Figure 1. Model for the evolution of bacterial resistance in plants. Left to right, recognition of pathogen-associated molecular pattern (PAMP) triggers basal immunity, which requires signalling through MAP kinase cascades and transcriptional reprogramming mediated by plant transcription factors. In a second step of the co-evolution, a cocktail of effector proteins is produced and delivered within plant cells via the Type III secretion system. The effectors target multiple host proteins to suppress basal immune responses. In a third step of co-evolution, plant resistance proteins (CC-NB-LRR and TIR-NB-LRR here) detect effector activity or the presence of the effector protein, and restore resistance through effector-triggered immune response. In a fourth step (not represented here), the bacteria alters its effectors to alter their functions or structure to eventually prevent the specific recognition by the resistance proteins. Reproduced from Chisholm et al. 2006 with permission of Elsevier.