Abstract: 【Aim】 To investigate the relationship between bacteria, catalase (CAT) and the pea aphid, Acyrthosiphon pisum, and the role of catalase in the immune responses of the pea aphid. 【Methods】 The bacterial load in the aphid was determined by in vitro culturing and colony counting after Micrococcus luteus and Escherichia coli infection. H₂O₂ concentration and catalase gene transcriptional level of the aphid after bacterial infection were measured by microwell-plate fluorescence assay and real-time quantitative PCR, respectively. The catalase gene (Cat) of the pea aphid was knock-downed by RNA interference, and the H₂O₂  concentration, bacterial load and survival rate of the aphids were assayed after bacterial infection. 【Results】 The H2O2 level and the expression level of the catalase gene in the pea aphid increased markedly 12 h post infection by M. luteus. After E. coli infection, the expression level of the catalase gene increased 12 h post infection, but the H₂O₂ level had no obvious change. After the catalase gene was knock-downed, the H₂O₂  level of the aphid was up-regulated significantly 12 h post M. luteus infection, the bacterial load of dsCat-injected pea aphid was about half that of dsGFP-injected pea aphid 24 h post M. luteus infection, but the survival rate of aphids had no change after M. luteus infection. However, knock-down of the catalase gene had no effects on the H₂O₂ level of the aphid, proliferation of bacteria inside the aphid, and the survival rate of the aphids infected by E. coli. 【Conclusion】 The catalase is involved in the immune defense of the pea aphid against infection by M. luteus, but is not a key enzyme in this process. To cope with E. coli infection, however, the aphid may have different defense strategies.

Key words: Acyrthosiphon pisum, bacteria, reactive oxygen species, catalase, RNA interference