Fig. 5. A hypothetical model of the function of the AvrPto class of type III effectors on the suppression of the papilla-associated cell wall-defense in plants. (A) The plant cell responds to an hrp mutant bacterium with a papilla-based extracellular defense by production and transport of cell wall and defense proteins through the secretory pathway. Extracellular defense compounds (blue triangles) and a large papilla beneath the hrp mutant bacterium inhibit the bacterial metabolism and/or produce a "desolate zone" that isolates the hrp mutant bacterium from access to nutrients/water. Wavy lines above the cell wall (CW) indicate nutrients/water. Golgi, Golgi apparatus; ER, endoplasmic reticulum. (B) Wild-type (WT) DC3000 delivers the AvrPto class of type III effectors (red circles) into the plant cell. Mechanism 1: effectors suppress the extracellular secretory pathway, which could lead to feedback repression of the genes encoding secreted cell wall and defense proteins that enter this particular secretory pathway. Mechanism 2: effectors directly inhibit the transcription of the genes encoding cell wall and defense proteins, which are components of the papilla-associated defense.