Activation of plant immunity by components of the type III secretion system from Xanthomonas campestris pv. vesicatoria
Final Report Abstract
The Gram-negative bacterium Xanthomonas euvesicatoria (Xe) causes bacterial spot disease in pepper and tomato plants and employs a type III secretion (T3S) system to translocate effector proteins into plant cells. The T3S system spans both bacterial membranes and is associated with an extracellular pilus, which serves as a transport channel for secreted proteins. A channel-like translocon in the plant plasma membrane allows the transkingdom delivery of type III effectors (T3Es) which promote bacterial survival and often suppress plant defense responses. In the present project, we show that the pilus of the T3S system acts as a PAMP (pathogen-associated molecular pattern) and activates PTI (PAMP-triggered immunity) responses in both pepper and N. benthamiana in the absence of translocated T3Es. PTI responses render the leaf tissue resistant against subsequent infections, which highlights a potential use of Xe translocon mutants as biocontrol agents for disease management. The protective effect induced by the T3S pilus suppressed T3E delivery by pathogenic Xe strains and Agrobacterium-mediated gene transfer during subsequent infections. In addition to PTI responses induced by the T3S system, we observed a protective effect of translocated T3Es against subsequent infections. In N. benthamiana, this effect was due to the recognition of the T3E XopQ by the plant immune system, which revealed that both PTI and ETI (effectortriggered immunity) render plant cells resistant against the delivery of bacterial pathogenicity factors. Surprisingly, a protective effect was also monitored during compatible Xe – plant interactions, indicating that translocated T3Es prevent the superinfection of infected plant tissue and thus presumably a premature death of host cells. A similar phenomenon was previously described for T3Es from animal-pathogenic bacteria. The underlying molecular mechanisms in plant cells remain to be investigated.
