title: Uptake of theFusariumEffector Avr2 by Tomato Is Not a Cell Autonomous Event.
abstract: Pathogens secrete effector proteins to manipulate the host for their own proliferation. Currently it is unclear whether the uptake of effector proteins from extracellular spaces is a host autonomous process. We study this process using the Avr2 effector protein fromFusarium oxysporumf. sp.lycopersici(Fol). Avr2 is an important virulence factor that is secreted into the xylem sap of tomato following infection. Besides that, it is also an avirulence factor triggering immune responses in plants carrying theI-2resistance gene. Recognition of Avr2 by I-2 occurs inside the plant nucleus. Here, we show that pathogenicity of anAvr2knockoutFusarium(FolΔAvr2) strain is fully complemented on transgenic tomato lines that express either a secreted (Avr2) or cytosolic Avr2 (ΔspAvr2) protein, indicating that Avr2 exerts its virulence functions inside the host cells. Furthermore, our data imply that secreted Avr2 is taken up from the extracellular spaces in the presence of the fungus. Grafting studies were performed in which scions ofI-2tomato plants were grafted onto either aΔspAvr2or on anAvr2rootstock. Although the Avr2 protein could readily be detected in the xylem sap of the grafted plant tissues, no I-2-mediated immune responses were induced suggesting thatI-2-expressing tomato cells cannot autonomously take up the effector protein from the xylem sap. Additionally,ΔspAvr2andAvr2plants were crossed withI-2plants. WhereasΔspAvr2/I-2F1 plants showed a constitutive immune response, immunity was not triggered in theAvr2/I-2plants confirming that Avr2 is not autonomously taken up from the extracellular spaces to trigger I-2. Intriguingly, infiltration ofAgrobacterium tumefaciensin leaves ofAvr2/I-2plants triggered I-2 mediated cell death, which indicates thatAgrobacteriumtriggers effector uptake. To test whether, besidesFol, effector uptake could also be induced by other fungal pathogens theΔspAvr2andAvr2transgenic lines were inoculated withVerticillium dahliae.WhereasΔspAvr2plants became hyper-susceptible to infection, no difference in disease development was found in theAvr2plants as compared to wild-type plants. These data suggest that effector uptake is not a host autonomous process and thatFolandA. tumefaciens, but notV. dahliae, facilitate Avr2 uptake by tomato cells from extracellular spaces.
authors.affiliation: Molecular Plant Pathology, Faculty of Science, Swammerdam Institute for Life Sciences, University of Amsterdam Amsterdam, Netherlands.
authors.forename: Xiaotang
authors.initials: X
authors.lastname: Di
authors.affiliation: Molecular Plant Pathology, Faculty of Science, Swammerdam Institute for Life Sciences, University of Amsterdam Amsterdam, Netherlands.
authors.forename: Jo
authors.initials: J
authors.lastname: Gomila
authors.affiliation: Molecular Plant Pathology, Faculty of Science, Swammerdam Institute for Life Sciences, University of Amsterdam Amsterdam, Netherlands.
authors.forename: Lisong
authors.initials: L
authors.lastname: Ma
authors.affiliation: Molecular Plant Pathology, Faculty of Science, Swammerdam Institute for Life Sciences, University of Amsterdam Amsterdam, Netherlands.
authors.forename: Harrold A
authors.initials: HA
authors.lastname: van den Burg
authors.affiliation: Molecular Plant Pathology, Faculty of Science, Swammerdam Institute for Life Sciences, University of Amsterdam Amsterdam, Netherlands.
authors.forename: Frank L W
authors.initials: FL
authors.lastname: Takken
keywords: Agrobacterium tumefaciens; Fusarium oxysporum; Verticillium dahliae; effector uptake; grafting; tomato