Infection and plant defense responses during plant-bacterial interaction

To exploit plant nutrients, phytopathogenic bacteria have evolved sophisticated infection strategies. A number of biotrophic Gram-negative bacteria are able to cause diseases in plants through a type III secretion system, composed of a protruding surface appendage, known as the hrp pilus, throughwhich the bacterium injects effector proteins into the host cells to manipulate plant cells, in particular to suppress plant defenses. A unique infection strategy in plant-bacterial interactions is adopted by Agrobacterium tumefaciens,which genetically transforms its host by transferring T-DNA from its tumor-inducing plasmid to the chromosome of a plant cell. This transfer is mediated by the pilus T, belonging to the type IV secretion system. Other important virulence factors of phytopathogenic bacteria are phytotoxins, extracellular polysaccharides, phytohormones and plant cell-wall-degrading enzymes. These latter factors are fundamental for the pathogenesis of necrotrophic bacteria. Plants defend themselves from bacterial attacks through a multilayered system of passive and active defense mechanisms, which can interfere with entry of bacteria into the plant tissue and restrict bacterial growth when the ingress has been gained. A number of hypotheses have been advanced to explain the restriction of the bacterial growth observed during the hypersensitive reaction and systemic acquired resistance development.