Elucidating in planta target genes of Ralstonia solanacearum TALE-like proteins

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Ralstonia solanacearum, the causal agent of bacterial wilt disease, is a quarantine organism in the USA and EU that ranks among the most devastating microbial pathogens in crop plants. We aim to study the interaction between R. solanacearum and its host plants at the molecular level and anticipate that our findings will enable us, in the long run, to develop effective countermeasures against this devastating pathogen. The effector proteins injected by bacterial pathogens into host cells either promote disease or trigger plant immunity. They are thus key determinants for the outcome of plant-microbe interactions. In the framework of this proposal we will study RipTALs, one class of effectors that are widespread in R. solanacearum and that share homology with Transcription Activator Like Effectors (TALEs) from the bacterial genus Xanthomonas.The TALE code describes the correlation of defined amino acids in a given TALE DNA binding domain with specific DNA bases in its effector binding element (EBE). This allows in silico prediction of EBEs for TALEs of known sequence. In preliminary studies we showed that, like for TALEs, RipTAL EBEs can be predicted with the TALE code. Furthermore RipTALs, like TALEs are injected into host cells and are capable of transcriptionally activating promoters bearing a predicted EBE. It is thus conceivable that RipTALs, like TALEs, transcriptionally activate host genes to promote susceptibility in host plants. Given that R. solanacearum is a root pathogen, while xanthomonads typically infect aerial tissues, it is conceivable that they promote disease by activation of host susceptibility (S) genes that encode functionally distinct proteins.We aim to identify RipTAL activated host S genes that promote bacterial disease when being transcriptionally upregulated by a given RipTAL. Once host S genes have been isolated and functionally validated we want to clarify how S gene products promote R. solanacearum disease at the molecular level.R. solanacearum strains isolated from all over the world can be subdivided into four phylotypes, which correlate with their geographical origin. Notably R. solanacearum phylotypes I, II and IV contain RipTALs that differ in their N- and C-terminal regions and their predicted EBEs. Till now molecular studies of RipTALs have been conducted almost exclusively on RipTALs from phylotype I. In the framework of this proposal we aim to also characterize RipTALs from phylotype II and IV strains with the long term goal to identify and characterize corresponding host S genes and gene products.In summary we aim to identify and characterize host S genes that are activated by RipTALs from diverse R. solanacearum phylotypes. We anticipate that our findings will provide insights into the disease strategies of R. solanacearum, aiding the development of efficient countermeasures to protect crop plants.