Characterisation of TAL-effector-mediated resistance to citrus canker using a new variant of Xanthomonas citri

ROXANA A. ROESCHLIN; MARÍA A. FAVARO; MARÍA R. MARANO; LUCILA GARCÍA; JAVIER FORMENT; FACUNDO UVIEDO; MARÍA A. CHIESA; JOSÉ GADEA

Banyuls sur Mer

Workshop; Effectome-Resistance Network Meeting; 2017

Ccost European Cooperation in Science and Technology

Xanthomonas citri subspecie citri (X. citri) is the causal agent of Asiatic citrus canker. We have previously identified a natural variant, X. citri AT, that triggers a host-specific defense response in Citrus limon and C. sinensis. In this work, this defense response is assessed by transcriptomic, physiological and ultrastructural analyses. X. citri AT triggers in resistant plants a hypersensitive response (HR-like) associated with the interference on biofilm development and arrest of bacterial growth. This response involves an extensive transcriptional re-programming setting in motion cell wall reinforcement, oxidative burst and accumulation of salicylic acid and secondary metabolites. Ultrastructural analyses revealed subcellular changes involving the activation of autophagy-associated vacuolar processes. Furthermore, this defense response protects plants from disease upon subsequent challenges by pathogenic Xanthomonas. The X. citri AT bacterial gene causing the deployment of HR-like responses in the plant appears to be a short (7.5 repeats) variant of the pathogenic pthA4 TAL-effector. The mode of action of this short TAL is also mediating transcriptional activation of plant genes, as mutated version of the TAL effector in nuclear localization or activation domains hampers the triggering of the HR-like response. DNA-binding assays using double-stranded microarrays indicate that this short TAL-effector is able to bind DNA. Identification of plant targets involved in the triggering of this canker resistance phenotype is underway. This knowledge will help to rationally exploit the plant immune system as a biotechnological approach to manage citrus canker, and reveals biological functionality for short TAL-effectors.