Uptake of the Effector Avr2 by Tomato Is Not a Cell Autonomous Event.

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 from f. sp. (). 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 the resistance gene. Recognition of Avr2 by I-2 occurs inside the plant nucleus. Here, we show that pathogenicity of an knockout () 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 of tomato plants were grafted onto either a or on an rootstock. 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 that expressing tomato cells cannot autonomously take up the effector protein from the xylem sap. Additionally, and plants were crossed with plants. Whereas F1 plants showed a constitutive immune response, immunity was not triggered in the plants confirming that Avr2 is not autonomously taken up from the extracellular spaces to trigger I-2. Intriguingly, infiltration of in leaves of plants triggered I-2 mediated cell death, which indicates that triggers effector uptake. To test whether, besides , effector uptake could also be induced by other fungal pathogens the and transgenic lines were inoculated with Whereas plants became hyper-susceptible to infection, no difference in disease development was found in the plants as compared to wild-type plants. These data suggest that effector uptake is not a host autonomous process and that and , but not , facilitate Avr2 uptake by tomato cells from extracellular spaces.