Location: Subtropical Plant Pathology Research
2013 Annual Report
Candidatus Liberibacter solanacearum (Lso) is a phloem-limited alphaproteobacterium associated with the devastating zebra chip disease of potato. Like other members of Liberibacter, Lso-ZC1 encodes a flagellin domain-containing protein (FlaLso,) with a conserved 22 amino-acid domain (flg22Lso). To understand the innate immune responses triggered by this unculturable intracellular bacterium, we studied the pathogen-associated molecular patterns (PAMPs) triggered immunity in a non-host plant, Nicotiana benthamiana, using the flg22Lso peptide and the full length flaLso gene. Our results showed that the expression of flaLso via Agrobacterium-mediated transient expression induced a slow 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 flg22Lso and flaLso in N. benthamiana. The flaLso, however, induced stronger up-regulations, especially that of FLS2 and RboH in N. benthamiana. Although neither cell death nor ROS were induced by the synthetic flg22Lso, weak callose deposition was observed in infiltrated leaves of tobacco, tomato, and potato plants. These results revealed that in order to avoid detection during its intracellular life cycle, the Lso bacterium not only lacks avirulence proteins but has also evolved to only weakly activate PAMP triggered immunity. We have screened ninety two genotypes/varieties with the flg22las and flg22lso peptides. Four of the 92 had the species activity of 20- to 40-fold greater than the other genotypes. Further characterization of these varieties is underway.