Aphids are devastating insect pests and cause great losses to agriculture worldwide. The sap-feeding plant pests harbour in their body cavity certain bacteria, which are essential for the aphids' fecundity and survival. Buchnera, the bacterium in question, can only grow inside aphids.

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New research findings, published this week, have shown that the mutually beneficial relationship is, however, inadvertently sabotaged by the bacterium which ‘betrays' the aphid by alerting target plants to its presence.

A research team led by scientists at the University of California, Riverside made the discovery. Nematology professor Isgouhi Kaloshian, who led the project, commented, "Although this betrayal is unintentional, it nevertheless alerts the plant about the aphid's presence and [means] the aphids are unable to reproduce in large numbers. A protein from the bacterium, found in the aphid saliva and likely delivered inside the plant host by the aphid, triggers plant immune responses against the aphid. It seems that the plant immune system targets the bacterium and exploits the strict mutual dependency between the plant and aphid to recognize the aphid as the intruder."

While feeding, aphids secrete saliva into plants. The researchers collected saliva from 100,000 aphids and analysed them to establish the general make-up of aphid saliva. They detected 105 proteins in the saliva and found that one, called GroEL, which comes from Buchnera bacteria, triggered a reaction from plants' immune systems.

"GroEL was known previously to trigger immunity in animals," Kaloshian continued. "However, our finding that it induces immunity in plants is new. Since most aphids harbor Buchnera, and likely have GroEL in their saliva, this bacterial protein may generally alert plants of the presence of aphids. How it is recognized by plants is still unknown."

Kaloshian said new knowledge gained from the research could be used to protect crops from aphids in future. The UC professor also said that, since Buchnera-related bacteria are present in a number of insects (other than aphids), the findings could be widely applicable to other pests.

The team are now working to identify the plant receptor that initiates the immune response to GroEL.