to transcription activator-like effectors (TALEs) from the bacterial genus Xanthomonas. TALEs bind to effector binding elements (EBEs) present in host promoters and transcriptionally activate downstream host susceptibility (S) genes to promote disease. Given its relatedness to TALEs it’s conceivable that Brg11 promotes disease by transcriptional activation of host S genes. Thus, we aimed to identify host genes that are transcriptionally activated by Brg11 to clarify how it promotes disease.To identify Brg11 host target genes we determined a preferred target sequence of Brg11 (Brg11-EBE). A 18bp-motif resembling Brg11-EBE was identified upstream of all arginine decarboxylase (ADC) genes in R. solanacearum host genomes and accordingly this motif was designated as ADC-box. Combined analysis of RNA-Seq data and target site prediction suggests that ADC1 and ADC2 are the only direct target genes of Brg11 in tomato. Inspection of Brg11-induced versus intrinsic ADC transcripts revealed short (100 bps) and long (450 bps) 5’UTRs, respectively. Preliminary data suggest that differences in their 5’UTRs cause that translation of intrinsic but not Brg11-induced transcripts is feedback regulated by metabolites that accumulate due to activity of ADC enzymes. Thus, Brg11-induced ADC transcripts likely evade translational control mechanisms that limit translation from intrinsic transcripts. In summary, our findings suggest that R. solanacearum Brg11 promotes bacterial wilt disease by transcriptional activation of host ADC genes.In the framework of this proposal we aim to:- study translational regulation of intrinsic versus Brg11-induced ADC transcripts- establish a catalogue of metabolites that change as consequence of elevated ADC protein levels- Analyze growth-inhibiting/promoting activity of Brg11-induced metabolites on plant associated microbes- Establish cellular reporters to visualize Brg11 target cells and Brg11-induced changes- Investigate Brg11-induced changes at the transcriptional and translational levelIn summary, these studies shall provide insights into how Brg11-induced activation of ADC genes promotes bacterial wilt disease.