Induction of innate immune responses by flagellin from the intracellular bacterium, ‘Candidatus Liberibacter solanacearum’

BACKGROUND: ‘Candidatus Liberibacter solanacearum’ (Lso) is a phloem-limited alphaproteobacterium associated with the devastating zebra chip disease of potato (Solanum tuberosum). Like other members of Liberibacter, Lso-ZC1 encodes a flagellin domain-containing protein (Fla(Lso)) with a conserved 22 amino-acid peptide (flg22(Lso)). To understand the innate immune responses triggered by this unculturable intracellular bacterium, we studied the pathogen-associated molecular patterns (PAMPs) that triggered immunity in Nicotiana benthamiana, using the flg22(Lso) peptide and the full length fla(Lso) gene. RESULTS: Our results showed that the expression of fla(Lso) via Agrobacterium-mediated transient expression induced a slow necrotic cell death in the inoculated leaves of N. benthamiana, which was coupled with a burst of reactive oxygen species (ROS) production. Moreover, the expression of several representative genes involved in innate immunity was transiently up-regulated by the flg22(Lso) in N. benthamiana. The Fla(Lso), however, induced stronger up-regulation of these representative genes compared to the flg22(Lso), especially that of flagellin receptor FLAGELLIN SENSING2 (FLS2) and respiratory burst oxidase (RbohB) in N. benthamiana. Although neither cell death nor ROS were induced by the synthetic flg22(Lso), a weak callose deposition was observed in infiltrated leaves of tobacco, tomato, and potato plants. CONCLUSION: The flagellin of Lso and its functional domain, flg22(Lso) share characteristics of pathogen-associated molecular patterns, and trigger unique innate immune responses in N. benthamiana. Slow and weak activation of the innate immune response in host plants by the flagellin of Lso may reflect the nature of its intracellular life cycle. Our findings provide new insights into the role of the Lso flagellin in the development of potato zebra chip disease and potential application in breeding for resistance.