Hydrogen cyanide produced by Pseudomonas chlororaphis O6 is a key aphicidal metabolite.

A biocontrol bacterium, Pseudomonas chlororaphis O6 promotes plant health through multifaceted mechanisms. In this study, we used P. chlororaphis O6 mutants to examine metabolites with aphicidal activity. Direct application of intact P. chlororaphis cells to the surface of second-instar nymphs of the green peach aphid resulted in no mortality. However, nymphs died when exposed only to the volatiles produced by the P. chlororaphis O6 wild-type strain grown on rich media. Mutants lacking the production of two antibiotics, phenazine and pyrrolnitrin, or the insect toxin FitD retained the aphicidal potential of the wild-type strain. However, the volatiles produced by mutants deficient in the production of hydrogen cyanide (HCN) or defective in the synthesis of the global regulator GacS, which regulates HCN synthesis, showed no aphicidal activity. Direct application of potassium cyanide caused mortality of green peach aphid nymphs. These results indicate that HCN production by a plant probiotic is involved in preventing insect growth.