Pea aphid as both host and vector for the phytopathogenic bacterium Pseudomonas syringae

Aphids are widespread agricultural pests that are capable of disseminating plant viral diseases; however, despite coming into frequent contact with epiphytic bacteria, aphids are considered to have no role in bacterial transmission. Here, we demonstrate the ability of pea aphids to vector the phytopathogen Pseudomonas syringae pv. syringae B728a (PsyB728a). While feeding on plants colonized by epiphytic bacteria, aphids acquire the bacteria, which colonize the digestive tract, multiply, and are excreted in the aphid honeydew, resulting in inoculation of the phyllosphere with up to 10(7) phytopathogenic bacteria per cm(2). Within days of ingesting bacteria, aphids succumb to bacterial sepsis, indicating that aphids serve as an alternative, nonplant host for PsyB728a. The related strain Pseudomonas syringae pv. tomato DC3000 is >1,000-fold less virulent than PsyB728a in the pea aphid, suggesting that PsyB728a possesses strain-specific pathogenicity factors that allow it to exploit aphids as hosts. To identify these factors, we performed a mutagenesis screen and recovered PsyB728a mutants that were hypovirulent, including one defective in a gene required for flagellum formation and motility. These interactions illustrate that aphids can also vector bacterial pathogens and that even seemingly host-restricted pathogens can have alternative host specificities and lifestyles.